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SPECIAL AMPLIFIERS
LEARNING OBJECTIVES

• Upon completion of this chapter, you will be able to:
• Describe the basic operation of a differential amplifier.
• Describe the operation of a differential amplifier under the following conditions:
• Single Input, Single Output
• Single input, differential output
• Differential input, differential output
• List the characteristics of an operational amplifier.
• Identify the symbol for an operational amplifier.
• Label the blocks on a block diagram of an operational amplifier.
• Describe the operation of an operational amplifier with inverting and noninverting configurations.
• Describe the bandwidth of a typical operational amplifier and methods to modify the bandwidth.
• Identify the following applications of operational amplifiers:
• Adder
• Subtractor
• State the common usage for a magnetic amplifier.
• Describe the basic operation of a magnetic amplifier.
• Describe various methods of changing inductance.
• Identify the purpose of components in a simple magnetic amplifier.

INTRODUCTION

If you were to make a quick review of the subjects discussed in this module up to this point, you would see that you have been given a considerable amount of information about amplifiers. You have been shown what amplification is and how the different classes of amplifiers affect amplification. You also have been shown that many factors must be considered when working with amplifiers, such as impedance, feedback, Frequency response, and coupling. With all this information behind you, you might ask yourself "what more can there be to know about amplifiers?"
There is a great deal more to learn about amplifiers. Even after you finish this chapter you will have only "scratched the surface" of the study of amplifiers. But, you will have prepared yourself for the remainder of the NEETS. This, in turn, should prepare you for further study and, perhaps, a career in electronics.
As in chapter 2, the circuits shown in this chapter are intended to present particular concepts to you. Therefore, the circuits may be incomplete or not practical for use in an actual piece of electronic equipment. You should keep in mind the fact that this text is intended to teach certain facts about amplifiers, and in order to simplify the illustrations used, complete operational circuits are not always shown.
In this chapter three types of special amplifiers are discussed. These are: DIFFERENTIAL AMPLIFIERS, OPERATIONAL AMPLIFIERS, and MAGNETIC AMPLIFIERS. These are called special amplifiers because they are used only in certain types of equipment.
The names of each of these special amplifiers describe the operation of the amplifier, NOT what is amplified. For example, a magnetic amplifier does not amplify magnetism but uses magnetic effects to produce amplification of an electronic signal.
A differential amplifier is an amplifier that can have two input signals and/or two output signals. This amplifier can amplify the difference between two input signals. A differential amplifier will also "cancel out" common signals at the two inputs.
One of the more interesting aspects of an operational amplifier is that it can be used to perform mathematical operations electronically. Properly connected, an operational amplifier can add, subtract, multiply, divide, and even perform the calculus operations of integration and differentiation. These amplifiers were originally used in a type of computer known as the "analog computer" but are now used in many electronic applications.
The magnetic amplifier uses a device called a "saturable core reactor" to control an a.c.output signal. The primary use of magnetic amplifiers is in power control systems.
These brief descriptions of the three special amplifiers are intended to provide you with a general idea of what these amplifiers are and how they can be used. The remaining sections of this chapter will provide you with more detailed information on these special amplifiers.
DIFFERENTIAL AMPLIFIERS
A differential amplifier has two possible inputs and two possible outputs. This arrangement means that the differential amplifier can be used in a variety of ways. Before examining the three basic configurations that are possible with a differential amplifier, you need to be familiar with the basic circuitry of a differential amplifier.
BASIC DIFFERENTIAL AMPLIFIER CIRCUIT
Before you are shown the operation of a differential amplifier, you will be shown how a simpler circuit works. This simpler circuit, known as the DIFFERENCE AMPLIFIER, has one thing in common with the differential amplifier: It operates on the difference between two inputs. However, the difference amplifier has only one output while the differential amplifier can have two outputs.
Format By now, you should be familiar with some amplifier circuits, which should give you an idea of what a difference amplifier is like. In NEETS, module 7, you were shown the basic configurations for transistor amplifiers. Figure 3-1 shows two of these configurations: the common emitter and the common base. In view (A) of figure 3-1 a common-emitter amplifier is shown. The output signal is an amplified version of the input signal and is 180 degrees out of phase with the input signal. View (B) is a common-base amplifier. In this circuit the output signal is an amplified version of the input signal and is in phase with the input signal. In both of these circuits, the output signal is controlled by the base-to-emitter bias. As this bias changes (because of the input signal) the current through the transistor changes. This causes the output signal developed across the collector load (R2) to change. None of this information is new, it is just a review of what you have already been shown regarding transistor amplifiers.
Figure 3-1A. – Common-emitter and common-base amplifiers.

Figure 3-1B. – Common-emitter and common-base amplifiers.

NOTE: Bias arrangements for the following explanations will be termed base-to- emitter. In other publications you will see the term emitter-to-base used to describe the same bias arrangement.
THE TWO-INPUT, SINGLE-OUTPUT, DIFFERENCE AMPLIFIER
If you combine the common-base and common-emitter configurations into a single transistor amplifier, you will have a circuit like the one shown in figure 3-2. This circuit is the two-input, single-output, difference amplifier.
Figure 3-2. – Two-input, single-output, difference amplifier.

In figure 3-2, the transistor has two inputs (the emitter and the base) and one output (the collector). Remember, the current through the transistor (and therefore the output signal) is controlled by the base-to-emitter bias. In the circuit shown in figure 3-2, the combination of the two input signals controls the output signal. In fact, the DIFFERENCE BETWEEN THE INPUT SIGNALS determines the base-to-emitter bias.
For the purpose of examining the operation of the circuit shown in figure 3-2, assume that the circuit has a gain of -10. This means that for each 1-volt change in the base-to-emitter bias, there would be a 10-volt change in the output signal. Assume, also, that the input signals will peak at 1-volt levels (+1 volt for the positive peak and -1 volt for the negative peak). The secret to understanding this circuit (or any transistor amplifier circuit) is to realize that the collector current is controlled by the base-to-emitter bias. In other words, in this circuit the output signal (the voltage developed across R3) is determined by the difference between the voltage on the base and the voltage on the emitter.

Figure 3-3 shows this two-input, single-output amplifier with input signals that are equal in amplitude and 180 degrees out of phase. Input number one has a positive alternation when input number two has a negative alternation and vice versa.
Figure 3-3. – Input signals 180° out of phase.

The circuit and the input and output signals are shown at the top of the figure. The lower portion of the figure is a comparison of the input signals and the output signal. Notice the vertical lines marked "T0" through "T8." These represent "time zero" through "time eight." In other words, these lines provide a way to examine the two input signals and the output signal at various instants of time.
In figure 3-3 at time zero (T0) both input signals are at 0 volts. The output signal is also at 0 volts. Between time zero (T0) and time one (T1), input signal number one goes positive and input signal number two goes negative. Each of these voltage changes causes an increase in the base-to-emitter bias which causes current through Q1 to increase. Increased current through Q1 results in a greater voltage drop across the collector load (R3) which causes the output signal to go negative.
By time one (T1), input signal number one has reached +1 volt and input signal number two has reached -1 volt. This is an overall increase in base-to-emitter bias of 2 volts. Since the gain of the circuit is -10, the output signal has decreased by 20 volts. As you can see, the output signal has been determined by the difference between the two input signals. In fact, the base-to-emitter bias can be found by subtracting the value of input signal number two from the value of input signal number one.
Between time one (T1) and time two (T2), input signal number one goes from +1 volt to 0 volts and input signal number two goes from -1 volt to 0 volts. At time two (T2) both input signals are at 0 volts and the base-to-emitter bias has returned to 0 volts. The output signal is also 0 volts.
Between time two (T2) and time three (T3), input signal number one goes negative and input signal number two goes positive. At time three (T3), the value of the base-to-emitter bias is -2 volts.
This causes the output signal to be +20 volts at time three (T3).
Between time three (T3) and time four (T4), input signal #1 goes from -1 volt to 0 volts and input signal #2 goes from +1 volt to 0 volts. At time four (T4) both input signals are 0 volts, the bias is 0 volts, and the output is 0 volts.
During time four (T4) through time eight (T8), the circuit repeats the sequence of events that took place from time zero (T0) through time four (T4).
You can see that when the input signals are equal in amplitude and 180 degrees out of phase, the output signal is twice as large (40 volts peak to peak) as it would be from either input signal alone (if the other input signal were held at 0 volts).

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Input signals in phase

Figure 3-4 shows the two-input, single-output, difference amplifier with two input signals that are equal in amplitude and in phase.
Figure 3-4. – Input signals in phase.

Notice, that the output signal remains at 0 volts for the entire time (T0 – T8). Since the two input signals are equal in amplitude and in phase, the difference between them (the base-to-emitter bias) is always 0 volts. This causes a 0-volt output signal.
If you compute the bias at any time period (T0 – T8), you will see that the output of the circuit remains at a constant zero.

For example:

From the above example, you can see that when the input signals are equal in amplitude and in phase, there is no output from the difference amplifier because there is no difference between the two inputs. You also know that when the input signals are equal in amplitude but 180 degrees out of phase, the output looks just like the input except for amplitude and a 180-degree phase reversal with respect to input signal number one. What happens if the input signals are equal in amplitude but different in phase by something other than 180 degrees? This would mean that sometimes one signal would be going negative while the other would be going positive; sometimes both signals would be going positive; and sometimes both signals would be going negative. Would the output signal still look like the input signals? The answer is "no," because figure 3-5 shows a difference amplifier with two input signals that are equal in amplitude but 90 degrees out of phase. From the figure you can see that at time zero (T0) input number one is at 0 volts and input number two is at -1 volt. The base-to-emitter bias is found to be +1 volt.
Figure 3-5. – Input signals 90° out of phase.

This +1-volt bias signal causes the output signal to be -10 volts at time zero (T0). Between time zero (T0) and time one (T1), both input signals go positive. The difference between the input signals stays constant. The effect of this is to keep the bias at +1 volt for the entire time between T0 and T1. This, in turn, keeps the output signal at -10 volts.
Between time one (T1) and time two (T2), input signal number one goes in a negative direction but input signal number two continues to go positive. Now the difference between the input signals decreases rapidly from +1 volt. Halfway between T1 and T2 (the dotted vertical line), input signal number one and input signal number two are equal in amplitude. The difference between the input signals is 0 volts and this causes the output signal to be 0 volts. From this point to T2 the difference between the input signals is a negative value. At T2:

From time two (T2) to time three (T3), input signal number one goes negative and input signal number two goes to zero. The difference between them stays constant at -1 volt. Therefore, the output signal stays at a +10-volt level for the entire time period from T2 to T3. At T3 the bias condition will be:
Between T3 and T4 input signal number one goes to zero while input signal number two goes negative. This, again, causes a rapid change in the difference between the input signals. Halfway between T3 and T4 (the dotted vertical line) the two input signals are equal in amplitude; therefore, the difference between the input signals is 0 volts, and the output signal becomes 0 volts. From that point to T4, the difference between the input signals becomes a positive voltage. At T4:

(The sequence of events from T4 to T8 are the same as those of T0 to T4.)
As you have seen, this amplifier amplifies the difference between two input signals. But this is NOT a differential amplifier. A differential amplifier has two inputs and two outputs. The circuit you have just been shown has only one output. Well then, how does a differential amplifier schematic look?
TYPICAL DIFFERENTIAL AMPLIFIER CIRCUIT
Figure 3-6 is the schematic diagram of a typical differential amplifier. Notice that there are two inputs and two outputs. This circuit requires two transistors to provide the two inputs and two outputs. If you look at one input and the transistor with which it is associated, you will see that each transistor is a common-emitter amplifier for that input (input one and Q1; input two and Q2). R1 develops the signal at input one for Q1, and R5 develops the signal at input two for Q2. R3 is the emitter resistor for both Q1 and Q2. Notice that R3 is NOT bypassed. This means that when a signal at input one affects the current through Q1, that signal is developed by R3. (The current through Q1 must flow through R3; as this current changes, the voltage developed across R3 changes.) When a signal is developed by R3, it is applied to the emitter of Q2. In the same way, signals at input two affect the current of Q2, are developed by R3, and are felt on the emitter of Q1. R2 develops the signal for output one, and R4 develops the signal for output two.
Figure 3-6. – Differential amplifier.

Even though this circuit is designed to have two inputs and two outputs, it is not necessary to use both inputs and both outputs. (Remember, a differential amplifier was defined as having two possible inputs and two possible outputs.) A differential amplifier can be connected as a single-input, single-output device; a single-input, differential-output device; or a differential-input, differential-output device.

Q.1 How many inputs and outputs are possible with a differential amplifier?

Q.2 What two transistor amplifier configurations are combined in the single-transistor, two-input, single-output difference amplifier?

Q.3 If the two input signals of a difference amplifier are in phase and equal in amplitude, what will the output signal be?

Q.4 If the two input signals to a difference amplifier are equal in amplitude and 180 degrees out of phase, what will the output signal be?

Q.5 If only one input signal is used with a difference amplifier, what will the output signal be?

Q.6 If the two input signals to a difference amplifier are equal in amplitude but neither in phase nor 180 degrees out of phase, what will the output signal be?

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SINGLE-INPUT, SINGLE-OUTPUT, DIFFERENTIAL AMPLIFIER

Figure 3-7 shows a differential amplifier with one input (the base of Q1) and one output (the collector of Q2). The second input (the base of Q2) is grounded and the second output (the collector of Q1) is not used.
Figure 3-7. – Single-input, single-output differential amplifier.

When the input signal developed by R1 goes positive, the current through Q1 increases. This increased current causes a positive-going signal at the top of R3. This signal is felt on the emitter of Q2. Since the base of Q2 is grounded, the current through Q2 decreases with a positive-going signal on the emitter. This decreased current causes less voltage drop across R4. Therefore, the voltage at the bottom of R4 increases and a positive-going signal is felt at the output.
When the input signal developed by R1 goes negative, the current through Q1 decreases. This decreased current causes a negative-going signal at the top of R3. This signal is felt on the emitter of Q2. When the emitter of Q2 goes negative, the current through Q2 increases. This increased current causes more of a voltage drop across R4. Therefore, the voltage at the bottom of R4 decreases and a negative-going signal is felt at the output.
This single-input, single-output, differential amplifier is very similar to a single-transistor amplifier as far as input and output signals are concerned. This use of a differential amplifier does provide amplification of a.c. or d.c. signals but does not take full advantage of the characteristics of a differential amplifier.
SINGLE-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
In chapter one of this module you were shown several phase splitters. You should remember that a phase splitter provides two outputs from a single input. These two outputs are 180 degrees out of phase with each other. The single-input, differential-output, differential amplifier will do the same thing.
Figure 3-8 shows a differential amplifier with one input (the base of Q1) and two outputs (the collectors of Q1 and Q2). One output is in phase with the input signal, and the other output is 180 degrees out of phase with the input signal. The outputs are differential outputs.
Figure 3-8. – Single-input, differential-output differential amplifier.

This circuit’s operation is the same as for the single-input, single-output differential

amplifier just described. However, another output is obtained from the bottom of R2. As the input signal goes positive, thus causing increased current through Q1, R2 has a greater voltage drop. The output signal at the bottom of R2 therefore is negative going. A negative-going input signal will decrease current and reverse the polarities of both output signals.
Now you see how a differential amplifier can produce two amplified, differential output signals from a single-input signal. One further point of interest about this configuration is that if a combined output signal is taken between outputs number one and two, this single output will be twice the amplitude of the individual outputs. In other words, you can double the gain of the differential amplifier (single output) by taking the output signal between the two output terminals. This single-output signal will be in phase with the input signal. This is shown by the phantom signal above R5 (the phantom resistor connected between outputs number one and two would be used to develop this signal).
DIFFERENTIAL-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
When a differential amplifier is connected with a differential input and a differential output, the full potential of the circuit is used. Figure 3-9 shows a differential amplifier with this type of configuration (differential-input, differential-output).
Figure 3-9. – Differential-input, differential-output differential amplifier.]

Normally, this configuration uses two input signals that are 180 degrees out of phase. This causes the difference (differential) signal to be twice as large as either input alone. (This is just like the two-input, single-output difference amplifier with input signals that are 180 degrees out of phase.)
Output number one is a signal that is in phase with input number two, and output number two is a signal that is in phase with input number one. The amplitude of each output signal is the input signal multiplied by the gain of the amplifier. With 180-degree-out-of-phase input signals, each output signal is greater in amplitude than either input signal by a factor of the gain of the amplifier.
When an output signal is taken between the two output terminals of the amplifier (as shown by the phantom connections, resistor, and signal), the combined output signal is twice as great in amplitude as either signal at output number one or output number two. (This is because output number one and output number two are 180 degrees out of phase with each other.) When the input signals are 180 degrees out of phase, the amplitude of the combined output signal is equal to the amplitude of one input signal multiplied by two times the gain of the amplifier.
When the input signals are not 180 degrees out of phase, the combined output signal taken across output one and output two is similar to the output that you were shown for the two-input, single-output, difference amplifier. The differential amplifier can have two outputs (180 degrees out of phase with each other), or the outputs can be combined as shown in figure 3-9.
In answering Q7 through Q9 use the following information: All input signals are sine waves with a peak-to-peak amplitude of 10 millivolts. The gain of the differential amplifier is 10.

Q.7 If the differential amplifier is configured with a single input and a single output, what will the peak-to-peak amplitude of the output signal be?

Q.8 If the differential amplifier is configured with a single input and differential outputs, what will the output signals be?

Q.9 If the single-input, differential-output, differential amplifier has an output signal taken between the two output terminals, what will the peak-to-peak amplitude of this combined output be?
In answering Q10 through Q14 use the following information: A differential amplifier is configured with a differential input and a differential output. All input signals are sine waves with a peak-to-peak amplitude of 10 millivolts. The gain of the differential amplifier is 10.
Q.10 If the input signals are in phase, what will be the peak-to-peak amplitude of the output signals?

Q.11 If the input signals are 180 degrees out of phase with each other, what will be the peak-to-peak amplitude of the output signals?

Q.12 If the input signals are 180 degrees out of phase with each other, what will the
phase relationship be between (a) the output signals and (b) the input and output signals?

Q.13 If the input signals are 180 degrees out of phase with each other and a combined output is taken between the two output terminals, what will the amplitude of the combined output signal be?

Q.14 If the input signals are 90 degrees out of phase with each other and a combined output is taken between the two output terminals, (a) what will the peak-to-peak amplitude of the combined output signal be, and (b) will the combined output signal be a sine wave?

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OPERATIONAL AMPLIFIERS

An OPERATIONAL AMPLIFIER (OP AMP) is an amplifier which is designed to be used with other circuit components to perform either computing functions (addition, subtraction) or some type of transfer operation, such as filtering. Operational amplifiers are usually high-gain amplifiers with the amount of gain determined by feedback.
Operational amplifiers have been in use for some time. They were originally developed for analog (non-digital) computers and used to perform mathematical functions. Operational amplifiers were not used in other devices very much because they were expensive and more complicated than other circuits.
Today many devices use operational amplifiers. Operational amplifiers are used as d.c. amplifiers, a.c. amplifiers, comparators, oscillators (which are covered in NEETS, module 9), filter circuits, and many other applications. The reason for this widespread use of the operational amplifier is that it is a very versatile and efficient device. As an integrated circuit (chip) the operational amplifier has become an inexpensive and readily available "building block" for many devices. In fact, an operational amplifier in integrated circuit form is no more expensive than a good transistor.
CHARACTERISTICS OF AN OPERATIONAL AMPLIFIER
The schematic symbols for an operational amplifier are shown in figure 3-10. View (A) shows the power supply requirements while view (B) shows only the input and output terminals. An operational amplifier is a special type of high-gain, d.c. amplifier. To be classified as an operational amplifier, the circuit must have certain characteristics. The three most important characteristics of an operational amplifier are:

• Very high gain
• Very high input impedance
• Very low output impedance

Figure 3-10A. – Schematic symbols of an operational amplifier.

Figure 3-10B. – Schematic symbols of an operational amplifier.

Since no single amplifier stage can provide all these characteristics well enough to be considered an operational amplifier, various amplifier stages are connected together. The total circuit made up of these individual stages is called an operational amplifier. This circuit (the operational amplifier) can be made up of individual components (transistors, resistors, capacitors, etc.), but the most common form of the operational amplifier is an integrated circuit. The integrated circuit (chip) will contain the various stages of the operational amplifier and can be treated and used as if it were a single stage.
BLOCK DIAGRAM OF AN OPERATIONAL AMPLIFIER
Figure 3-11 is a block diagram of an operational amplifier. Notice that there are three stages within the operational amplifier.
Figure 3-11. – Block diagram of an operational amplifier.

The input stage is a differential amplifier. The differential amplifier used as an input stage provides differential inputs and a Frequency response down to d.c. Special techniques are used to provide the high input impedance necessary for the operational amplifier.
The second stage is a high-gain voltage amplifier. This stage may be made from several transistors to provide high gain. A typical operational amplifier could have a voltage gain of 200,000. Most of this gain comes from the voltage amplifier stage.
The final stage of the OP AMP is an output amplifier. The output amplifier provides low output impedance. The actual circuit used could be an emitter follower. The output stage should allow the operational amplifier to deliver several milliamperes to a load.
Notice that the operational amplifier has a positive power supply (+V CC) and a negative power supply (-VEE). This arrangement enables the operational amplifier to produce either a positive or a negative output.
The two input terminals are labeled "inverting input" (-) and "noninverting input" (+). The operational amplifier can be used with three different input conditions (modes). With differential inputs (first mode), both input terminals are used and two input signals which are 180 degrees out of phase with each other are used. This produces an output signal that is in phase with the signal on the noninverting input. If the noninverting input is grounded and a signal is applied to the inverting input (second mode), the output signal will be 180 degrees out of phase with the input signal (and one-half the amplitude of the first mode output). If the inverting input is grounded and a signal is applied to the noninverting input (third mode), the
output signal will be in phase with the input signal (and one-half the amplitude of the first mode output).

Q.15 What are the three requirements for an operational amplifier?

Q.16 What is the most commonly used form of the operational amplifier?

Q.17 Draw the schematic symbol for an operational amplifier.

Q.18 Label the parts of the operational amplifier shown in figure 3-12.

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Operational amplifier

Figure 3-12. – Operational amplifier.

CLOSED-LOOP OPERATION OF AN OPERATIONAL AMPLIFIER
Operational amplifiers can have either a closed-loop operation or an open-loop operation. The operation (closed-loop or open-loop) is determined by whether or not feedback is used. Without feedback the operational amplifier has an open-loop operation. This open-loop operation is practical only when the operational amplifier is used as a comparator (a circuit which compares two input signals or compares an input signal to some fixed level of voltage). As an amplifier, the open-loop operation is not practical because the very high gain of the operational amplifier creates poor stability. (Noise and other unwanted signals are amplified so much in open-loop operation that the operational amplifier is usually not used in this way.) Therefore, most operational amplifiers are used with feedback (closed-loop operation).
Operational amplifiers are used with degenerative (or negative) feedback which reduces the gain of the operational amplifier but greatly increases the stability of the circuit. In the closed-loop configuration, the output signal is applied back to one of the input terminals. This feedback is always degenerative (negative). In other words, the feedback signal always opposes the effects of the original input signal. One result of degenerative feedback is that the inverting and noninverting inputs to the operational amplifier will be kept at the same potential.
Closed-loop circuits can be of the inverting configuration or noninverting configuration. Since the inverting configuration is used more often than the noninverting configuration, the inverting configuration will be shown first.
Inverting Configuration
Figure 3-13 shows an operational amplifier in a closed-loop, inverting configuration. Resistor R2 is used to feed part of the output signal back to the input of the operational amplifier.
Figure 3-13. – Inverting configuration.

At this point it is important to keep in mind the difference between the entire circuit (or operational circuit) and the operational amplifier. The operational amplifier is represented by the triangle-like symbol while the operational circuit includes the resistors and any other components as well as the operational amplifier. In other words, the input to the circuit is shown in figure 3-13, but the signal at the inverting input of the operational amplifier is determined by the feedback signal as well as by the circuit input signal.
As you can see in figure 3-13, the output signal is 180 degrees out of phase with the input signal. The feedback signal is a portion of the output signal and, therefore, also 180 degrees out of phase with the input signal. Whenever the input signal goes positive, the output signal and the feedback signal go negative. The result of this is that the inverting input to the operational amplifier is always very close to 0 volts with this configuration. In fact, with the noninverting input grounded, the voltage at the inverting input to the operational amplifier is so small compared to other voltages in the circuit that it is considered to be VIRTUAL GROUND. (Remember, in a closed-loop operation the inverting and noninverting inputs are at the same potential.)
Virtual ground is a point in a circuit which is at ground potential (0 volts) but is NOT connected to ground. Figure 3-14, (view A) (view B) and (view C), shows an example of several circuits with points at virtual ground.
Figure 3-14A. – Virtual ground circuits.

Figure 3-14B. – Virtual ground circuits.

Figure 3-14C. – Virtual ground circuits.

In view (A), V1 (the left-hand battery) supplies +10 volts to the circuit while V2 (the right-hand battery) supplies -10 volts to the circuit. The total difference in potential in the circuit is 20 volts.
The total resistance of the circuit can be calculated:
Now that the total resistance is known, the circuit current can be calculated:
The voltage drop across R1 can be computed:
The voltage at point A would be equal to the voltage of V1 minus the voltage drop of R1.
To check this result, compute the voltage drop across R2 and subtract this from the voltage at point A. The result should be the voltage of V2.
It is not necessary that the voltage supplies be equal to create a point of virtual ground. In view (B) V1 supplies +1 volt to the circuit while V2 supplies -10 volts. The total difference in potential is 11 volts. The total resistance of this circuit (R1 + R2) is 11 ohms. The total current (IT) is 1 ampere. The voltage drop across R1 (E R1 = R1 X IT) is 1 volt. The voltage drop across R2 (ER2 = R2 X I T) is 10 volts. The voltage at
point A can be computed:
So point A is at virtual ground in this circuit also. To check the results, compute the voltage at V2.

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SPECIAL AMPLIFIERS
LEARNING OBJECTIVES

• Upon completion of this chapter, you will be able to:
• Describe the basic operation of a differential amplifier.
• Describe the operation of a differential amplifier under the following conditions:
• Single Input, Single Output
• Single input, differential output
• Differential input, differential output
• List the characteristics of an operational amplifier.
• Identify the symbol for an operational amplifier.
• Label the blocks on a block diagram of an operational amplifier.
• Describe the operation of an operational amplifier with inverting and noninverting configurations.
• Describe the bandwidth of a typical operational amplifier and methods to modify the bandwidth.
• Identify the following applications of operational amplifiers:
• Adder
• Subtractor
• State the common usage for a magnetic amplifier.
• Describe the basic operation of a magnetic amplifier.
• Describe various methods of changing inductance.
• Identify the purpose of components in a simple magnetic amplifier.

INTRODUCTION

If you were to make a quick review of the subjects discussed in this module up to this point, you would see that you have been given a considerable amount of information about amplifiers. You have been shown what amplification is and how the different classes of amplifiers affect amplification. You also have been shown that many factors must be considered when working with amplifiers, such as impedance, feedback, Frequency response, and coupling. With all this information behind you, you might ask yourself "what more can there be to know about amplifiers?"
There is a great deal more to learn about amplifiers. Even after you finish this chapter you will have only "scratched the surface" of the study of amplifiers. But, you will have prepared yourself for the remainder of the NEETS. This, in turn, should prepare you for further study and, perhaps, a career in electronics.
As in chapter 2, the circuits shown in this chapter are intended to present particular concepts to you. Therefore, the circuits may be incomplete or not practical for use in an actual piece of electronic equipment. You should keep in mind the fact that this text is intended to teach certain facts about amplifiers, and in order to simplify the illustrations used, complete operational circuits are not always shown.
In this chapter three types of special amplifiers are discussed. These are: DIFFERENTIAL AMPLIFIERS, OPERATIONAL AMPLIFIERS, and MAGNETIC AMPLIFIERS. These are called special amplifiers because they are used only in certain types of equipment.
The names of each of these special amplifiers describe the operation of the amplifier, NOT what is amplified. For example, a magnetic amplifier does not amplify magnetism but uses magnetic effects to produce amplification of an electronic signal.
A differential amplifier is an amplifier that can have two input signals and/or two output signals. This amplifier can amplify the difference between two input signals. A differential amplifier will also "cancel out" common signals at the two inputs.
One of the more interesting aspects of an operational amplifier is that it can be used to perform mathematical operations electronically. Properly connected, an operational amplifier can add, subtract, multiply, divide, and even perform the calculus operations of integration and differentiation. These amplifiers were originally used in a type of computer known as the "analog computer" but are now used in many electronic applications.
The magnetic amplifier uses a device called a "saturable core reactor" to control an a.c.output signal. The primary use of magnetic amplifiers is in power control systems.
These brief descriptions of the three special amplifiers are intended to provide you with a general idea of what these amplifiers are and how they can be used. The remaining sections of this chapter will provide you with more detailed information on these special amplifiers.
DIFFERENTIAL AMPLIFIERS
A differential amplifier has two possible inputs and two possible outputs. This arrangement means that the differential amplifier can be used in a variety of ways. Before examining the three basic configurations that are possible with a differential amplifier, you need to be familiar with the basic circuitry of a differential amplifier.
BASIC DIFFERENTIAL AMPLIFIER CIRCUIT
Before you are shown the operation of a differential amplifier, you will be shown how a simpler circuit works. This simpler circuit, known as the DIFFERENCE AMPLIFIER, has one thing in common with the differential amplifier: It operates on the difference between two inputs. However, the difference amplifier has only one output while the differential amplifier can have two outputs.
Format By now, you should be familiar with some amplifier circuits, which should give you an idea of what a difference amplifier is like. In NEETS, module 7, you were shown the basic configurations for transistor amplifiers. Figure 3-1 shows two of these configurations: the common emitter and the common base. In view (A) of figure 3-1 a common-emitter amplifier is shown. The output signal is an amplified version of the input signal and is 180 degrees out of phase with the input signal. View (B) is a common-base amplifier. In this circuit the output signal is an amplified version of the input signal and is in phase with the input signal. In both of these circuits, the output signal is controlled by the base-to-emitter bias. As this bias changes (because of the input signal) the current through the transistor changes. This causes the output signal developed across the collector load (R2) to change. None of this information is new, it is just a review of what you have already been shown regarding transistor amplifiers.
Figure 3-1A. – Common-emitter and common-base amplifiers.

Figure 3-1B. – Common-emitter and common-base amplifiers.

NOTE: Bias arrangements for the following explanations will be termed base-to- emitter. In other publications you will see the term emitter-to-base used to describe the same bias arrangement.
THE TWO-INPUT, SINGLE-OUTPUT, DIFFERENCE AMPLIFIER
If you combine the common-base and common-emitter configurations into a single transistor amplifier, you will have a circuit like the one shown in figure 3-2. This circuit is the two-input, single-output, difference amplifier.
Figure 3-2. – Two-input, single-output, difference amplifier.

In figure 3-2, the transistor has two inputs (the emitter and the base) and one output (the collector). Remember, the current through the transistor (and therefore the output signal) is controlled by the base-to-emitter bias. In the circuit shown in figure 3-2, the combination of the two input signals controls the output signal. In fact, the DIFFERENCE BETWEEN THE INPUT SIGNALS determines the base-to-emitter bias.
For the purpose of examining the operation of the circuit shown in figure 3-2, assume that the circuit has a gain of -10. This means that for each 1-volt change in the base-to-emitter bias, there would be a 10-volt change in the output signal. Assume, also, that the input signals will peak at 1-volt levels (+1 volt for the positive peak and -1 volt for the negative peak). The secret to understanding this circuit (or any transistor amplifier circuit) is to realize that the collector current is controlled by the base-to-emitter bias. In other words, in this circuit the output signal (the voltage developed across R3) is determined by the difference between the voltage on the base and the voltage on the emitter.

Figure 3-3 shows this two-input, single-output amplifier with input signals that are equal in amplitude and 180 degrees out of phase. Input number one has a positive alternation when input number two has a negative alternation and vice versa.
Figure 3-3. – Input signals 180° out of phase.

The circuit and the input and output signals are shown at the top of the figure. The lower portion of the figure is a comparison of the input signals and the output signal. Notice the vertical lines marked "T0" through "T8." These represent "time zero" through "time eight." In other words, these lines provide a way to examine the two input signals and the output signal at various instants of time.
In figure 3-3 at time zero (T0) both input signals are at 0 volts. The output signal is also at 0 volts. Between time zero (T0) and time one (T1), input signal number one goes positive and input signal number two goes negative. Each of these voltage changes causes an increase in the base-to-emitter bias which causes current through Q1 to increase. Increased current through Q1 results in a greater voltage drop across the collector load (R3) which causes the output signal to go negative.
By time one (T1), input signal number one has reached +1 volt and input signal number two has reached -1 volt. This is an overall increase in base-to-emitter bias of 2 volts. Since the gain of the circuit is -10, the output signal has decreased by 20 volts. As you can see, the output signal has been determined by the difference between the two input signals. In fact, the base-to-emitter bias can be found by subtracting the value of input signal number two from the value of input signal number one.
Between time one (T1) and time two (T2), input signal number one goes from +1 volt to 0 volts and input signal number two goes from -1 volt to 0 volts. At time two (T2) both input signals are at 0 volts and the base-to-emitter bias has returned to 0 volts. The output signal is also 0 volts.
Between time two (T2) and time three (T3), input signal number one goes negative and input signal number two goes positive. At time three (T3), the value of the base-to-emitter bias is -2 volts.
This causes the output signal to be +20 volts at time three (T3).
Between time three (T3) and time four (T4), input signal #1 goes from -1 volt to 0 volts and input signal #2 goes from +1 volt to 0 volts. At time four (T4) both input signals are 0 volts, the bias is 0 volts, and the output is 0 volts.
During time four (T4) through time eight (T8), the circuit repeats the sequence of events that took place from time zero (T0) through time four (T4).
You can see that when the input signals are equal in amplitude and 180 degrees out of phase, the output signal is twice as large (40 volts peak to peak) as it would be from either input signal alone (if the other input signal were held at 0 volts).

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Input signals in phase

Figure 3-4 shows the two-input, single-output, difference amplifier with two input signals that are equal in amplitude and in phase.
Figure 3-4. – Input signals in phase.

Notice, that the output signal remains at 0 volts for the entire time (T0 – T8). Since the two input signals are equal in amplitude and in phase, the difference between them (the base-to-emitter bias) is always 0 volts. This causes a 0-volt output signal.
If you compute the bias at any time period (T0 – T8), you will see that the output of the circuit remains at a constant zero.

For example:

From the above example, you can see that when the input signals are equal in amplitude and in phase, there is no output from the difference amplifier because there is no difference between the two inputs. You also know that when the input signals are equal in amplitude but 180 degrees out of phase, the output looks just like the input except for amplitude and a 180-degree phase reversal with respect to input signal number one. What happens if the input signals are equal in amplitude but different in phase by something other than 180 degrees? This would mean that sometimes one signal would be going negative while the other would be going positive; sometimes both signals would be going positive; and sometimes both signals would be going negative. Would the output signal still look like the input signals? The answer is "no," because figure 3-5 shows a difference amplifier with two input signals that are equal in amplitude but 90 degrees out of phase. From the figure you can see that at time zero (T0) input number one is at 0 volts and input number two is at -1 volt. The base-to-emitter bias is found to be +1 volt.
Figure 3-5. – Input signals 90° out of phase.

This +1-volt bias signal causes the output signal to be -10 volts at time zero (T0). Between time zero (T0) and time one (T1), both input signals go positive. The difference between the input signals stays constant. The effect of this is to keep the bias at +1 volt for the entire time between T0 and T1. This, in turn, keeps the output signal at -10 volts.
Between time one (T1) and time two (T2), input signal number one goes in a negative direction but input signal number two continues to go positive. Now the difference between the input signals decreases rapidly from +1 volt. Halfway between T1 and T2 (the dotted vertical line), input signal number one and input signal number two are equal in amplitude. The difference between the input signals is 0 volts and this causes the output signal to be 0 volts. From this point to T2 the difference between the input signals is a negative value. At T2:

From time two (T2) to time three (T3), input signal number one goes negative and input signal number two goes to zero. The difference between them stays constant at -1 volt. Therefore, the output signal stays at a +10-volt level for the entire time period from T2 to T3. At T3 the bias condition will be:
Between T3 and T4 input signal number one goes to zero while input signal number two goes negative. This, again, causes a rapid change in the difference between the input signals. Halfway between T3 and T4 (the dotted vertical line) the two input signals are equal in amplitude; therefore, the difference between the input signals is 0 volts, and the output signal becomes 0 volts. From that point to T4, the difference between the input signals becomes a positive voltage. At T4:

(The sequence of events from T4 to T8 are the same as those of T0 to T4.)
As you have seen, this amplifier amplifies the difference between two input signals. But this is NOT a differential amplifier. A differential amplifier has two inputs and two outputs. The circuit you have just been shown has only one output. Well then, how does a differential amplifier schematic look?
TYPICAL DIFFERENTIAL AMPLIFIER CIRCUIT
Figure 3-6 is the schematic diagram of a typical differential amplifier. Notice that there are two inputs and two outputs. This circuit requires two transistors to provide the two inputs and two outputs. If you look at one input and the transistor with which it is associated, you will see that each transistor is a common-emitter amplifier for that input (input one and Q1; input two and Q2). R1 develops the signal at input one for Q1, and R5 develops the signal at input two for Q2. R3 is the emitter resistor for both Q1 and Q2. Notice that R3 is NOT bypassed. This means that when a signal at input one affects the current through Q1, that signal is developed by R3. (The current through Q1 must flow through R3; as this current changes, the voltage developed across R3 changes.) When a signal is developed by R3, it is applied to the emitter of Q2. In the same way, signals at input two affect the current of Q2, are developed by R3, and are felt on the emitter of Q1. R2 develops the signal for output one, and R4 develops the signal for output two.
Figure 3-6. – Differential amplifier.

Even though this circuit is designed to have two inputs and two outputs, it is not necessary to use both inputs and both outputs. (Remember, a differential amplifier was defined as having two possible inputs and two possible outputs.) A differential amplifier can be connected as a single-input, single-output device; a single-input, differential-output device; or a differential-input, differential-output device.

Q.1 How many inputs and outputs are possible with a differential amplifier?

Q.2 What two transistor amplifier configurations are combined in the single-transistor, two-input, single-output difference amplifier?

Q.3 If the two input signals of a difference amplifier are in phase and equal in amplitude, what will the output signal be?

Q.4 If the two input signals to a difference amplifier are equal in amplitude and 180 degrees out of phase, what will the output signal be?

Q.5 If only one input signal is used with a difference amplifier, what will the output signal be?

Q.6 If the two input signals to a difference amplifier are equal in amplitude but neither in phase nor 180 degrees out of phase, what will the output signal be?

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SINGLE-INPUT, SINGLE-OUTPUT, DIFFERENTIAL AMPLIFIER

Figure 3-7 shows a differential amplifier with one input (the base of Q1) and one output (the collector of Q2). The second input (the base of Q2) is grounded and the second output (the collector of Q1) is not used.
Figure 3-7. – Single-input, single-output differential amplifier.

When the input signal developed by R1 goes positive, the current through Q1 increases. This increased current causes a positive-going signal at the top of R3. This signal is felt on the emitter of Q2. Since the base of Q2 is grounded, the current through Q2 decreases with a positive-going signal on the emitter. This decreased current causes less voltage drop across R4. Therefore, the voltage at the bottom of R4 increases and a positive-going signal is felt at the output.
When the input signal developed by R1 goes negative, the current through Q1 decreases. This decreased current causes a negative-going signal at the top of R3. This signal is felt on the emitter of Q2. When the emitter of Q2 goes negative, the current through Q2 increases. This increased current causes more of a voltage drop across R4. Therefore, the voltage at the bottom of R4 decreases and a negative-going signal is felt at the output.
This single-input, single-output, differential amplifier is very similar to a single-transistor amplifier as far as input and output signals are concerned. This use of a differential amplifier does provide amplification of a.c. or d.c. signals but does not take full advantage of the characteristics of a differential amplifier.
SINGLE-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
In chapter one of this module you were shown several phase splitters. You should remember that a phase splitter provides two outputs from a single input. These two outputs are 180 degrees out of phase with each other. The single-input, differential-output, differential amplifier will do the same thing.
Figure 3-8 shows a differential amplifier with one input (the base of Q1) and two outputs (the collectors of Q1 and Q2). One output is in phase with the input signal, and the other output is 180 degrees out of phase with the input signal. The outputs are differential outputs.
Figure 3-8. – Single-input, differential-output differential amplifier.

This circuit’s operation is the same as for the single-input, single-output differential

amplifier just described. However, another output is obtained from the bottom of R2. As the input signal goes positive, thus causing increased current through Q1, R2 has a greater voltage drop. The output signal at the bottom of R2 therefore is negative going. A negative-going input signal will decrease current and reverse the polarities of both output signals.
Now you see how a differential amplifier can produce two amplified, differential output signals from a single-input signal. One further point of interest about this configuration is that if a combined output signal is taken between outputs number one and two, this single output will be twice the amplitude of the individual outputs. In other words, you can double the gain of the differential amplifier (single output) by taking the output signal between the two output terminals. This single-output signal will be in phase with the input signal. This is shown by the phantom signal above R5 (the phantom resistor connected between outputs number one and two would be used to develop this signal).
DIFFERENTIAL-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
When a differential amplifier is connected with a differential input and a differential output, the full potential of the circuit is used. Figure 3-9 shows a differential amplifier with this type of configuration (differential-input, differential-output).
Figure 3-9. – Differential-input, differential-output differential amplifier.]

Normally, this configuration uses two input signals that are 180 degrees out of phase. This causes the difference (differential) signal to be twice as large as either input alone. (This is just like the two-input, single-output difference amplifier with input signals that are 180 degrees out of phase.)
Output number one is a signal that is in phase with input number two, and output number two is a signal that is in phase with input number one. The amplitude of each output signal is the input signal multiplied by the gain of the amplifier. With 180-degree-out-of-phase input signals, each output signal is greater in amplitude than either input signal by a factor of the gain of the amplifier.
When an output signal is taken between the two output terminals of the amplifier (as shown by the phantom connections, resistor, and signal), the combined output signal is twice as great in amplitude as either signal at output number one or output number two. (This is because output number one and output number two are 180 degrees out of phase with each other.) When the input signals are 180 degrees out of phase, the amplitude of the combined output signal is equal to the amplitude of one input signal multiplied by two times the gain of the amplifier.
When the input signals are not 180 degrees out of phase, the combined output signal taken across output one and output two is similar to the output that you were shown for the two-input, single-output, difference amplifier. The differential amplifier can have two outputs (180 degrees out of phase with each other), or the outputs can be combined as shown in figure 3-9.
In answering Q7 through Q9 use the following information: All input signals are sine waves with a peak-to-peak amplitude of 10 millivolts. The gain of the differential amplifier is 10.

Q.7 If the differential amplifier is configured with a single input and a single output, what will the peak-to-peak amplitude of the output signal be?

Q.8 If the differential amplifier is configured with a single input and differential outputs, what will the output signals be?

Q.9 If the single-input, differential-output, differential amplifier has an output signal taken between the two output terminals, what will the peak-to-peak amplitude of this combined output be?
In answering Q10 through Q14 use the following information: A differential amplifier is configured with a differential input and a differential output. All input signals are sine waves with a peak-to-peak amplitude of 10 millivolts. The gain of the differential amplifier is 10.
Q.10 If the input signals are in phase, what will be the peak-to-peak amplitude of the output signals?

Q.11 If the input signals are 180 degrees out of phase with each other, what will be the peak-to-peak amplitude of the output signals?

Q.12 If the input signals are 180 degrees out of phase with each other, what will the
phase relationship be between (a) the output signals and (b) the input and output signals?

Q.13 If the input signals are 180 degrees out of phase with each other and a combined output is taken between the two output terminals, what will the amplitude of the combined output signal be?

Q.14 If the input signals are 90 degrees out of phase with each other and a combined output is taken between the two output terminals, (a) what will the peak-to-peak amplitude of the combined output signal be, and (b) will the combined output signal be a sine wave?

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OPERATIONAL AMPLIFIERS

An OPERATIONAL AMPLIFIER (OP AMP) is an amplifier which is designed to be used with other circuit components to perform either computing functions (addition, subtraction) or some type of transfer operation, such as filtering. Operational amplifiers are usually high-gain amplifiers with the amount of gain determined by feedback.
Operational amplifiers have been in use for some time. They were originally developed for analog (non-digital) computers and used to perform mathematical functions. Operational amplifiers were not used in other devices very much because they were expensive and more complicated than other circuits.
Today many devices use operational amplifiers. Operational amplifiers are used as d.c. amplifiers, a.c. amplifiers, comparators, oscillators (which are covered in NEETS, module 9), filter circuits, and many other applications. The reason for this widespread use of the operational amplifier is that it is a very versatile and efficient device. As an integrated circuit (chip) the operational amplifier has become an inexpensive and readily available "building block" for many devices. In fact, an operational amplifier in integrated circuit form is no more expensive than a good transistor.
CHARACTERISTICS OF AN OPERATIONAL AMPLIFIER
The schematic symbols for an operational amplifier are shown in figure 3-10. View (A) shows the power supply requirements while view (B) shows only the input and output terminals. An operational amplifier is a special type of high-gain, d.c. amplifier. To be classified as an operational amplifier, the circuit must have certain characteristics. The three most important characteristics of an operational amplifier are:

• Very high gain
• Very high input impedance
• Very low output impedance

Figure 3-10A. – Schematic symbols of an operational amplifier.

Figure 3-10B. – Schematic symbols of an operational amplifier.

Since no single amplifier stage can provide all these characteristics well enough to be considered an operational amplifier, various amplifier stages are connected together. The total circuit made up of these individual stages is called an operational amplifier. This circuit (the operational amplifier) can be made up of individual components (transistors, resistors, capacitors, etc.), but the most common form of the operational amplifier is an integrated circuit. The integrated circuit (chip) will contain the various stages of the operational amplifier and can be treated and used as if it were a single stage.
BLOCK DIAGRAM OF AN OPERATIONAL AMPLIFIER
Figure 3-11 is a block diagram of an operational amplifier. Notice that there are three stages within the operational amplifier.
Figure 3-11. – Block diagram of an operational amplifier.

The input stage is a differential amplifier. The differential amplifier used as an input stage provides differential inputs and a Frequency response down to d.c. Special techniques are used to provide the high input impedance necessary for the operational amplifier.
The second stage is a high-gain voltage amplifier. This stage may be made from several transistors to provide high gain. A typical operational amplifier could have a voltage gain of 200,000. Most of this gain comes from the voltage amplifier stage.
The final stage of the OP AMP is an output amplifier. The output amplifier provides low output impedance. The actual circuit used could be an emitter follower. The output stage should allow the operational amplifier to deliver several milliamperes to a load.
Notice that the operational amplifier has a positive power supply (+V CC) and a negative power supply (-VEE). This arrangement enables the operational amplifier to produce either a positive or a negative output.
The two input terminals are labeled "inverting input" (-) and "noninverting input" (+). The operational amplifier can be used with three different input conditions (modes). With differential inputs (first mode), both input terminals are used and two input signals which are 180 degrees out of phase with each other are used. This produces an output signal that is in phase with the signal on the noninverting input. If the noninverting input is grounded and a signal is applied to the inverting input (second mode), the output signal will be 180 degrees out of phase with the input signal (and one-half the amplitude of the first mode output). If the inverting input is grounded and a signal is applied to the noninverting input (third mode), the
output signal will be in phase with the input signal (and one-half the amplitude of the first mode output).

Q.15 What are the three requirements for an operational amplifier?

Q.16 What is the most commonly used form of the operational amplifier?

Q.17 Draw the schematic symbol for an operational amplifier.

Q.18 Label the parts of the operational amplifier shown in figure 3-12.

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Operational amplifier

Figure 3-12. – Operational amplifier.

CLOSED-LOOP OPERATION OF AN OPERATIONAL AMPLIFIER
Operational amplifiers can have either a closed-loop operation or an open-loop operation. The operation (closed-loop or open-loop) is determined by whether or not feedback is used. Without feedback the operational amplifier has an open-loop operation. This open-loop operation is practical only when the operational amplifier is used as a comparator (a circuit which compares two input signals or compares an input signal to some fixed level of voltage). As an amplifier, the open-loop operation is not practical because the very high gain of the operational amplifier creates poor stability. (Noise and other unwanted signals are amplified so much in open-loop operation that the operational amplifier is usually not used in this way.) Therefore, most operational amplifiers are used with feedback (closed-loop operation).
Operational amplifiers are used with degenerative (or negative) feedback which reduces the gain of the operational amplifier but greatly increases the stability of the circuit. In the closed-loop configuration, the output signal is applied back to one of the input terminals. This feedback is always degenerative (negative). In other words, the feedback signal always opposes the effects of the original input signal. One result of degenerative feedback is that the inverting and noninverting inputs to the operational amplifier will be kept at the same potential.
Closed-loop circuits can be of the inverting configuration or noninverting configuration. Since the inverting configuration is used more often than the noninverting configuration, the inverting configuration will be shown first.
Inverting Configuration
Figure 3-13 shows an operational amplifier in a closed-loop, inverting configuration. Resistor R2 is used to feed part of the output signal back to the input of the operational amplifier.
Figure 3-13. – Inverting configuration.

At this point it is important to keep in mind the difference between the entire circuit (or operational circuit) and the operational amplifier. The operational amplifier is represented by the triangle-like symbol while the operational circuit includes the resistors and any other components as well as the operational amplifier. In other words, the input to the circuit is shown in figure 3-13, but the signal at the inverting input of the operational amplifier is determined by the feedback signal as well as by the circuit input signal.
As you can see in figure 3-13, the output signal is 180 degrees out of phase with the input signal. The feedback signal is a portion of the output signal and, therefore, also 180 degrees out of phase with the input signal. Whenever the input signal goes positive, the output signal and the feedback signal go negative. The result of this is that the inverting input to the operational amplifier is always very close to 0 volts with this configuration. In fact, with the noninverting input grounded, the voltage at the inverting input to the operational amplifier is so small compared to other voltages in the circuit that it is considered to be VIRTUAL GROUND. (Remember, in a closed-loop operation the inverting and noninverting inputs are at the same potential.)
Virtual ground is a point in a circuit which is at ground potential (0 volts) but is NOT connected to ground. Figure 3-14, (view A) (view B) and (view C), shows an example of several circuits with points at virtual ground.
Figure 3-14A. – Virtual ground circuits.

Figure 3-14B. – Virtual ground circuits.

Figure 3-14C. – Virtual ground circuits.

In view (A), V1 (the left-hand battery) supplies +10 volts to the circuit while V2 (the right-hand battery) supplies -10 volts to the circuit. The total difference in potential in the circuit is 20 volts.
The total resistance of the circuit can be calculated:
Now that the total resistance is known, the circuit current can be calculated:
The voltage drop across R1 can be computed:
The voltage at point A would be equal to the voltage of V1 minus the voltage drop of R1.
To check this result, compute the voltage drop across R2 and subtract this from the voltage at point A. The result should be the voltage of V2.
It is not necessary that the voltage supplies be equal to create a point of virtual ground. In view (B) V1 supplies +1 volt to the circuit while V2 supplies -10 volts. The total difference in potential is 11 volts. The total resistance of this circuit (R1 + R2) is 11 ohms. The total current (IT) is 1 ampere. The voltage drop across R1 (E R1 = R1 X IT) is 1 volt. The voltage drop across R2 (ER2 = R2 X I T) is 10 volts. The voltage at
point A can be computed:
So point A is at virtual ground in this circuit also. To check the results, compute the voltage at V2.

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TYPICAL RF AMPLIFIER CIRCUITS.

As a technician, you will see many different rf amplifiers in many different pieces of equipment. The particular circuit configuration used for an rf amplifier will depend upon how that amplifier is used. In the final part of this chapter, you will be shown some typical rf amplifier circuits.
Figure 2-19 is the schematic diagram of a typical rf amplifier that is used in an AM radio receiver. In figure 2-19, the input circuit is the antenna of the radio (L1-a coil) which forms part of an LC circuit which is tuned to the desired station by variable capacitor C1. L1 is wound on the same core as L2, which couples the input signal through C2 to the transistor (Q1). R1 is used to provide proper bias to Q1 from the base power supply (VBB). R2 provides proper bias to the emitter of Q1, and C3 is used to bypass R2. The primary of T1 and capacitor C4 form a parallel LC circuit which acts as the load for Q1. This LC circuit is tuned by C4, which is ganged to C1 allowing the antenna and the LC circuit to be tuned together. The primary of T1 is center-tapped to provide proper impedance matching with Q1.
Figure 2-19. – Typical AM radio rf amplifier.

You may notice that no neutralization is shown in this circuit. This circuit is designed for the AM broadcast band (535 kHz – 1605 kHz).
At these relatively low rf frequencies the degenerative feedback caused by base-to-collector interelectrode capacitance is minor and, therefore, the amplifier does not need neutralization.
Figure 2-20 is a typical rf amplifier used in a vhf television receiver. The input-signal-developing circuit for this amplifier is made up of L1, C1, and C2. The inductor tunes the input-signal-developing circuit for the proper TV channel. (L1 can be switched out of the circuit and another inductor switched in to the circuit by the channel selector.) R1 provides proper bias to Q1 from the base supply voltage (VBB). Q1 is the transistor. Notice that the case of Q1 (the dotted circle around the transistor symbol) is shown to be grounded. The case must be grounded because of the high frequencies (54 MHz – 217 MHz) used by the circuit. R2 provides proper bias from the emitter of Q1, and C3 is used to bypass R2. C5 and L2 are a parallel LC circuit which acts as the load for Q1. The LC circuit is tuned by L2 which is switched in to and out of the LC circuit by the channel selector. L3 and C6 are a parallel LC circuit which develops the signal for the next stage. The parallel LC circuit is tuned by L3 which is switched in to and out of the LC circuit by the channel selector along with L1 and L2. (L1, L2, and L3 are actually part of a bank of inductors. L1, L2, and L3 are in the circuit when the channel selector is on channel 2. For other channels, another group of three inductors would be used in the circuit.) R3 develops a signal which is fed through C4 to provide neutralization. This counteracts the effects of the interelectrode capacitance from the base to the collector of Q1. C7 is used to isolate the rf signal from the collector power supply (V_CC).

Figure 2-20. – Typical vhf television rf amplifier.

The following questions refer to figure 2-21.
Figure 2-21. – Typical rf amplifier.

Q.35 What components form the input-signal-developing impedance for the amplifier?

Q.36 What is the purpose of R1?

Q.37 What is the purpose of R2?

Q.38 If C4 were removed from the circuit, what would happen to the output of the amplifier?

Q.39 What components form the load for Q1?

Q.40 How many tuned parallel LC circuits are shown in this schematic?

Q.41 What do the dotted lines connecting C1, C2, C5, and C6 indicate?

Q.42 What is the purpose of C3?

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SUMMARY OF VIDEO AND RF AMPLIFIERS

This chapter has presented information on video and rf amplifiers. The information that follows summarizes the important points of this chapter.
A FREQUENCY-RESPONSE CURVE will enable you to determine the BANDWIDTH and the UPPER and LOWER FREQUENCY LIMITS of an amplifier.

THE BANDWIDTH of an amplifier is determined by the formula:THE UPPER-Frequency response of an amplifier is limited by the inductance and capacitance of the circuit.

THE INTERELECTRODE CAPACITANCE of a transistor causes DEGENERATIVE FEEDBACK at high frequencies.

VIDEO AMPLIFIERS must have a Frequency response of 10 hertz to 6 megahertz (10 Hz – 6 MHz). To provide this Frequency response, both high- and low-frequency compensation must be used.
PEAKING COILS are used in video amplifiers to overcome the high-frequency limitations caused by the capacitance of the circuit.
SERIES PEAKING is accomplished by a peaking coil in series with the output-signal path.

SHUNT PEAKING is accomplished by a peaking coil in parallel (shunt) with the output-signal path.

COMBINATION PEAKING is accomplished by using both series and shunt peaking.

LOW-FREQUENCY COMPENSATION is accomplished in a video amplifier by the use of a parallel RC circuit in series with the load resistor.

A RADIO-FREQUENCY (RF) AMPLIFIER uses FREQUENCY-DETERMINING NETWORKS to provide the required response at a given frequency.

THE FREQUENCY-DETERMINING NETWORK in an rf amplifier provides maximum impedance at the desired frequency. It is a parallel LC circuit which is called a TUNED CIRCUIT.

TRANSFORMER COUPLING is the most common form of coupling in rf amplifiers. This coupling is accomplished by the use of rf transformers as part of the frequency-determining network for the amplifier.
ADEQUATE BANDPASS is accomplished by optimum coupling in the rf transformer or by the use of a SWAMPING RESISTOR.
NEUTRALIZATION in an rf amplifier provides feedback (usually positive) to overcome the effects caused by the base-to-collector interelectrode capacitance.

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ANSWERS TO QUESTIONS Q1. THROUGH Q42.

A1. The difference between the upper and lower frequency limits of an amplifier.

A2. The half-power points of a frequency-response curve. The upper and lower limits of the band of frequencies for which the amplifier is most effective.

A3. (A) f2 = 80 kHz, f1 = 30 kHz, BW = 50 kHz(B) f2 = 4 kHz, f1 = 2 kHz, BW = 2 kHz

A4. The capacitance and inductance of the circuit and the interelectrode capacitance of the transistor.

A5. Negative (degenerative) feedback.

A6. It decreases.

A7. It increases.

A8. The capacitance of the circuit.

A9. Peaking coils.

A10. The relationship of the components to the output-signal path.

A11. Combination peaking.

A12. The coupling capacitor (C3).

A13. A shunt peaking coil for Q2.

A14. A decoupling capacitor for the effects of R2.

A15. A part of the low-frequency compensation network for Q1.

A16. A series peaking coil for Q1.

A17. A swamping resistor for L2.

A18. L1, L2, and R5.

A19. R9 and C5.

A20. The gain increases.

A21. The gain decreases.

A22. To provide maximum impedance at the desired frequency.

A23. Yes.

A24. By changing the value.

A25. Transformer coupling.

A26. It uses fewer components than capacitive coupling and can provide an increase in gain.

A27. A step-down transformer.

A28. A too-narrow bandpass.

A29. By using an optimumly-coupled transformer.

A30. Low gain at the center frequency.

A31. A swamping resistor in parallel with the tuned circuit.

A32. RF transformers are used and the transistor is neutralized.

A33. Degenerative or negative.

A34. By neutralization such as the use of a capacitor to provide regenerative (positive) feedback.

A35. C2 and the secondary of T1.

A36. R1 provides the proper bias to the base of Q1 from V BB.

A37. R2 provides the proper bias to the emitter of Q1.

A38. The output would decrease. (C4 decouples R2 preventing degenerative feedback from R2.)

A39. C5 and the primary of T2.

A40. Four.

A41. The dotted lines indicate that these capacitors are "ganged" and are tuned together with a single control.

A42. C3 provides neutralization for Q1.

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TYPICAL RF AMPLIFIER CIRCUITS.

As a technician, you will see many different rf amplifiers in many different pieces of equipment. The particular circuit configuration used for an rf amplifier will depend upon how that amplifier is used. In the final part of this chapter, you will be shown some typical rf amplifier circuits.
Figure 2-19 is the schematic diagram of a typical rf amplifier that is used in an AM radio receiver. In figure 2-19, the input circuit is the antenna of the radio (L1-a coil) which forms part of an LC circuit which is tuned to the desired station by variable capacitor C1. L1 is wound on the same core as L2, which couples the input signal through C2 to the transistor (Q1). R1 is used to provide proper bias to Q1 from the base power supply (VBB). R2 provides proper bias to the emitter of Q1, and C3 is used to bypass R2. The primary of T1 and capacitor C4 form a parallel LC circuit which acts as the load for Q1. This LC circuit is tuned by C4, which is ganged to C1 allowing the antenna and the LC circuit to be tuned together. The primary of T1 is center-tapped to provide proper impedance matching with Q1.
Figure 2-19. – Typical AM radio rf amplifier.

You may notice that no neutralization is shown in this circuit. This circuit is designed for the AM broadcast band (535 kHz – 1605 kHz).
At these relatively low rf frequencies the degenerative feedback caused by base-to-collector interelectrode capacitance is minor and, therefore, the amplifier does not need neutralization.
Figure 2-20 is a typical rf amplifier used in a vhf television receiver. The input-signal-developing circuit for this amplifier is made up of L1, C1, and C2. The inductor tunes the input-signal-developing circuit for the proper TV channel. (L1 can be switched out of the circuit and another inductor switched in to the circuit by the channel selector.) R1 provides proper bias to Q1 from the base supply voltage (VBB). Q1 is the transistor. Notice that the case of Q1 (the dotted circle around the transistor symbol) is shown to be grounded. The case must be grounded because of the high frequencies (54 MHz – 217 MHz) used by the circuit. R2 provides proper bias from the emitter of Q1, and C3 is used to bypass R2. C5 and L2 are a parallel LC circuit which acts as the load for Q1. The LC circuit is tuned by L2 which is switched in to and out of the LC circuit by the channel selector. L3 and C6 are a parallel LC circuit which develops the signal for the next stage. The parallel LC circuit is tuned by L3 which is switched in to and out of the LC circuit by the channel selector along with L1 and L2. (L1, L2, and L3 are actually part of a bank of inductors. L1, L2, and L3 are in the circuit when the channel selector is on channel 2. For other channels, another group of three inductors would be used in the circuit.) R3 develops a signal which is fed through C4 to provide neutralization. This counteracts the effects of the interelectrode capacitance from the base to the collector of Q1. C7 is used to isolate the rf signal from the collector power supply (V_CC).

Figure 2-20. – Typical vhf television rf amplifier.

The following questions refer to figure 2-21.
Figure 2-21. – Typical rf amplifier.

Q.35 What components form the input-signal-developing impedance for the amplifier?

Q.36 What is the purpose of R1?

Q.37 What is the purpose of R2?

Q.38 If C4 were removed from the circuit, what would happen to the output of the amplifier?

Q.39 What components form the load for Q1?

Q.40 How many tuned parallel LC circuits are shown in this schematic?

Q.41 What do the dotted lines connecting C1, C2, C5, and C6 indicate?

Q.42 What is the purpose of C3?

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SUMMARY OF VIDEO AND RF AMPLIFIERS

This chapter has presented information on video and rf amplifiers. The information that follows summarizes the important points of this chapter.
A FREQUENCY-RESPONSE CURVE will enable you to determine the BANDWIDTH and the UPPER and LOWER FREQUENCY LIMITS of an amplifier.

THE BANDWIDTH of an amplifier is determined by the formula:THE UPPER-Frequency response of an amplifier is limited by the inductance and capacitance of the circuit.

THE INTERELECTRODE CAPACITANCE of a transistor causes DEGENERATIVE FEEDBACK at high frequencies.

VIDEO AMPLIFIERS must have a Frequency response of 10 hertz to 6 megahertz (10 Hz – 6 MHz). To provide this Frequency response, both high- and low-frequency compensation must be used.
PEAKING COILS are used in video amplifiers to overcome the high-frequency limitations caused by the capacitance of the circuit.
SERIES PEAKING is accomplished by a peaking coil in series with the output-signal path.

SHUNT PEAKING is accomplished by a peaking coil in parallel (shunt) with the output-signal path.

COMBINATION PEAKING is accomplished by using both series and shunt peaking.

LOW-FREQUENCY COMPENSATION is accomplished in a video amplifier by the use of a parallel RC circuit in series with the load resistor.

A RADIO-FREQUENCY (RF) AMPLIFIER uses FREQUENCY-DETERMINING NETWORKS to provide the required response at a given frequency.

THE FREQUENCY-DETERMINING NETWORK in an rf amplifier provides maximum impedance at the desired frequency. It is a parallel LC circuit which is called a TUNED CIRCUIT.

TRANSFORMER COUPLING is the most common form of coupling in rf amplifiers. This coupling is accomplished by the use of rf transformers as part of the frequency-determining network for the amplifier.
ADEQUATE BANDPASS is accomplished by optimum coupling in the rf transformer or by the use of a SWAMPING RESISTOR.
NEUTRALIZATION in an rf amplifier provides feedback (usually positive) to overcome the effects caused by the base-to-collector interelectrode capacitance.

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ANSWERS TO QUESTIONS Q1. THROUGH Q42.

A1. The difference between the upper and lower frequency limits of an amplifier.

A2. The half-power points of a frequency-response curve. The upper and lower limits of the band of frequencies for which the amplifier is most effective.

A3. (A) f2 = 80 kHz, f1 = 30 kHz, BW = 50 kHz(B) f2 = 4 kHz, f1 = 2 kHz, BW = 2 kHz

A4. The capacitance and inductance of the circuit and the interelectrode capacitance of the transistor.

A5. Negative (degenerative) feedback.

A6. It decreases.

A7. It increases.

A8. The capacitance of the circuit.

A9. Peaking coils.

A10. The relationship of the components to the output-signal path.

A11. Combination peaking.

A12. The coupling capacitor (C3).

A13. A shunt peaking coil for Q2.

A14. A decoupling capacitor for the effects of R2.

A15. A part of the low-frequency compensation network for Q1.

A16. A series peaking coil for Q1.

A17. A swamping resistor for L2.

A18. L1, L2, and R5.

A19. R9 and C5.

A20. The gain increases.

A21. The gain decreases.

A22. To provide maximum impedance at the desired frequency.

A23. Yes.

A24. By changing the value.

A25. Transformer coupling.

A26. It uses fewer components than capacitive coupling and can provide an increase in gain.

A27. A step-down transformer.

A28. A too-narrow bandpass.

A29. By using an optimumly-coupled transformer.

A30. Low gain at the center frequency.

A31. A swamping resistor in parallel with the tuned circuit.

A32. RF transformers are used and the transistor is neutralized.

A33. Degenerative or negative.

A34. By neutralization such as the use of a capacitor to provide regenerative (positive) feedback.

A35. C2 and the secondary of T1.

A36. R1 provides the proper bias to the base of Q1 from V BB.

A37. R2 provides the proper bias to the emitter of Q1.

A38. The output would decrease. (C4 decouples R2 preventing degenerative feedback from R2.)

A39. C5 and the primary of T2.

A40. Four.

A41. The dotted lines indicate that these capacitors are "ganged" and are tuned together with a single control.

A42. C3 provides neutralization for Q1.

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TYPICAL RF AMPLIFIER CIRCUITS.

As a technician, you will see many different rf amplifiers in many different pieces of equipment. The particular circuit configuration used for an rf amplifier will depend upon how that amplifier is used. In the final part of this chapter, you will be shown some typical rf amplifier circuits.
Figure 2-19 is the schematic diagram of a typical rf amplifier that is used in an AM radio receiver. In figure 2-19, the input circuit is the antenna of the radio (L1-a coil) which forms part of an LC circuit which is tuned to the desired station by variable capacitor C1. L1 is wound on the same core as L2, which couples the input signal through C2 to the transistor (Q1). R1 is used to provide proper bias to Q1 from the base power supply (VBB). R2 provides proper bias to the emitter of Q1, and C3 is used to bypass R2. The primary of T1 and capacitor C4 form a parallel LC circuit which acts as the load for Q1. This LC circuit is tuned by C4, which is ganged to C1 allowing the antenna and the LC circuit to be tuned together. The primary of T1 is center-tapped to provide proper impedance matching with Q1.
Figure 2-19. – Typical AM radio rf amplifier.

You may notice that no neutralization is shown in this circuit. This circuit is designed for the AM broadcast band (535 kHz – 1605 kHz).
At these relatively low rf frequencies the degenerative feedback caused by base-to-collector interelectrode capacitance is minor and, therefore, the amplifier does not need neutralization.
Figure 2-20 is a typical rf amplifier used in a vhf television receiver. The input-signal-developing circuit for this amplifier is made up of L1, C1, and C2. The inductor tunes the input-signal-developing circuit for the proper TV channel. (L1 can be switched out of the circuit and another inductor switched in to the circuit by the channel selector.) R1 provides proper bias to Q1 from the base supply voltage (VBB). Q1 is the transistor. Notice that the case of Q1 (the dotted circle around the transistor symbol) is shown to be grounded. The case must be grounded because of the high frequencies (54 MHz – 217 MHz) used by the circuit. R2 provides proper bias from the emitter of Q1, and C3 is used to bypass R2. C5 and L2 are a parallel LC circuit which acts as the load for Q1. The LC circuit is tuned by L2 which is switched in to and out of the LC circuit by the channel selector. L3 and C6 are a parallel LC circuit which develops the signal for the next stage. The parallel LC circuit is tuned by L3 which is switched in to and out of the LC circuit by the channel selector along with L1 and L2. (L1, L2, and L3 are actually part of a bank of inductors. L1, L2, and L3 are in the circuit when the channel selector is on channel 2. For other channels, another group of three inductors would be used in the circuit.) R3 develops a signal which is fed through C4 to provide neutralization. This counteracts the effects of the interelectrode capacitance from the base to the collector of Q1. C7 is used to isolate the rf signal from the collector power supply (V_CC).

Figure 2-20. – Typical vhf television rf amplifier.

The following questions refer to figure 2-21.
Figure 2-21. – Typical rf amplifier.

Q.35 What components form the input-signal-developing impedance for the amplifier?

Q.36 What is the purpose of R1?

Q.37 What is the purpose of R2?

Q.38 If C4 were removed from the circuit, what would happen to the output of the amplifier?

Q.39 What components form the load for Q1?

Q.40 How many tuned parallel LC circuits are shown in this schematic?

Q.41 What do the dotted lines connecting C1, C2, C5, and C6 indicate?

Q.42 What is the purpose of C3?

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SUMMARY OF VIDEO AND RF AMPLIFIERS

This chapter has presented information on video and rf amplifiers. The information that follows summarizes the important points of this chapter.
A FREQUENCY-RESPONSE CURVE will enable you to determine the BANDWIDTH and the UPPER and LOWER FREQUENCY LIMITS of an amplifier.

THE BANDWIDTH of an amplifier is determined by the formula:THE UPPER-Frequency response of an amplifier is limited by the inductance and capacitance of the circuit.

THE INTERELECTRODE CAPACITANCE of a transistor causes DEGENERATIVE FEEDBACK at high frequencies.

VIDEO AMPLIFIERS must have a Frequency response of 10 hertz to 6 megahertz (10 Hz – 6 MHz). To provide this Frequency response, both high- and low-frequency compensation must be used.
PEAKING COILS are used in video amplifiers to overcome the high-frequency limitations caused by the capacitance of the circuit.
SERIES PEAKING is accomplished by a peaking coil in series with the output-signal path.

SHUNT PEAKING is accomplished by a peaking coil in parallel (shunt) with the output-signal path.

COMBINATION PEAKING is accomplished by using both series and shunt peaking.

LOW-FREQUENCY COMPENSATION is accomplished in a video amplifier by the use of a parallel RC circuit in series with the load resistor.

A RADIO-FREQUENCY (RF) AMPLIFIER uses FREQUENCY-DETERMINING NETWORKS to provide the required response at a given frequency.

THE FREQUENCY-DETERMINING NETWORK in an rf amplifier provides maximum impedance at the desired frequency. It is a parallel LC circuit which is called a TUNED CIRCUIT.

TRANSFORMER COUPLING is the most common form of coupling in rf amplifiers. This coupling is accomplished by the use of rf transformers as part of the frequency-determining network for the amplifier.
ADEQUATE BANDPASS is accomplished by optimum coupling in the rf transformer or by the use of a SWAMPING RESISTOR.
NEUTRALIZATION in an rf amplifier provides feedback (usually positive) to overcome the effects caused by the base-to-collector interelectrode capacitance.

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ANSWERS TO QUESTIONS Q1. THROUGH Q42.

A1. The difference between the upper and lower frequency limits of an amplifier.

A2. The half-power points of a frequency-response curve. The upper and lower limits of the band of frequencies for which the amplifier is most effective.

A3. (A) f2 = 80 kHz, f1 = 30 kHz, BW = 50 kHz(B) f2 = 4 kHz, f1 = 2 kHz, BW = 2 kHz

A4. The capacitance and inductance of the circuit and the interelectrode capacitance of the transistor.

A5. Negative (degenerative) feedback.

A6. It decreases.

A7. It increases.

A8. The capacitance of the circuit.

A9. Peaking coils.

A10. The relationship of the components to the output-signal path.

A11. Combination peaking.

A12. The coupling capacitor (C3).

A13. A shunt peaking coil for Q2.

A14. A decoupling capacitor for the effects of R2.

A15. A part of the low-frequency compensation network for Q1.

A16. A series peaking coil for Q1.

A17. A swamping resistor for L2.

A18. L1, L2, and R5.

A19. R9 and C5.

A20. The gain increases.

A21. The gain decreases.

A22. To provide maximum impedance at the desired frequency.

A23. Yes.

A24. By changing the value.

A25. Transformer coupling.

A26. It uses fewer components than capacitive coupling and can provide an increase in gain.

A27. A step-down transformer.

A28. A too-narrow bandpass.

A29. By using an optimumly-coupled transformer.

A30. Low gain at the center frequency.

A31. A swamping resistor in parallel with the tuned circuit.

A32. RF transformers are used and the transistor is neutralized.

A33. Degenerative or negative.

A34. By neutralization such as the use of a capacitor to provide regenerative (positive) feedback.

A35. C2 and the secondary of T1.

A36. R1 provides the proper bias to the base of Q1 from V BB.

A37. R2 provides the proper bias to the emitter of Q1.

A38. The output would decrease. (C4 decouples R2 preventing degenerative feedback from R2.)

A39. C5 and the primary of T2.

A40. Four.

A41. The dotted lines indicate that these capacitors are "ganged" and are tuned together with a single control.

A42. C3 provides neutralization for Q1.

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VIDEO AMPLIFIERS

As you have seen, a transistor amplifier is limited in its Frequency response. You should also remember from chapter 1 that a VIDEO AMPLIFIER should have a frequency response of 10 hertz (10 Hz) to 6 megahertz (6 MHz). The question has probably occurred to you: How is it possible to "extend" the range of Frequency response of an amplifier?

HIGH-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS

If the frequency-response range of an audio amplifier must be extended to 6 megahertz (6 MHz) for use as a video amplifier, some means must be found to overcome the limitations of the audio amplifier. As you have seen, the capacitance of an amplifier circuit and the interelectrode capacitance of the transistor (or electronic tube) cause the higher Frequency response to be limited.
In some ways capacitance and inductance can be thought of as opposites.
As stated before, as frequency increases, capacitive reactance decreases, and inductive reactance increases. Capacitance opposes changes in voltage, and inductance opposes changes in current. Capacitance causes current to lead voltage, and inductance causes voltage to lead current.
Since frequency affects capacitive reactance and inductive reactance in opposite ways, and since it is the capacitive reactance that causes the problem with high-frequency response, inductors are added to an amplifier circuit to improve the high-frequency response. This is called HIGH-FREQUENCY COMPENSATION. Inductors (coils), when used for high-frequency compensation, are called PEAKING COILS. Peaking coils can be added to a circuit so they are in series with the output signal path or in parallel to the output signal path. Instead of only in series or parallel, a combination of peaking coils in series and parallel with the output signal path can also be used for high-frequency compensation.
As in all electronic circuits, nothing comes free. The use of peaking coils WILL increase the Frequency response of an amplifier circuit, but it will ALSO lower the gain of the amplifier.
Series Peaking
The use of a peaking coil in series with the output signal path is known as SERIES PEAKING. Figure 2-6 shows a transistor amplifier circuit with a series peaking coil. In this figure, R1 is the input-signal-developing resistor. R2 is used for bias and temperature stability of Q1. C1 is the bypass capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling capacitor which couples the output signal to the next stage. "Phantom" capacitor COUT represents the output capacitance of the circuit, and "phantom" capacitor CIN represents the input capacitance of the next stage.
Figure 2-6. – Series peaking coil.

You know that the capacitive reactance of COUT and CIN will limit the high-Frequency response of the circuit. L1 is the series peaking coil. It is in series with the output-signal path and isolates COUT from CIN.
R4 is called a "swamping" resistor and is used to keep L1 from overcompensating at a narrow range of frequencies. In other words, R4 is used to keep the frequency-response curve flat. If R4 were not used with L1, there could be a "peak" in the frequency-response curve. (Remember, L1 is called a peaking coil.)
Shunt Peaking
If a coil is placed in parallel (shunt) with the output signal path, the technique is called SHUNT PEAKING. Figure 2-7 shows a circuit with a shunt peaking coil. With the exceptions of the "phantom" capacitor and the inductor, the components in this circuit are the same as those in figure 2-6. R1 is the input-signal-developing resistor. R2 is used for bias and temperature stability. C1 is the bypass capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling capacitor which couples the output signal to the next stage.

Figure 2-7. – Shunt peaking coil.

The "phantom" capacitor, CT, represents the total capacitance of the circuit. Notice that it tends to couple the output signal to ground.
L1 is the shunt peaking coil. While it is in series with the load resistor (R3), it is in parallel (shunt) with the output-signal path.
Since inductive reactance increases as frequency increases, the reactance of L1 develops more output signal as the frequency increases. At the same time, the capacitive reactance of CT is decreasing as frequency increases. This tends to couple more of the output signal to ground. The increased inductive reactance counters the effect of the decreased capacitive reactance and this increases the high-Frequency response of the amplifier.
Combination Peaking
You have seen how a series peaking coil isolates the output capacitance of an amplifier from the input capacitance of the next stage. You have also seen how a shunt peaking coil will counteract the effects of the total capacitance of an amplifier. If these two techniques are used together, the combination is more effective than the use of either one alone. The use of both series and shunt peaking coils is known as COMBINATION PEAKING. An amplifier circuit with combination peaking is shown in figure 2-8. In figure 2-8 the peaking coils are L1 and L2. L1 is a shunt peaking coil, and L2 is a series peaking coil.
Figure 2-8. – Combination peaking.

The "phantom" capacitor CT represents the total capacitance of the amplifier circuit. "Phantom" capacitor CIN represents the input capacitance of the next stage. Combination peaking will easily allow an amplifier to have a high-Frequency response of 6 megahertz (6 MHz).

Q.8 What is the major factor that limits the high-Frequency response of an amplifier circuits?

Q.9 What components can be used to increase the high-Frequency response of an amplifier?

Q.10 What determines whether these components are considered series or shunt?

Q.11 What is the arrangement of both series and shunt components called?

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LOW-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS

Now that you have seen how the high-Frequency response of an amplifier can be extended to 6 megahertz (6 MHz), you should realize that it is only necessary to extend the low-Frequency response to 10 hertz (10 Hz) in order to have a video amplifier.
Once again, the culprit in low-Frequency response is capacitance (or capacitive reactance). But this time the problem is the coupling capacitor between the stages.
At low frequencies the capacitive reactance of the coupling capacitor (C2 in figure 2-8) is high. This high reactance limits the amount of output signal that is coupled to the next stage. In addition, the RC network of the coupling capacitor and the signal-developing resistor of the next stage cause a phase shift in the output signal. (Refer to NEETS, module 2, for a discussion of phase shifts in RC networks.) Both of these problems (poor low-Frequency response and phase shift) can be solved by adding a parallel RC network in series with the load resistor. This is shown in figure 2-9.
Figure 2-9. – Low frequency compensation network.

The complete circuitry for Q2 is not shown in this figure, as the main concern is the signal-developing resistor (R5) for Q2. The coupling capacitor (C2) and the resistor (R5) limit the low-Frequency response of the amplifier and cause a phase shift. The amount of the phase shift will depend upon the amount of resistance and capacitance. The RC network of R4 and C3 compensates for the effects of C2 and R5 and extends the low-frequency response of the amplifier.
At low frequencies, R4 adds to the load resistance (R3) and increases the gain of the amplifier. As frequency increases, the reactance of C3 decreases. C3 then provides a path around R4 and the gain of the transistor decreases. At the same time, the reactance of the coupling capacitor (C2) decreases and more signal is coupled to Q2.
Because the circuit shown in figure 2-9 has no high-frequency compensation, it would not be a very practical video amplifier.
TYPICAL VIDEO-AMPLIFIER CIRCUIT
There are many different ways in which video amplifiers can be built. The particular configuration of a video amplifier depends upon the equipment in which the video amplifier is used. The circuit shown in figure 2-10 is only one of many possible video-amplifier circuits. Rather than reading about what each component does in this circuit, you can see how well you have learned about video amplifiers by answering the following questions. You should have no problem identifying the purpose of the components because similar circuits have been explained to you earlier in the text.
Figure 2-10. – Video amplifier circuit.

The following questions refer to figure 2-10.

Q.12 What component in an amplifier circuit tends to limit the low-Frequency response of the amplifier?

Q.13 What is the purpose of L3?
Q.14 What is the purpose of C1?

Q.15 What is the purpose of R4?

Q.16 What is the purpose of L2?

Q.17 What is the purpose of R5?

Q.18 What component(s) is/are used for high-frequency compensation for Q1?

Q.19 What component(s) is/are used for low-frequency compensation for Q2?

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RADIO-FREQUENCY AMPLIFIERS

Now that you have seen the way in which a broadband, or video, amplifier can be constructed, you may be wondering about radio-frequency (rf) amplifiers. Do they use the same techniques? Are they just another type of broadband amplifier?
The answer to both questions is "no." Radio-frequency amplifiers use different techniques than video amplifiers and are very different from them.
Before you study the specific techniques used in rf amplifiers, you should review some information on the relationship between the input and output impedance of an amplifier and the gain of the amplifier stage.
AMPLIFIER INPUT/OUTPUT IMPEDANCE AND GAIN
You should remember that the gain of a stage is calculated by using the input and output signals. The formula used to calculate the gain of a stage is:

Voltage gain is calculated using input and output voltage; current gain uses input and output current; and power gain uses input and output power. For the purposes of our discussion, we will only be concerned with voltage gain.
Figure 2-11 shows a simple amplifier circuit with the input- and output-signal-developing impedances represented by variable resistors. In this circuit, C1 and C2 are the input and output coupling capacitors. R1 represents the impedance of the input circuit. R2 represents the input-signal-developing impedance, and R3 represents the output impedance.
Figure 2-11. – Variable input and output impedances.

R1 and R2 form a voltage-divider network for the input signal. When R2 is increased in value, the input signal to the transistor (Q1) increases. This causes a larger output signal, and the gain of the stage increases.
Now look at the output resistor, R3. As R3 is increased in value, the output signal increases. This also increases the gain of the stage.
As you can see, increasing the input-signal-developing impedance, the output impedance, or both will increase the gain of the stage. Of course there are limits to this process. The transistor must not be overdriven with too high an input signal or distortion will result.
With this principle in mind, if you could design a circuit that had maximum impedance at a specific frequency (or band of frequencies), that circuit could be used in an rf amplifier. This FREQUENCY-DETERMINING NETWORK could be used as the input-signal-developing impedance, the output impedance, or both. The rf amplifier circuit would then be as shown in figure 2-12.
Figure 2-12. – Semiblock diagram of rf amplifier.

In this "semi-block" diagram, C1 and C2 are the input and output coupling capacitors. R1 represents the impedance of the input circuit. The blocks marked FDN represent the frequency-determining networks. They are used as input-signal-developing and output impedances for Q1.

FREQUENCY-DETERMINING NETWORK FOR AN RF AMPLIFIER

What kind of circuit would act as a frequency-determining network? In general, a frequency-determining network is a circuit that provides the desired response at a particular frequency. This response could be maximum impedance or minimum impedance; it all depends on how the frequency-determining network is used. You will see more about frequency-determining networks in NEETS, module 9 – Introduction to Wave-Generation and Shaping Circuits. As you have seen, the frequency-determining network needed for an rf amplifier should have maximum impedance at the desired frequency.
Before you are shown the actual components that make up the frequency-determining network for an rf amplifier, look at figure 2-13, which is a simple parallel circuit. The resistors in this circuit are variable and are connected together (ganged) in such a way that as the resistance of R1 increases, the resistance of R2 decreases, and vice versa.
Figure 2-13. – Parallel variable resistors (ganged).

If each resistor has a range from 0 to 200 ohms, the following relationship will exist between the individual resistances and the resistance of the network (RT). (All values are in ohms, RT rounded off to two decimal places. These are selected values; there are an infinite number of possible combinations.)

As you can see, this circuit has maximum resistance (RT) when the individual resistors are of equal value. If the variable resistors represented impedances and if components could be found that varied their impedance in the same way as the ganged resistors in figure 2-13 , you would have the frequency-determining network needed for an rf amplifier.
There are components that will vary their impedance (reactance) like the ganged resistors. As you know, the reactance of an inductor and a capacitor vary as frequency changes. As frequency increases, inductive reactance increases, and capacitive reactance decreases.
At some frequency, inductive and capacitive reactance will be equal. That frequency will depend upon the value of the inductor and capacitor. If the inductor and capacitor are connected as a parallel LC circuit, you will have the ideal frequency-determining network for an rf amplifier.
The parallel LC circuit used as a frequency-determining network is called a TUNED CIRCUIT. This circuit is "tuned" to give the proper response at the desired frequency by selecting the proper values of inductance and capacitance. A circuit using this principle is shown in figure 2-14 which shows an rf amplifier with parallel LC circuits used as frequency-determining networks. This rf amplifier will only be effective in amplifying the frequency determined by the parallel LC circuits.
Figure 2-14. – Simple rf amplifier.

In many electronic devices, such as radio or television receivers or radar systems, a particular frequency must be selected from a band of frequencies. This could be done by using a separate rf amplifier for each frequency and then turning on the appropriate rf amplifier. It would be more efficient if a single rf amplifier could be "tuned" to the particular frequency as that frequency is needed. This is what happens when you select a channel on your television set or tune to a station on your radio. To accomplish this "tuning," you need only change the value of inductance or capacitance in the parallel LC circuits (tuned circuits).
In most cases, the capacitance is changed by the use of variable capacitors. The capacitors in the input and output portions of all the rf amplifier stages are ganged together in order that they can all be changed at one time with a single device, such as the tuning dial on a radio. (This technique will be shown on a schematic a little later in this chapter.)

Q.20 If the input-signal-developing impedance of an amplifier is increased, what is the effect on the gain?

Q.21 If the output impedance of an amplifier circuit is decreased, what is the effect on the gain?

Q.22 What is the purpose of a frequency-determining network in an rf amplifier?

Q.23 Can a parallel LC circuit be used as the frequency-determining network for an rf amplifier?

Q.24 How can the frequency be changed in the frequency-determining network?

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RF AMPLIFIER COUPLING

Figure 2-14 and the other circuits you have been shown use capacitors to couple the signal in to and out of the circuit (C1 and C4 in figure 2-14). As you remember from chapter 1, there are also other methods of coupling signals from one stage to another. Transformer coupling is the most common method used to couple rf amplifiers. Transformer coupling has many advantages over RC coupling for rf amplifiers; for example, transformer coupling uses fewer components than capacitive coupling. It can also provide a means of increasing the gain of the stage by using a step-up transformer for voltage gain. If a current gain is required, a step-down transformer can be used.
You should also remember that the primary and secondary windings of a transformer are inductors. With these factors in mind, an rf amplifier could be constructed like the one shown in figure 2-15.
Figure 2-15. – Transformer-coupled rf amplifier.

In this circuit, the secondary of T1 and capacitor C1 form a tuned circuit which is the input-signal-developing impedance. The primary of T2 and capacitor C2 are a tuned circuit which acts as the output impedance of
Q1. (Both T1 and T2 must be rf transformers in order to operate at rf frequencies.)
The input signal applied to the primary of T1 could come from the previous stage or from some input device, such as a receiving antenna. In either case, the input device would have a capacitor connected across a coil to form a tuned circuit. In the same way, the secondary of T2 represents the output of this circuit. A capacitor connected across the secondary of T2 would form a parallel LC network. This network could act as the input-
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signal-developing impedance for the next stage, or the network could represent some type of output device, such as a transmitting antenna.
The tuned circuits formed by the transformer and capacitors may not have the bandwidth required for the amplifier. In other words, the bandwidth of the tuned circuit may be too "narrow" for the requirements of the amplifier. (For example, the rf amplifiers used in television receivers usually require a bandwidth of 6 MHz.)
One way of "broadening" the bandpass of a tuned circuit is to use a swamping resistor. This is similar to the use of the swamping resistor that was shown with the series peaking coil in a video amplifier. A swamping resistor connected in parallel with the tuned circuit will cause a much broader bandpass. (This technique and the theory behind it are discussed in more detail in NEETS, module 9.)
Another technique used to broaden the bandpass involves the amount of coupling in the transformers. For transformers, the term "coupling" refers to the amount of energy transferred from the primary to the secondary of the transformer. This depends upon the number of flux lines from the primary that intersect, or cut, the secondary. When more flux lines cut the secondary, more energy is transferred.
Coupling is mainly a function of the space between the primary and secondary windings. A transformer can be loosely coupled (having little transfer of energy), optimumly coupled (just the right amount of energy transferred), or overcoupled (to the point that the flux lines of primary and secondary windings interfere with each other).
Figure 2-16, (view A) (view B) (view C), shows the effect of coupling on frequency response when parallel LC circuits are made from the primary and secondary windings of transformers.
Figure 2-16A. – Effect of coupling on Frequency response. LOOSE COUPLING
Figure 2-16B. – Effect of coupling on Frequency response. OPTIMUM COUPLING

Figure 2-16C. – Effect of coupling on Frequency response. OVER-COUPLING

In view (A) the transformer is loosely coupled; the Frequency response curve shows a narrow bandwidth. In view (B) the transformer has optimum coupling; the bandwidth is wider and the curve is relatively flat. In view (C) the transformer is overcoupled; the Frequency response curve shows a broad bandpass, but the curve "dips" in the middle showing that these frequencies are not developed as well as others in the bandwidth.
Optimum coupling will usually provide the necessary bandpass for the frequency-determining network (and therefore the rf amplifier). For some uses, such as rf amplifiers in a television receiver, the bandpass available from optimum coupling is not wide enough. In these cases, a swamping resistor (as mentioned earlier) will be used with the optimum coupling to broaden the bandpass.
COMPENSATION OF RF AMPLIFIERS
Now you have been shown the way in which an rf amplifier is configured to amplify a band of frequencies and the way in which an rf amplifier can be "tuned" for a particular band of frequencies. You have also seen some ways in which the bandpass of an rf amplifier can be adjusted. However, the frequencies at which rf amplifiers operate are so high that certain problems exist.
One of these problems is the losses that can occur in a transformer at these high frequencies. Another problem is with interelectrode capacitance in the transistor. The process of overcoming these problems is known as COMPENSATION.
Transformers in RF Amplifiers
As you recall from NEETS, module 1, the losses in a transformer are classified as copper loss, eddy-current loss, and hysteresis loss. Copper loss is not affected by frequency, as it depends upon the resistance of the winding and the current through the winding. Similarly, eddy-current loss is mostly a function of induced voltage rather than the frequency of that voltage. Hysteresis loss, however, increases as frequency increases.
Hysteresis loss is caused by the realignment of the magnetic domains in the core of the transformer each time the polarity of the magnetic field changes. As the frequency of the a.c. increases, the number of shifts in the magnetic field also increases (two shifts for each cycle of a.c.);

therefore, the "molecular friction" increases and the hysteresis loss is greater. This increase in hysteresis loss causes the efficiency of the transformer (and therefore the amplifier) to decrease. The energy that goes into hysteresis loss is taken away from energy that could go into the signal.
RF TRANSFORMERS, specially designed for use with rf, are used to correct the problem of excessive hysteresis loss in the transformer of an rf amplifier. The windings of rf transformers are wound onto a tube of nonmagnetic material and the core is either powdered iron or air. These types of cores also reduce eddy-current loss.
Neutralization of RF Amplifiers
The problem of interelectrode capacitance in the transistor of an rf amplifier is solved by NEUTRALIZATION. Neutralization is the process of counteracting or "neutralizing" the effects of interelectrode capacitance.
Figure 2-17 shows the effect of the base-to-collector interelectrode capacitance in an rf amplifier. The "phantom" capacitor (CBC) represents the interelectrode capacitance between the base and the collector of Q1. This is the interelectrode capacitance that has the most effect in an rf amplifier. As you can see, CBC causes a degenerative (negative) feedback which decreases the gain of the amplifier. (There are some special cases in which CBC can cause regenerative (positive) feedback. In this case, the technique described below will provide negative feedback which will accomplish the neutralization of the amplifier.)
Figure 2-17. – Interelectrode capacitance in an rf amplifier.

As you may recall, unwanted degenerative feedback can be counteracted (neutralized) by using positive feedback. This is exactly what is done to neutralize an rf amplifier.
Positive feedback is accomplished by the use of a feedback capacitor. This capacitor must feed back a signal that is in phase with the signal on the base of Q1. One method of doing this is shown in figure 2-18.
Figure 2-18. – Neutralized rf amplifier.

In figure 2-18, a feedback capacitor (C4) has been added to neutralize the amplifier. This solves the problem of unwanted degenerative feedback. Except for capacitor C4, this circuit is identical to the circuit shown in figure 2-17. (When CBC causes regenerative feedback, C4 will still neutralize the amplifier. This is true because C4 always provides a feedback signal which is 180 degrees out of phase with the feedback signal caused by CBC.)

Q.25 What is the most common form of coupling for an rf amplifier?

Q.26 What are two advantages of this type of coupling?

Q.27 If current gain is required from an rf amplifier, what type of component should be used as an output coupling element?

Q.28 What problem is caused in an rf amplifier by a loosely coupled transformer?

Q.29 How is this problem corrected?

Q.30 What problem is caused by overcoupling in a transformer?

Q.31 What method provides the widest bandpass?

Q.32 What two methods are used to compensate for the problems that cause low gain in an rf amplifier?

Q.33 What type of feedback is usually caused by the base-to-collector interelectrode capacitance? Q.34 How is this compensated for?

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VIDEO AMPLIFIERS

As you have seen, a transistor amplifier is limited in its Frequency response. You should also remember from chapter 1 that a VIDEO AMPLIFIER should have a frequency response of 10 hertz (10 Hz) to 6 megahertz (6 MHz). The question has probably occurred to you: How is it possible to "extend" the range of Frequency response of an amplifier?

HIGH-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS

If the frequency-response range of an audio amplifier must be extended to 6 megahertz (6 MHz) for use as a video amplifier, some means must be found to overcome the limitations of the audio amplifier. As you have seen, the capacitance of an amplifier circuit and the interelectrode capacitance of the transistor (or electronic tube) cause the higher Frequency response to be limited.
In some ways capacitance and inductance can be thought of as opposites.
As stated before, as frequency increases, capacitive reactance decreases, and inductive reactance increases. Capacitance opposes changes in voltage, and inductance opposes changes in current. Capacitance causes current to lead voltage, and inductance causes voltage to lead current.
Since frequency affects capacitive reactance and inductive reactance in opposite ways, and since it is the capacitive reactance that causes the problem with high-frequency response, inductors are added to an amplifier circuit to improve the high-frequency response. This is called HIGH-FREQUENCY COMPENSATION. Inductors (coils), when used for high-frequency compensation, are called PEAKING COILS. Peaking coils can be added to a circuit so they are in series with the output signal path or in parallel to the output signal path. Instead of only in series or parallel, a combination of peaking coils in series and parallel with the output signal path can also be used for high-frequency compensation.
As in all electronic circuits, nothing comes free. The use of peaking coils WILL increase the Frequency response of an amplifier circuit, but it will ALSO lower the gain of the amplifier.
Series Peaking
The use of a peaking coil in series with the output signal path is known as SERIES PEAKING. Figure 2-6 shows a transistor amplifier circuit with a series peaking coil. In this figure, R1 is the input-signal-developing resistor. R2 is used for bias and temperature stability of Q1. C1 is the bypass capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling capacitor which couples the output signal to the next stage. "Phantom" capacitor COUT represents the output capacitance of the circuit, and "phantom" capacitor CIN represents the input capacitance of the next stage.
Figure 2-6. – Series peaking coil.

You know that the capacitive reactance of COUT and CIN will limit the high-Frequency response of the circuit. L1 is the series peaking coil. It is in series with the output-signal path and isolates COUT from CIN.
R4 is called a "swamping" resistor and is used to keep L1 from overcompensating at a narrow range of frequencies. In other words, R4 is used to keep the frequency-response curve flat. If R4 were not used with L1, there could be a "peak" in the frequency-response curve. (Remember, L1 is called a peaking coil.)
Shunt Peaking
If a coil is placed in parallel (shunt) with the output signal path, the technique is called SHUNT PEAKING. Figure 2-7 shows a circuit with a shunt peaking coil. With the exceptions of the "phantom" capacitor and the inductor, the components in this circuit are the same as those in figure 2-6. R1 is the input-signal-developing resistor. R2 is used for bias and temperature stability. C1 is the bypass capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling capacitor which couples the output signal to the next stage.

Figure 2-7. – Shunt peaking coil.

The "phantom" capacitor, CT, represents the total capacitance of the circuit. Notice that it tends to couple the output signal to ground.
L1 is the shunt peaking coil. While it is in series with the load resistor (R3), it is in parallel (shunt) with the output-signal path.
Since inductive reactance increases as frequency increases, the reactance of L1 develops more output signal as the frequency increases. At the same time, the capacitive reactance of CT is decreasing as frequency increases. This tends to couple more of the output signal to ground. The increased inductive reactance counters the effect of the decreased capacitive reactance and this increases the high-Frequency response of the amplifier.
Combination Peaking
You have seen how a series peaking coil isolates the output capacitance of an amplifier from the input capacitance of the next stage. You have also seen how a shunt peaking coil will counteract the effects of the total capacitance of an amplifier. If these two techniques are used together, the combination is more effective than the use of either one alone. The use of both series and shunt peaking coils is known as COMBINATION PEAKING. An amplifier circuit with combination peaking is shown in figure 2-8. In figure 2-8 the peaking coils are L1 and L2. L1 is a shunt peaking coil, and L2 is a series peaking coil.
Figure 2-8. – Combination peaking.

The "phantom" capacitor CT represents the total capacitance of the amplifier circuit. "Phantom" capacitor CIN represents the input capacitance of the next stage. Combination peaking will easily allow an amplifier to have a high-Frequency response of 6 megahertz (6 MHz).

Q.8 What is the major factor that limits the high-Frequency response of an amplifier circuits?

Q.9 What components can be used to increase the high-Frequency response of an amplifier?

Q.10 What determines whether these components are considered series or shunt?

Q.11 What is the arrangement of both series and shunt components called?

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LOW-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS

Now that you have seen how the high-Frequency response of an amplifier can be extended to 6 megahertz (6 MHz), you should realize that it is only necessary to extend the low-Frequency response to 10 hertz (10 Hz) in order to have a video amplifier.
Once again, the culprit in low-Frequency response is capacitance (or capacitive reactance). But this time the problem is the coupling capacitor between the stages.
At low frequencies the capacitive reactance of the coupling capacitor (C2 in figure 2-8) is high. This high reactance limits the amount of output signal that is coupled to the next stage. In addition, the RC network of the coupling capacitor and the signal-developing resistor of the next stage cause a phase shift in the output signal. (Refer to NEETS, module 2, for a discussion of phase shifts in RC networks.) Both of these problems (poor low-Frequency response and phase shift) can be solved by adding a parallel RC network in series with the load resistor. This is shown in figure 2-9.
Figure 2-9. – Low frequency compensation network.

The complete circuitry for Q2 is not shown in this figure, as the main concern is the signal-developing resistor (R5) for Q2. The coupling capacitor (C2) and the resistor (R5) limit the low-Frequency response of the amplifier and cause a phase shift. The amount of the phase shift will depend upon the amount of resistance and capacitance. The RC network of R4 and C3 compensates for the effects of C2 and R5 and extends the low-frequency response of the amplifier.
At low frequencies, R4 adds to the load resistance (R3) and increases the gain of the amplifier. As frequency increases, the reactance of C3 decreases. C3 then provides a path around R4 and the gain of the transistor decreases. At the same time, the reactance of the coupling capacitor (C2) decreases and more signal is coupled to Q2.
Because the circuit shown in figure 2-9 has no high-frequency compensation, it would not be a very practical video amplifier.
TYPICAL VIDEO-AMPLIFIER CIRCUIT
There are many different ways in which video amplifiers can be built. The particular configuration of a video amplifier depends upon the equipment in which the video amplifier is used. The circuit shown in figure 2-10 is only one of many possible video-amplifier circuits. Rather than reading about what each component does in this circuit, you can see how well you have learned about video amplifiers by answering the following questions. You should have no problem identifying the purpose of the components because similar circuits have been explained to you earlier in the text.
Figure 2-10. – Video amplifier circuit.

The following questions refer to figure 2-10.

Q.12 What component in an amplifier circuit tends to limit the low-Frequency response of the amplifier?

Q.13 What is the purpose of L3?
Q.14 What is the purpose of C1?

Q.15 What is the purpose of R4?

Q.16 What is the purpose of L2?

Q.17 What is the purpose of R5?

Q.18 What component(s) is/are used for high-frequency compensation for Q1?

Q.19 What component(s) is/are used for low-frequency compensation for Q2?

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RADIO-FREQUENCY AMPLIFIERS

Now that you have seen the way in which a broadband, or video, amplifier can be constructed, you may be wondering about radio-frequency (rf) amplifiers. Do they use the same techniques? Are they just another type of broadband amplifier?
The answer to both questions is "no." Radio-frequency amplifiers use different techniques than video amplifiers and are very different from them.
Before you study the specific techniques used in rf amplifiers, you should review some information on the relationship between the input and output impedance of an amplifier and the gain of the amplifier stage.
AMPLIFIER INPUT/OUTPUT IMPEDANCE AND GAIN
You should remember that the gain of a stage is calculated by using the input and output signals. The formula used to calculate the gain of a stage is:

Voltage gain is calculated using input and output voltage; current gain uses input and output current; and power gain uses input and output power. For the purposes of our discussion, we will only be concerned with voltage gain.
Figure 2-11 shows a simple amplifier circuit with the input- and output-signal-developing impedances represented by variable resistors. In this circuit, C1 and C2 are the input and output coupling capacitors. R1 represents the impedance of the input circuit. R2 represents the input-signal-developing impedance, and R3 represents the output impedance.
Figure 2-11. – Variable input and output impedances.

R1 and R2 form a voltage-divider network for the input signal. When R2 is increased in value, the input signal to the transistor (Q1) increases. This causes a larger output signal, and the gain of the stage increases.
Now look at the output resistor, R3. As R3 is increased in value, the output signal increases. This also increases the gain of the stage.
As you can see, increasing the input-signal-developing impedance, the output impedance, or both will increase the gain of the stage. Of course there are limits to this process. The transistor must not be overdriven with too high an input signal or distortion will result.
With this principle in mind, if you could design a circuit that had maximum impedance at a specific frequency (or band of frequencies), that circuit could be used in an rf amplifier. This FREQUENCY-DETERMINING NETWORK could be used as the input-signal-developing impedance, the output impedance, or both. The rf amplifier circuit would then be as shown in figure 2-12.
Figure 2-12. – Semiblock diagram of rf amplifier.

In this "semi-block" diagram, C1 and C2 are the input and output coupling capacitors. R1 represents the impedance of the input circuit. The blocks marked FDN represent the frequency-determining networks. They are used as input-signal-developing and output impedances for Q1.

FREQUENCY-DETERMINING NETWORK FOR AN RF AMPLIFIER

What kind of circuit would act as a frequency-determining network? In general, a frequency-determining network is a circuit that provides the desired response at a particular frequency. This response could be maximum impedance or minimum impedance; it all depends on how the frequency-determining network is used. You will see more about frequency-determining networks in NEETS, module 9 – Introduction to Wave-Generation and Shaping Circuits. As you have seen, the frequency-determining network needed for an rf amplifier should have maximum impedance at the desired frequency.
Before you are shown the actual components that make up the frequency-determining network for an rf amplifier, look at figure 2-13, which is a simple parallel circuit. The resistors in this circuit are variable and are connected together (ganged) in such a way that as the resistance of R1 increases, the resistance of R2 decreases, and vice versa.
Figure 2-13. – Parallel variable resistors (ganged).

If each resistor has a range from 0 to 200 ohms, the following relationship will exist between the individual resistances and the resistance of the network (RT). (All values are in ohms, RT rounded off to two decimal places. These are selected values; there are an infinite number of possible combinations.)

As you can see, this circuit has maximum resistance (RT) when the individual resistors are of equal value. If the variable resistors represented impedances and if components could be found that varied their impedance in the same way as the ganged resistors in figure 2-13 , you would have the frequency-determining network needed for an rf amplifier.
There are components that will vary their impedance (reactance) like the ganged resistors. As you know, the reactance of an inductor and a capacitor vary as frequency changes. As frequency increases, inductive reactance increases, and capacitive reactance decreases.
At some frequency, inductive and capacitive reactance will be equal. That frequency will depend upon the value of the inductor and capacitor. If the inductor and capacitor are connected as a parallel LC circuit, you will have the ideal frequency-determining network for an rf amplifier.
The parallel LC circuit used as a frequency-determining network is called a TUNED CIRCUIT. This circuit is "tuned" to give the proper response at the desired frequency by selecting the proper values of inductance and capacitance. A circuit using this principle is shown in figure 2-14 which shows an rf amplifier with parallel LC circuits used as frequency-determining networks. This rf amplifier will only be effective in amplifying the frequency determined by the parallel LC circuits.
Figure 2-14. – Simple rf amplifier.

In many electronic devices, such as radio or television receivers or radar systems, a particular frequency must be selected from a band of frequencies. This could be done by using a separate rf amplifier for each frequency and then turning on the appropriate rf amplifier. It would be more efficient if a single rf amplifier could be "tuned" to the particular frequency as that frequency is needed. This is what happens when you select a channel on your television set or tune to a station on your radio. To accomplish this "tuning," you need only change the value of inductance or capacitance in the parallel LC circuits (tuned circuits).
In most cases, the capacitance is changed by the use of variable capacitors. The capacitors in the input and output portions of all the rf amplifier stages are ganged together in order that they can all be changed at one time with a single device, such as the tuning dial on a radio. (This technique will be shown on a schematic a little later in this chapter.)

Q.20 If the input-signal-developing impedance of an amplifier is increased, what is the effect on the gain?

Q.21 If the output impedance of an amplifier circuit is decreased, what is the effect on the gain?

Q.22 What is the purpose of a frequency-determining network in an rf amplifier?

Q.23 Can a parallel LC circuit be used as the frequency-determining network for an rf amplifier?

Q.24 How can the frequency be changed in the frequency-determining network?

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RF AMPLIFIER COUPLING

Figure 2-14 and the other circuits you have been shown use capacitors to couple the signal in to and out of the circuit (C1 and C4 in figure 2-14). As you remember from chapter 1, there are also other methods of coupling signals from one stage to another. Transformer coupling is the most common method used to couple rf amplifiers. Transformer coupling has many advantages over RC coupling for rf amplifiers; for example, transformer coupling uses fewer components than capacitive coupling. It can also provide a means of increasing the gain of the stage by using a step-up transformer for voltage gain. If a current gain is required, a step-down transformer can be used.
You should also remember that the primary and secondary windings of a transformer are inductors. With these factors in mind, an rf amplifier could be constructed like the one shown in figure 2-15.
Figure 2-15. – Transformer-coupled rf amplifier.

In this circuit, the secondary of T1 and capacitor C1 form a tuned circuit which is the input-signal-developing impedance. The primary of T2 and capacitor C2 are a tuned circuit which acts as the output impedance of
Q1. (Both T1 and T2 must be rf transformers in order to operate at rf frequencies.)
The input signal applied to the primary of T1 could come from the previous stage or from some input device, such as a receiving antenna. In either case, the input device would have a capacitor connected across a coil to form a tuned circuit. In the same way, the secondary of T2 represents the output of this circuit. A capacitor connected across the secondary of T2 would form a parallel LC network. This network could act as the input-
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signal-developing impedance for the next stage, or the network could represent some type of output device, such as a transmitting antenna.
The tuned circuits formed by the transformer and capacitors may not have the bandwidth required for the amplifier. In other words, the bandwidth of the tuned circuit may be too "narrow" for the requirements of the amplifier. (For example, the rf amplifiers used in television receivers usually require a bandwidth of 6 MHz.)
One way of "broadening" the bandpass of a tuned circuit is to use a swamping resistor. This is similar to the use of the swamping resistor that was shown with the series peaking coil in a video amplifier. A swamping resistor connected in parallel with the tuned circuit will cause a much broader bandpass. (This technique and the theory behind it are discussed in more detail in NEETS, module 9.)
Another technique used to broaden the bandpass involves the amount of coupling in the transformers. For transformers, the term "coupling" refers to the amount of energy transferred from the primary to the secondary of the transformer. This depends upon the number of flux lines from the primary that intersect, or cut, the secondary. When more flux lines cut the secondary, more energy is transferred.
Coupling is mainly a function of the space between the primary and secondary windings. A transformer can be loosely coupled (having little transfer of energy), optimumly coupled (just the right amount of energy transferred), or overcoupled (to the point that the flux lines of primary and secondary windings interfere with each other).
Figure 2-16, (view A) (view B) (view C), shows the effect of coupling on frequency response when parallel LC circuits are made from the primary and secondary windings of transformers.
Figure 2-16A. – Effect of coupling on Frequency response. LOOSE COUPLING
Figure 2-16B. – Effect of coupling on Frequency response. OPTIMUM COUPLING

Figure 2-16C. – Effect of coupling on Frequency response. OVER-COUPLING

In view (A) the transformer is loosely coupled; the Frequency response curve shows a narrow bandwidth. In view (B) the transformer has optimum coupling; the bandwidth is wider and the curve is relatively flat. In view (C) the transformer is overcoupled; the Frequency response curve shows a broad bandpass, but the curve "dips" in the middle showing that these frequencies are not developed as well as others in the bandwidth.
Optimum coupling will usually provide the necessary bandpass for the frequency-determining network (and therefore the rf amplifier). For some uses, such as rf amplifiers in a television receiver, the bandpass available from optimum coupling is not wide enough. In these cases, a swamping resistor (as mentioned earlier) will be used with the optimum coupling to broaden the bandpass.
COMPENSATION OF RF AMPLIFIERS
Now you have been shown the way in which an rf amplifier is configured to amplify a band of frequencies and the way in which an rf amplifier can be "tuned" for a particular band of frequencies. You have also seen some ways in which the bandpass of an rf amplifier can be adjusted. However, the frequencies at which rf amplifiers operate are so high that certain problems exist.
One of these problems is the losses that can occur in a transformer at these high frequencies. Another problem is with interelectrode capacitance in the transistor. The process of overcoming these problems is known as COMPENSATION.
Transformers in RF Amplifiers
As you recall from NEETS, module 1, the losses in a transformer are classified as copper loss, eddy-current loss, and hysteresis loss. Copper loss is not affected by frequency, as it depends upon the resistance of the winding and the current through the winding. Similarly, eddy-current loss is mostly a function of induced voltage rather than the frequency of that voltage. Hysteresis loss, however, increases as frequency increases.
Hysteresis loss is caused by the realignment of the magnetic domains in the core of the transformer each time the polarity of the magnetic field changes. As the frequency of the a.c. increases, the number of shifts in the magnetic field also increases (two shifts for each cycle of a.c.);

therefore, the "molecular friction" increases and the hysteresis loss is greater. This increase in hysteresis loss causes the efficiency of the transformer (and therefore the amplifier) to decrease. The energy that goes into hysteresis loss is taken away from energy that could go into the signal.
RF TRANSFORMERS, specially designed for use with rf, are used to correct the problem of excessive hysteresis loss in the transformer of an rf amplifier. The windings of rf transformers are wound onto a tube of nonmagnetic material and the core is either powdered iron or air. These types of cores also reduce eddy-current loss.
Neutralization of RF Amplifiers
The problem of interelectrode capacitance in the transistor of an rf amplifier is solved by NEUTRALIZATION. Neutralization is the process of counteracting or "neutralizing" the effects of interelectrode capacitance.
Figure 2-17 shows the effect of the base-to-collector interelectrode capacitance in an rf amplifier. The "phantom" capacitor (CBC) represents the interelectrode capacitance between the base and the collector of Q1. This is the interelectrode capacitance that has the most effect in an rf amplifier. As you can see, CBC causes a degenerative (negative) feedback which decreases the gain of the amplifier. (There are some special cases in which CBC can cause regenerative (positive) feedback. In this case, the technique described below will provide negative feedback which will accomplish the neutralization of the amplifier.)
Figure 2-17. – Interelectrode capacitance in an rf amplifier.

As you may recall, unwanted degenerative feedback can be counteracted (neutralized) by using positive feedback. This is exactly what is done to neutralize an rf amplifier.
Positive feedback is accomplished by the use of a feedback capacitor. This capacitor must feed back a signal that is in phase with the signal on the base of Q1. One method of doing this is shown in figure 2-18.
Figure 2-18. – Neutralized rf amplifier.

In figure 2-18, a feedback capacitor (C4) has been added to neutralize the amplifier. This solves the problem of unwanted degenerative feedback. Except for capacitor C4, this circuit is identical to the circuit shown in figure 2-17. (When CBC causes regenerative feedback, C4 will still neutralize the amplifier. This is true because C4 always provides a feedback signal which is 180 degrees out of phase with the feedback signal caused by CBC.)

Q.25 What is the most common form of coupling for an rf amplifier?

Q.26 What are two advantages of this type of coupling?

Q.27 If current gain is required from an rf amplifier, what type of component should be used as an output coupling element?

Q.28 What problem is caused in an rf amplifier by a loosely coupled transformer?

Q.29 How is this problem corrected?

Q.30 What problem is caused by overcoupling in a transformer?

Q.31 What method provides the widest bandpass?

Q.32 What two methods are used to compensate for the problems that cause low gain in an rf amplifier?

Q.33 What type of feedback is usually caused by the base-to-collector interelectrode capacitance? Q.34 How is this compensated for?

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VIDEO AMPLIFIERS

As you have seen, a transistor amplifier is limited in its Frequency response. You should also remember from chapter 1 that a VIDEO AMPLIFIER should have a frequency response of 10 hertz (10 Hz) to 6 megahertz (6 MHz). The question has probably occurred to you: How is it possible to "extend" the range of Frequency response of an amplifier?

HIGH-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS

If the frequency-response range of an audio amplifier must be extended to 6 megahertz (6 MHz) for use as a video amplifier, some means must be found to overcome the limitations of the audio amplifier. As you have seen, the capacitance of an amplifier circuit and the interelectrode capacitance of the transistor (or electronic tube) cause the higher Frequency response to be limited.
In some ways capacitance and inductance can be thought of as opposites.
As stated before, as frequency increases, capacitive reactance decreases, and inductive reactance increases. Capacitance opposes changes in voltage, and inductance opposes changes in current. Capacitance causes current to lead voltage, and inductance causes voltage to lead current.
Since frequency affects capacitive reactance and inductive reactance in opposite ways, and since it is the capacitive reactance that causes the problem with high-frequency response, inductors are added to an amplifier circuit to improve the high-frequency response. This is called HIGH-FREQUENCY COMPENSATION. Inductors (coils), when used for high-frequency compensation, are called PEAKING COILS. Peaking coils can be added to a circuit so they are in series with the output signal path or in parallel to the output signal path. Instead of only in series or parallel, a combination of peaking coils in series and parallel with the output signal path can also be used for high-frequency compensation.
As in all electronic circuits, nothing comes free. The use of peaking coils WILL increase the Frequency response of an amplifier circuit, but it will ALSO lower the gain of the amplifier.
Series Peaking
The use of a peaking coil in series with the output signal path is known as SERIES PEAKING. Figure 2-6 shows a transistor amplifier circuit with a series peaking coil. In this figure, R1 is the input-signal-developing resistor. R2 is used for bias and temperature stability of Q1. C1 is the bypass capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling capacitor which couples the output signal to the next stage. "Phantom" capacitor COUT represents the output capacitance of the circuit, and "phantom" capacitor CIN represents the input capacitance of the next stage.
Figure 2-6. – Series peaking coil.

You know that the capacitive reactance of COUT and CIN will limit the high-Frequency response of the circuit. L1 is the series peaking coil. It is in series with the output-signal path and isolates COUT from CIN.
R4 is called a "swamping" resistor and is used to keep L1 from overcompensating at a narrow range of frequencies. In other words, R4 is used to keep the frequency-response curve flat. If R4 were not used with L1, there could be a "peak" in the frequency-response curve. (Remember, L1 is called a peaking coil.)
Shunt Peaking
If a coil is placed in parallel (shunt) with the output signal path, the technique is called SHUNT PEAKING. Figure 2-7 shows a circuit with a shunt peaking coil. With the exceptions of the "phantom" capacitor and the inductor, the components in this circuit are the same as those in figure 2-6. R1 is the input-signal-developing resistor. R2 is used for bias and temperature stability. C1 is the bypass capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling capacitor which couples the output signal to the next stage.

Figure 2-7. – Shunt peaking coil.

The "phantom" capacitor, CT, represents the total capacitance of the circuit. Notice that it tends to couple the output signal to ground.
L1 is the shunt peaking coil. While it is in series with the load resistor (R3), it is in parallel (shunt) with the output-signal path.
Since inductive reactance increases as frequency increases, the reactance of L1 develops more output signal as the frequency increases. At the same time, the capacitive reactance of CT is decreasing as frequency increases. This tends to couple more of the output signal to ground. The increased inductive reactance counters the effect of the decreased capacitive reactance and this increases the high-Frequency response of the amplifier.
Combination Peaking
You have seen how a series peaking coil isolates the output capacitance of an amplifier from the input capacitance of the next stage. You have also seen how a shunt peaking coil will counteract the effects of the total capacitance of an amplifier. If these two techniques are used together, the combination is more effective than the use of either one alone. The use of both series and shunt peaking coils is known as COMBINATION PEAKING. An amplifier circuit with combination peaking is shown in figure 2-8. In figure 2-8 the peaking coils are L1 and L2. L1 is a shunt peaking coil, and L2 is a series peaking coil.
Figure 2-8. – Combination peaking.

The "phantom" capacitor CT represents the total capacitance of the amplifier circuit. "Phantom" capacitor CIN represents the input capacitance of the next stage. Combination peaking will easily allow an amplifier to have a high-Frequency response of 6 megahertz (6 MHz).

Q.8 What is the major factor that limits the high-Frequency response of an amplifier circuits?

Q.9 What components can be used to increase the high-Frequency response of an amplifier?

Q.10 What determines whether these components are considered series or shunt?

Q.11 What is the arrangement of both series and shunt components called?

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LOW-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS

Now that you have seen how the high-Frequency response of an amplifier can be extended to 6 megahertz (6 MHz), you should realize that it is only necessary to extend the low-Frequency response to 10 hertz (10 Hz) in order to have a video amplifier.
Once again, the culprit in low-Frequency response is capacitance (or capacitive reactance). But this time the problem is the coupling capacitor between the stages.
At low frequencies the capacitive reactance of the coupling capacitor (C2 in figure 2-8) is high. This high reactance limits the amount of output signal that is coupled to the next stage. In addition, the RC network of the coupling capacitor and the signal-developing resistor of the next stage cause a phase shift in the output signal. (Refer to NEETS, module 2, for a discussion of phase shifts in RC networks.) Both of these problems (poor low-Frequency response and phase shift) can be solved by adding a parallel RC network in series with the load resistor. This is shown in figure 2-9.
Figure 2-9. – Low frequency compensation network.

The complete circuitry for Q2 is not shown in this figure, as the main concern is the signal-developing resistor (R5) for Q2. The coupling capacitor (C2) and the resistor (R5) limit the low-Frequency response of the amplifier and cause a phase shift. The amount of the phase shift will depend upon the amount of resistance and capacitance. The RC network of R4 and C3 compensates for the effects of C2 and R5 and extends the low-frequency response of the amplifier.
At low frequencies, R4 adds to the load resistance (R3) and increases the gain of the amplifier. As frequency increases, the reactance of C3 decreases. C3 then provides a path around R4 and the gain of the transistor decreases. At the same time, the reactance of the coupling capacitor (C2) decreases and more signal is coupled to Q2.
Because the circuit shown in figure 2-9 has no high-frequency compensation, it would not be a very practical video amplifier.
TYPICAL VIDEO-AMPLIFIER CIRCUIT
There are many different ways in which video amplifiers can be built. The particular configuration of a video amplifier depends upon the equipment in which the video amplifier is used. The circuit shown in figure 2-10 is only one of many possible video-amplifier circuits. Rather than reading about what each component does in this circuit, you can see how well you have learned about video amplifiers by answering the following questions. You should have no problem identifying the purpose of the components because similar circuits have been explained to you earlier in the text.
Figure 2-10. – Video amplifier circuit.

The following questions refer to figure 2-10.

Q.12 What component in an amplifier circuit tends to limit the low-Frequency response of the amplifier?

Q.13 What is the purpose of L3?
Q.14 What is the purpose of C1?

Q.15 What is the purpose of R4?

Q.16 What is the purpose of L2?

Q.17 What is the purpose of R5?

Q.18 What component(s) is/are used for high-frequency compensation for Q1?

Q.19 What component(s) is/are used for low-frequency compensation for Q2?

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RADIO-FREQUENCY AMPLIFIERS

Now that you have seen the way in which a broadband, or video, amplifier can be constructed, you may be wondering about radio-frequency (rf) amplifiers. Do they use the same techniques? Are they just another type of broadband amplifier?
The answer to both questions is "no." Radio-frequency amplifiers use different techniques than video amplifiers and are very different from them.
Before you study the specific techniques used in rf amplifiers, you should review some information on the relationship between the input and output impedance of an amplifier and the gain of the amplifier stage.
AMPLIFIER INPUT/OUTPUT IMPEDANCE AND GAIN
You should remember that the gain of a stage is calculated by using the input and output signals. The formula used to calculate the gain of a stage is:

Voltage gain is calculated using input and output voltage; current gain uses input and output current; and power gain uses input and output power. For the purposes of our discussion, we will only be concerned with voltage gain.
Figure 2-11 shows a simple amplifier circuit with the input- and output-signal-developing impedances represented by variable resistors. In this circuit, C1 and C2 are the input and output coupling capacitors. R1 represents the impedance of the input circuit. R2 represents the input-signal-developing impedance, and R3 represents the output impedance.
Figure 2-11. – Variable input and output impedances.

R1 and R2 form a voltage-divider network for the input signal. When R2 is increased in value, the input signal to the transistor (Q1) increases. This causes a larger output signal, and the gain of the stage increases.
Now look at the output resistor, R3. As R3 is increased in value, the output signal increases. This also increases the gain of the stage.
As you can see, increasing the input-signal-developing impedance, the output impedance, or both will increase the gain of the stage. Of course there are limits to this process. The transistor must not be overdriven with too high an input signal or distortion will result.
With this principle in mind, if you could design a circuit that had maximum impedance at a specific frequency (or band of frequencies), that circuit could be used in an rf amplifier. This FREQUENCY-DETERMINING NETWORK could be used as the input-signal-developing impedance, the output impedance, or both. The rf amplifier circuit would then be as shown in figure 2-12.
Figure 2-12. – Semiblock diagram of rf amplifier.

In this "semi-block" diagram, C1 and C2 are the input and output coupling capacitors. R1 represents the impedance of the input circuit. The blocks marked FDN represent the frequency-determining networks. They are used as input-signal-developing and output impedances for Q1.

FREQUENCY-DETERMINING NETWORK FOR AN RF AMPLIFIER

What kind of circuit would act as a frequency-determining network? In general, a frequency-determining network is a circuit that provides the desired response at a particular frequency. This response could be maximum impedance or minimum impedance; it all depends on how the frequency-determining network is used. You will see more about frequency-determining networks in NEETS, module 9 – Introduction to Wave-Generation and Shaping Circuits. As you have seen, the frequency-determining network needed for an rf amplifier should have maximum impedance at the desired frequency.
Before you are shown the actual components that make up the frequency-determining network for an rf amplifier, look at figure 2-13, which is a simple parallel circuit. The resistors in this circuit are variable and are connected together (ganged) in such a way that as the resistance of R1 increases, the resistance of R2 decreases, and vice versa.
Figure 2-13. – Parallel variable resistors (ganged).

If each resistor has a range from 0 to 200 ohms, the following relationship will exist between the individual resistances and the resistance of the network (RT). (All values are in ohms, RT rounded off to two decimal places. These are selected values; there are an infinite number of possible combinations.)

As you can see, this circuit has maximum resistance (RT) when the individual resistors are of equal value. If the variable resistors represented impedances and if components could be found that varied their impedance in the same way as the ganged resistors in figure 2-13 , you would have the frequency-determining network needed for an rf amplifier.
There are components that will vary their impedance (reactance) like the ganged resistors. As you know, the reactance of an inductor and a capacitor vary as frequency changes. As frequency increases, inductive reactance increases, and capacitive reactance decreases.
At some frequency, inductive and capacitive reactance will be equal. That frequency will depend upon the value of the inductor and capacitor. If the inductor and capacitor are connected as a parallel LC circuit, you will have the ideal frequency-determining network for an rf amplifier.
The parallel LC circuit used as a frequency-determining network is called a TUNED CIRCUIT. This circuit is "tuned" to give the proper response at the desired frequency by selecting the proper values of inductance and capacitance. A circuit using this principle is shown in figure 2-14 which shows an rf amplifier with parallel LC circuits used as frequency-determining networks. This rf amplifier will only be effective in amplifying the frequency determined by the parallel LC circuits.
Figure 2-14. – Simple rf amplifier.

In many electronic devices, such as radio or television receivers or radar systems, a particular frequency must be selected from a band of frequencies. This could be done by using a separate rf amplifier for each frequency and then turning on the appropriate rf amplifier. It would be more efficient if a single rf amplifier could be "tuned" to the particular frequency as that frequency is needed. This is what happens when you select a channel on your television set or tune to a station on your radio. To accomplish this "tuning," you need only change the value of inductance or capacitance in the parallel LC circuits (tuned circuits).
In most cases, the capacitance is changed by the use of variable capacitors. The capacitors in the input and output portions of all the rf amplifier stages are ganged together in order that they can all be changed at one time with a single device, such as the tuning dial on a radio. (This technique will be shown on a schematic a little later in this chapter.)

Q.20 If the input-signal-developing impedance of an amplifier is increased, what is the effect on the gain?

Q.21 If the output impedance of an amplifier circuit is decreased, what is the effect on the gain?

Q.22 What is the purpose of a frequency-determining network in an rf amplifier?

Q.23 Can a parallel LC circuit be used as the frequency-determining network for an rf amplifier?

Q.24 How can the frequency be changed in the frequency-determining network?

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RF AMPLIFIER COUPLING

Figure 2-14 and the other circuits you have been shown use capacitors to couple the signal in to and out of the circuit (C1 and C4 in figure 2-14). As you remember from chapter 1, there are also other methods of coupling signals from one stage to another. Transformer coupling is the most common method used to couple rf amplifiers. Transformer coupling has many advantages over RC coupling for rf amplifiers; for example, transformer coupling uses fewer components than capacitive coupling. It can also provide a means of increasing the gain of the stage by using a step-up transformer for voltage gain. If a current gain is required, a step-down transformer can be used.
You should also remember that the primary and secondary windings of a transformer are inductors. With these factors in mind, an rf amplifier could be constructed like the one shown in figure 2-15.
Figure 2-15. – Transformer-coupled rf amplifier.

In this circuit, the secondary of T1 and capacitor C1 form a tuned circuit which is the input-signal-developing impedance. The primary of T2 and capacitor C2 are a tuned circuit which acts as the output impedance of
Q1. (Both T1 and T2 must be rf transformers in order to operate at rf frequencies.)
The input signal applied to the primary of T1 could come from the previous stage or from some input device, such as a receiving antenna. In either case, the input device would have a capacitor connected across a coil to form a tuned circuit. In the same way, the secondary of T2 represents the output of this circuit. A capacitor connected across the secondary of T2 would form a parallel LC network. This network could act as the input-
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signal-developing impedance for the next stage, or the network could represent some type of output device, such as a transmitting antenna.
The tuned circuits formed by the transformer and capacitors may not have the bandwidth required for the amplifier. In other words, the bandwidth of the tuned circuit may be too "narrow" for the requirements of the amplifier. (For example, the rf amplifiers used in television receivers usually require a bandwidth of 6 MHz.)
One way of "broadening" the bandpass of a tuned circuit is to use a swamping resistor. This is similar to the use of the swamping resistor that was shown with the series peaking coil in a video amplifier. A swamping resistor connected in parallel with the tuned circuit will cause a much broader bandpass. (This technique and the theory behind it are discussed in more detail in NEETS, module 9.)
Another technique used to broaden the bandpass involves the amount of coupling in the transformers. For transformers, the term "coupling" refers to the amount of energy transferred from the primary to the secondary of the transformer. This depends upon the number of flux lines from the primary that intersect, or cut, the secondary. When more flux lines cut the secondary, more energy is transferred.
Coupling is mainly a function of the space between the primary and secondary windings. A transformer can be loosely coupled (having little transfer of energy), optimumly coupled (just the right amount of energy transferred), or overcoupled (to the point that the flux lines of primary and secondary windings interfere with each other).
Figure 2-16, (view A) (view B) (view C), shows the effect of coupling on frequency response when parallel LC circuits are made from the primary and secondary windings of transformers.
Figure 2-16A. – Effect of coupling on Frequency response. LOOSE COUPLING
Figure 2-16B. – Effect of coupling on Frequency response. OPTIMUM COUPLING

Figure 2-16C. – Effect of coupling on Frequency response. OVER-COUPLING

In view (A) the transformer is loosely coupled; the Frequency response curve shows a narrow bandwidth. In view (B) the transformer has optimum coupling; the bandwidth is wider and the curve is relatively flat. In view (C) the transformer is overcoupled; the Frequency response curve shows a broad bandpass, but the curve "dips" in the middle showing that these frequencies are not developed as well as others in the bandwidth.
Optimum coupling will usually provide the necessary bandpass for the frequency-determining network (and therefore the rf amplifier). For some uses, such as rf amplifiers in a television receiver, the bandpass available from optimum coupling is not wide enough. In these cases, a swamping resistor (as mentioned earlier) will be used with the optimum coupling to broaden the bandpass.
COMPENSATION OF RF AMPLIFIERS
Now you have been shown the way in which an rf amplifier is configured to amplify a band of frequencies and the way in which an rf amplifier can be "tuned" for a particular band of frequencies. You have also seen some ways in which the bandpass of an rf amplifier can be adjusted. However, the frequencies at which rf amplifiers operate are so high that certain problems exist.
One of these problems is the losses that can occur in a transformer at these high frequencies. Another problem is with interelectrode capacitance in the transistor. The process of overcoming these problems is known as COMPENSATION.
Transformers in RF Amplifiers
As you recall from NEETS, module 1, the losses in a transformer are classified as copper loss, eddy-current loss, and hysteresis loss. Copper loss is not affected by frequency, as it depends upon the resistance of the winding and the current through the winding. Similarly, eddy-current loss is mostly a function of induced voltage rather than the frequency of that voltage. Hysteresis loss, however, increases as frequency increases.
Hysteresis loss is caused by the realignment of the magnetic domains in the core of the transformer each time the polarity of the magnetic field changes. As the frequency of the a.c. increases, the number of shifts in the magnetic field also increases (two shifts for each cycle of a.c.);

therefore, the "molecular friction" increases and the hysteresis loss is greater. This increase in hysteresis loss causes the efficiency of the transformer (and therefore the amplifier) to decrease. The energy that goes into hysteresis loss is taken away from energy that could go into the signal.
RF TRANSFORMERS, specially designed for use with rf, are used to correct the problem of excessive hysteresis loss in the transformer of an rf amplifier. The windings of rf transformers are wound onto a tube of nonmagnetic material and the core is either powdered iron or air. These types of cores also reduce eddy-current loss.
Neutralization of RF Amplifiers
The problem of interelectrode capacitance in the transistor of an rf amplifier is solved by NEUTRALIZATION. Neutralization is the process of counteracting or "neutralizing" the effects of interelectrode capacitance.
Figure 2-17 shows the effect of the base-to-collector interelectrode capacitance in an rf amplifier. The "phantom" capacitor (CBC) represents the interelectrode capacitance between the base and the collector of Q1. This is the interelectrode capacitance that has the most effect in an rf amplifier. As you can see, CBC causes a degenerative (negative) feedback which decreases the gain of the amplifier. (There are some special cases in which CBC can cause regenerative (positive) feedback. In this case, the technique described below will provide negative feedback which will accomplish the neutralization of the amplifier.)
Figure 2-17. – Interelectrode capacitance in an rf amplifier.

As you may recall, unwanted degenerative feedback can be counteracted (neutralized) by using positive feedback. This is exactly what is done to neutralize an rf amplifier.
Positive feedback is accomplished by the use of a feedback capacitor. This capacitor must feed back a signal that is in phase with the signal on the base of Q1. One method of doing this is shown in figure 2-18.
Figure 2-18. – Neutralized rf amplifier.

In figure 2-18, a feedback capacitor (C4) has been added to neutralize the amplifier. This solves the problem of unwanted degenerative feedback. Except for capacitor C4, this circuit is identical to the circuit shown in figure 2-17. (When CBC causes regenerative feedback, C4 will still neutralize the amplifier. This is true because C4 always provides a feedback signal which is 180 degrees out of phase with the feedback signal caused by CBC.)

Q.25 What is the most common form of coupling for an rf amplifier?

Q.26 What are two advantages of this type of coupling?

Q.27 If current gain is required from an rf amplifier, what type of component should be used as an output coupling element?

Q.28 What problem is caused in an rf amplifier by a loosely coupled transformer?

Q.29 How is this problem corrected?

Q.30 What problem is caused by overcoupling in a transformer?

Q.31 What method provides the widest bandpass?

Q.32 What two methods are used to compensate for the problems that cause low gain in an rf amplifier?

Q.33 What type of feedback is usually caused by the base-to-collector interelectrode capacitance? Q.34 How is this compensated for?

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Short Circuit Protection

The main disadvantage of a series regulator is that the pass transistor is in series with the load. If a short develops in the load, a large amount of current will flow in the regulator circuit. The pass transistor can be damaged by this excessive current flow. You could place a fuse in the circuit, but in many cases, the transistor will be damaged before the fuse blows. The best way to protect this circuit is to limit the current automatically to a safe value. A series regulator with a current-limiting circuit is shown in figure 4-50. You should recall that in order for a silicon NPN transistor to conduct, the base must be between 0.6 volt to 0.7 volt more positive than the emitter. Resistor R4 will develop a voltage drop of 0.6 volt when the load current reaches 600 milliamperes. This is illustrated using Ohm’s law:

Figure 4-50. – Series regulator with current limiting.

When load current is below 600 milliamperes, the base-to-emitter voltage on Q2 is not high enough to allow Q2 to conduct. With Q2 cut off, the circuit acts like a series regulator.
When the load current increases above 600 milliamperes, the voltage drop across R4 increases to more than 0.6 volt. This causes Q2 to conduct through resistor R2, thereby decreasing the voltage on the base of pass transistor Q1. This action causes Q1 to conduct less. Therefore, the current cannot increase above 600 to 700 milliamperes.
By increasing the value of R4, you can limit the current to almost any value. For example, a 100-ohm resistor develops a voltage drop of 0.6 volt at 6 milliamperes of current. You may encounter current-limiting circuits that are more sophisticated, but the theory of operation is always the same. If you understand this circuit, you should have no problem with the others.
TROUBLESHOOTING POWER SUPPLIES
Whenever you are working with electricity, the proper use of safety precautions is of the utmost importance to remember. In the front of all electronic technical manuals, you will always find a section on safety precautions. Also posted on each piece of equipment should be a sign listing the specific precautions for that equipment. One area that is sometimes overlooked, and is a hazard especially on board ship, is the method in which equipment is grounded. By grounding the return side of the power transformer to the metal chassis, the load being supplied by the power supply can be wired directly to the metal chassis. Thereby the necessity of wiring directly to the return side of the transformer is eliminated. This method saves wire and reduces the cost of building the equipment, and while it solves one of the problems of the manufacturer, it creates a problem for you, the technician. Unless the chassis is physically grounded to the ship’s ground (the hull), the chassis can be charged (or can float) several hundred volts above ship’s ground. If you come in contact with the metal chassis at the same time you are in contact with the ship’s hull, the current from the chassis can use your body as a low resistance path back to the ship’s ac generators. At best this can be an unpleasant experience; at worst it can be fatal. For this reason Navy electronic equipment is always grounded to the ship’s hull, and approved rubber mats are required in all spaces where electronic equipment is present. Therefore, before starting to work on any electronic or electrical equipment, ALWAYS ENSURE THAT THE EQUIPMENT AND ANY TEST EQUIPMENT YOU ARE USING IS PROPERLY GROUNDED AND THAT THE RUBBER MAT YOU ARE STANDING ON IS IN GOOD CONDITION. As long as you follow these simple rules, you should be able to avoid the possibility of becoming an electrical conductor.

TESTING

There are two widely used checks in testing electronic equipment, VISUAL and SIGNAL TRACING. The importance of the visual check should not be underestimated because many technicians find defects right away simply by looking for them. A visual check does not take long. In fact, you should be able to see the problem readily if it is the type of problem that can be seen. You should learn the following procedure. You could find yourself using it quite often. This procedure is not only for power supplies but also for any type of electronic equipment you may be troubleshooting. (Because diode and transistor testing was covered in chapter 1 and 2 of this module, it will not be discussed at this time. If you have problems in this area, refer to chapter 1 for diodes or chapter 2 for transistors.)

• BEFORE YOU ENERGIZE THE EQUIPMENT, LOOK FOR: SHORTS – Any terminal or connection that is close to the chassis or to any other terminal should be examined for the possibility of a short. A short in any part of the power supply can cause considerable damage. Look for and remove any stray drops of solder, bits of wire, nuts, or screws. It sometimes helps to shake the chassis and listen for any tell-tale rattles. Remember to correct any problem that may cause a short circuit; if it is not causing trouble now, it may cause problems in the future.

DISCOLORED OR LEAKING TRANSFORMER – This is a sure sign that there is a short somewhere. Locate it. If the equipment has a fuse, find out why the fuse did not blow; too large a size may have been installed, or there may be a short across the fuse holder.
LOOSE, BROKEN, OR CORRODED CONNECTION – Any connection that is not in good condition is a trouble spot. If it is not causing trouble now, it will probably cause problems in the future. Fix it.
DAMAGED RESISTORS OR CAPACITORS – A resistor that is discolored or charred has been subjected to an overload. An electrolytic capacitor will show a whitish deposit at the seal around the terminals. Check for a short whenever you notice a damaged resistor or a damaged capacitor. If there is no short, the trouble may be that the power supply has been overloaded in some way. Make a note to replace the part after signal tracing. There is no sense in risking a new part until the trouble has been located.

• ENERGIZE THE EQUIPMENT AND LOOK FOR:

SMOKING PARTS – If any part smokes or if you hear any boiling or sputtering sounds, remove the power immediately. There is a short circuit somewhere that you have missed in your first inspection. Use any ohmmeter to check the part once again. Start in the neighborhood of the smoking part. SPARKING – Tap or shake the chassis. If you see or hear sparking, you have located a loose connection or a short. Check and repair.
If you locate and repair any of the defects listed under the visual check, make a note of what you find and what you do to correct it. It is quite probable you have found the trouble. However, a good technician takes nothing for granted. You must prove to yourself that the equipment is operating properly and that no other troubles exist.
If you find none of the defects listed under the visual check, go ahead with the signal tracing procedure. The trouble is probably of such a nature that it cannot be seen directly-it may only be seen using an oscilloscope.

Tracing the ac signal through the equipment is the most rapid and accurate method of locating a trouble that cannot be found by a visual check, and it also serves as check on any repairs you may have made. The idea is to trace the ac voltage from the transformer, to see it change to pulsating dc at the rectifier output, and then see the pulsations smoothed out by the filter. The point where the signal stops or becomes distorted is the place look for the trouble. If you have no dc output voltage, you should look for an open or a short in your signal tracing. If you have a low dc voltage, you should look for a defective part and keep your eyes open for the place where the signal becomes distorted.
Signal tracing is one method used to localize trouble in a circuit. This is done by observing the waveform at the input and output of each part of a circuit.

Let’s review what each part of a good power supply does to a signal, as shown in figure 4-51. The ac voltage is brought in from the power line by means of the line cord. This voltage is connected to the primary of the transformer through the ON-OFF switch (S1). At the secondary winding of the transformer (points 1 and 2), the scope shows you a picture of the stepped-up voltage developed across each half of the secondary winding-the picture is that of a complete sine wave. Each of the two stepped-up voltages is connected between ground and one of the two anodes of the rectifier diodes. At the two rectifier anodes (points 4 and 5), there is still no change in the shape of the stepped-up voltage-the scope picture still shows a complete sine wave.
Figure 4-51. – Complete power supply (without regulator).

However, when you look at the scope pattern for point 6 (the voltage at the rectifier cathodes), you see the waveshape for pulsating direct current. This pulsating dc is fed through the first choke (L1) and filter capacitor (C1) which remove a large part of the ripple, or "hum," as shown by the waveform for point 7. Finally the dc voltage is fed through the second choke (L2) and filter capacitor (C2), which remove nearly all of the remaining ripple. (See the waveform for point 8, which shows almost no visible ripple.) You now have almost pure dc.

No matter what power supplies you use in the future, they all do the same thing – they change ac voltage into dc voltage.

Component Problems

The following paragraphs will give you an indication of troubles that occur with many different electronic circuit components.
TRANSFORMER AND CHOKE TROUBLES. – As you should know by now, the transformer and the choke are quite similar in construction. Likewise, the basic troubles that they may develop are comparable.

• A winding can open.
• Two or more turns of one winding can short together.
• A winding can short to the casing, which is usually grounded.
• Two windings(primary and secondary) can short together.
• This trouble is possible, of course, only in transformers.

When you have decided which of these four possible troubles could be causing the symptoms, you have definite steps to take. If you surmise that there is an open winding, or windings shorted together or to ground, an ohmmeter continuity check will locate the trouble. If the turns of a winding are shorted together, you may not be able to detect a difference in winding resistance. Therefore, you need to connect a good transformer in the place of the old one and see if the symptoms are eliminated. Keep in mind that transformers are difficult to replace. Make absolutely sure that the trouble is not elsewhere in the circuit before you change the transformer.
Occasionally, the shorts will only appear when the operating voltages are applied to the transformer. In this case you might find the trouble with a megger-an instrument which applies a high voltage as it reads resistance.

CAPACITOR AND RESISTOR TROUBLES. – Just two things can happen to a capacitor:

• It may open up, removing the capacitor completely from the circuit.
• It may develop an internal short circuit. This means that it begins to pass current as though it were a resistor or a direct short.
• 

You may check a capacitor suspected of being open by disconnecting it from the circuit and checking it with a capacitor analyzer. You can check a capacitor suspected of being leaky with an ohmmeter; if it reads less than 500 kilohms, it is more than likely bad. However, capacitor troubles are difficult to find since they may appear intermittently or only under operating voltages. Therefore, the best check for a faulty capacitor is to replace it with one known to be good. If this restores proper operation, the fault was in the capacitor.

Resistor troubles are the simplest. However, like the others, they must be considered.

• A resistor can open.
• A resistor can increase in value.
• A resistor can decrease in value.

You already know how to check possible resistor troubles. Just use an ohmmeter after making sure no parallel circuit is connected across the resistor you wish to measure. When you know a parallel circuit is connected across the resistor or when you are in doubt disconnect one end of the resistor before measuring it. The ohmmeter check will usually be adequate. However, never forget that occasionally intermittent troubles may develop in resistors as well as in any other electronic parts.
Although you may observe problems that have not been covered specifically in this chapter, you should have gained enough knowledge to localize and repair any problem that may occur.

Q.41 What is the most important thing to remember when troubleshooting?

Q.42 What is the main reason for grounding the return side of the transformer to the chassis?

Q.43 What are two types of checks used in troubleshooting power supplies?

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SUMMARY of FILTERS

This chapter has presented you with a basic description of the theory and operation of a basic power supply and its components. The following summary is provided to enhance your understanding of power supplies.
POWER SUPPLIES are electronic circuits designed to convert ac to dc at any desired level. Almost all power supplies are composed of four sections: transformer, rectifier, filter, and regulator.

The POWER TRANSFORMER is the input transformer for the power supply.

The RECTIFIER is the section of the power supply that contains the secondary windings of the power transformer and the rectifier circuit. The rectifier uses the ability of a diode to conduct during one half cycle of ac to convert ac to dc.
HALF-WAVE RECTIFIERS give an output on only one half cycle of the input ac. For this reason, the pulses of dc are separated by a period of one half cycle of zero potential voltage.

FULL-WAVE RECTIFIERS conduct on both halves of the input ac cycles. As a result, the dc pulses are not separated from each other. A characteristic of full-wave rectifiers is the use of a center-tapped, high-voltage secondary. Because of the center tap, the output of the rectifier is limited to one-half of the input voltage of the high-voltage secondary.

BRIDGE RECTIFIERS are full-wave rectifiers that do not use a center-tapped, high-voltage secondary. Because of this, their dc output voltage is equal to the input voltage from the high-voltage secondary of the power transformer. Bridge rectifiers use four diodes connected in a bridge network. Diodes conduct in diagonal pairs to give a full-wave pulsating dc output.

FILTER CIRCUITS are designed to smooth, or filter, the ripple voltage present on the pulsating dc output of the rectifier. This is done by an electrical device that has the ability to store energy and to release the stored energy.
CAPACITANCE FILTERS are nothing more than large capacitors placed across the output of the rectifier section. Because of the large size of the capacitors, fast charge paths, and slow discharge paths, the capacitor will charge to average value, which will keep the pulsating dc output from reaching zero volts.

INDUCTOR FILTERS use an inductor called a choke to filter the pulsating dc input. Because of the impedance offered to circuit current, the output of the filter is at a lower amplitude than the input.

PI-TYPE FILTERS use both capacitive and inductive filters connected in a pi-type configuration. By combining filtering devices, the ability of the pi filter to remove ripple voltage is superior to that of either the capacitance or inductance filter.

VOLTAGE REGULATORS are circuits designed to maintain the output of power supplies at a constant amplitude despite variations of the ac source voltage or changes of the resistance of the load. This is done by creating a voltage divider of a resistive element in the regulator and the resistance of the load. Regulation is achieved by varying the resistance of the resistive element in the regulator.
A SERIES REGULATOR uses a variable resistance in series with the load. Regulation is achieved by varying this resistance either to increase or to decrease the voltage drop across the resistive element of the regulator. Characteristically, the resistance of the variable resistance moves in the same direction as the load. When the resistance of the load increases, the variable resistance of the regulator increases; when load resistance decreases, the variable resistance of the regulator decreases.

SHUNT REGULATORS use a variable resistance placed in parallel with  the load. Regulation is achieved by keeping the resistance of the load constant. Characteristically the resistance of the shunt moves in the opposite direction of the resistance of the load.

The CURRENT LIMITER is a short-circuit protection device that automatically limits the current to a safe value. This is done when the current-limiting transistor senses an increase in load current. At this time the current-limiting transistor decreases the voltage on the base of the pass transistor in the regulator, causing a decrease in its conduction. Therefore, current cannot rise above a safe value.
TROUBLESHOOTING is a method of detecting and repairing problems in electronic equipment. Two methods commonly used are the VISUAL CHECK and SIGNAL TRACING. The visual check allows the technician to make a quick check of component problems, such as shorts, discolored or leaky transformers, loose or broken connections, damaged resistors or capacitors, smoking parts, or sparking. The signal tracing method is used when the technician cannot readily see the problem and needs to use test equipment. Component failure is also important in troubleshooting. In transformers and chokes, a winding can open, or two or more windings can short, either to themselves or to the case that is usually grounded. In a capacitor only two things can occur: either it can short and act as a resistor, or it can open, removing it from the circuit. A resistor can open, increase in value, or decrease in value.

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ANSWERS TO QUESTIONS Q1. THROUGH Q43.

A1. Transformer, rectifier, filter, regulator.

A2. To change ac to pulsating dc.

A3. To change pulsating dc to pure dc.

A4. To maintain a constant voltage to the load.

A5. The half-wave rectifier.

A6. 15.9 volts.

A7. It isolates the chassis from the power line.

A8. The fact that the full-wave rectifier uses the full output, both half cycles, of the transformer.

A9. 120 hertz. A10. 63.7 volts.

A11. Peak voltage is half that of the half-wave rectifier.

A12. The bridge rectifier can produce twice the voltage with the same size transformer.

A13. It will decrease. Capacitance is inversely proportional to:

A14. The capacitor filter.

A15. Parallel.

A16. At a high frequency.

A17. A filter circuit increases the average output voltage.

A18. Value of capacitance and load resistance.

A19. Good.

A20. Yes.

A21. The CEMF of the inductor.

A22. From 1 to 20 henries.

A23. Decrease.

A24. Expense.

A25. When ripple must be held at an absolute minimum.

A26. LC capacitor-input filter.

A27. Cost and size of the inductor.

A28. Regulators.

A29. Variation.

A30. Series and shunt.

A31. An increase.

A32. In parallel.

A33. Bias.

A34. Increases.

A35. Increases.

A36. Decreases.

A37. An increase.

A38. Two.

A39. Trippler.

A40. In parallel.

A41. Safety precautions.

A42. To eliminate shock hazard.

A43. Visual and signal tracing.

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Short Circuit Protection

The main disadvantage of a series regulator is that the pass transistor is in series with the load. If a short develops in the load, a large amount of current will flow in the regulator circuit. The pass transistor can be damaged by this excessive current flow. You could place a fuse in the circuit, but in many cases, the transistor will be damaged before the fuse blows. The best way to protect this circuit is to limit the current automatically to a safe value. A series regulator with a current-limiting circuit is shown in figure 4-50. You should recall that in order for a silicon NPN transistor to conduct, the base must be between 0.6 volt to 0.7 volt more positive than the emitter. Resistor R4 will develop a voltage drop of 0.6 volt when the load current reaches 600 milliamperes. This is illustrated using Ohm’s law:

Figure 4-50. – Series regulator with current limiting.

When load current is below 600 milliamperes, the base-to-emitter voltage on Q2 is not high enough to allow Q2 to conduct. With Q2 cut off, the circuit acts like a series regulator.
When the load current increases above 600 milliamperes, the voltage drop across R4 increases to more than 0.6 volt. This causes Q2 to conduct through resistor R2, thereby decreasing the voltage on the base of pass transistor Q1. This action causes Q1 to conduct less. Therefore, the current cannot increase above 600 to 700 milliamperes.
By increasing the value of R4, you can limit the current to almost any value. For example, a 100-ohm resistor develops a voltage drop of 0.6 volt at 6 milliamperes of current. You may encounter current-limiting circuits that are more sophisticated, but the theory of operation is always the same. If you understand this circuit, you should have no problem with the others.
TROUBLESHOOTING POWER SUPPLIES
Whenever you are working with electricity, the proper use of safety precautions is of the utmost importance to remember. In the front of all electronic technical manuals, you will always find a section on safety precautions. Also posted on each piece of equipment should be a sign listing the specific precautions for that equipment. One area that is sometimes overlooked, and is a hazard especially on board ship, is the method in which equipment is grounded. By grounding the return side of the power transformer to the metal chassis, the load being supplied by the power supply can be wired directly to the metal chassis. Thereby the necessity of wiring directly to the return side of the transformer is eliminated. This method saves wire and reduces the cost of building the equipment, and while it solves one of the problems of the manufacturer, it creates a problem for you, the technician. Unless the chassis is physically grounded to the ship’s ground (the hull), the chassis can be charged (or can float) several hundred volts above ship’s ground. If you come in contact with the metal chassis at the same time you are in contact with the ship’s hull, the current from the chassis can use your body as a low resistance path back to the ship’s ac generators. At best this can be an unpleasant experience; at worst it can be fatal. For this reason Navy electronic equipment is always grounded to the ship’s hull, and approved rubber mats are required in all spaces where electronic equipment is present. Therefore, before starting to work on any electronic or electrical equipment, ALWAYS ENSURE THAT THE EQUIPMENT AND ANY TEST EQUIPMENT YOU ARE USING IS PROPERLY GROUNDED AND THAT THE RUBBER MAT YOU ARE STANDING ON IS IN GOOD CONDITION. As long as you follow these simple rules, you should be able to avoid the possibility of becoming an electrical conductor.

TESTING

There are two widely used checks in testing electronic equipment, VISUAL and SIGNAL TRACING. The importance of the visual check should not be underestimated because many technicians find defects right away simply by looking for them. A visual check does not take long. In fact, you should be able to see the problem readily if it is the type of problem that can be seen. You should learn the following procedure. You could find yourself using it quite often. This procedure is not only for power supplies but also for any type of electronic equipment you may be troubleshooting. (Because diode and transistor testing was covered in chapter 1 and 2 of this module, it will not be discussed at this time. If you have problems in this area, refer to chapter 1 for diodes or chapter 2 for transistors.)

• BEFORE YOU ENERGIZE THE EQUIPMENT, LOOK FOR: SHORTS – Any terminal or connection that is close to the chassis or to any other terminal should be examined for the possibility of a short. A short in any part of the power supply can cause considerable damage. Look for and remove any stray drops of solder, bits of wire, nuts, or screws. It sometimes helps to shake the chassis and listen for any tell-tale rattles. Remember to correct any problem that may cause a short circuit; if it is not causing trouble now, it may cause problems in the future.

DISCOLORED OR LEAKING TRANSFORMER – This is a sure sign that there is a short somewhere. Locate it. If the equipment has a fuse, find out why the fuse did not blow; too large a size may have been installed, or there may be a short across the fuse holder.
LOOSE, BROKEN, OR CORRODED CONNECTION – Any connection that is not in good condition is a trouble spot. If it is not causing trouble now, it will probably cause problems in the future. Fix it.
DAMAGED RESISTORS OR CAPACITORS – A resistor that is discolored or charred has been subjected to an overload. An electrolytic capacitor will show a whitish deposit at the seal around the terminals. Check for a short whenever you notice a damaged resistor or a damaged capacitor. If there is no short, the trouble may be that the power supply has been overloaded in some way. Make a note to replace the part after signal tracing. There is no sense in risking a new part until the trouble has been located.

• ENERGIZE THE EQUIPMENT AND LOOK FOR:

SMOKING PARTS – If any part smokes or if you hear any boiling or sputtering sounds, remove the power immediately. There is a short circuit somewhere that you have missed in your first inspection. Use any ohmmeter to check the part once again. Start in the neighborhood of the smoking part. SPARKING – Tap or shake the chassis. If you see or hear sparking, you have located a loose connection or a short. Check and repair.
If you locate and repair any of the defects listed under the visual check, make a note of what you find and what you do to correct it. It is quite probable you have found the trouble. However, a good technician takes nothing for granted. You must prove to yourself that the equipment is operating properly and that no other troubles exist.
If you find none of the defects listed under the visual check, go ahead with the signal tracing procedure. The trouble is probably of such a nature that it cannot be seen directly-it may only be seen using an oscilloscope.

Tracing the ac signal through the equipment is the most rapid and accurate method of locating a trouble that cannot be found by a visual check, and it also serves as check on any repairs you may have made. The idea is to trace the ac voltage from the transformer, to see it change to pulsating dc at the rectifier output, and then see the pulsations smoothed out by the filter. The point where the signal stops or becomes distorted is the place look for the trouble. If you have no dc output voltage, you should look for an open or a short in your signal tracing. If you have a low dc voltage, you should look for a defective part and keep your eyes open for the place where the signal becomes distorted.
Signal tracing is one method used to localize trouble in a circuit. This is done by observing the waveform at the input and output of each part of a circuit.

Let’s review what each part of a good power supply does to a signal, as shown in figure 4-51. The ac voltage is brought in from the power line by means of the line cord. This voltage is connected to the primary of the transformer through the ON-OFF switch (S1). At the secondary winding of the transformer (points 1 and 2), the scope shows you a picture of the stepped-up voltage developed across each half of the secondary winding-the picture is that of a complete sine wave. Each of the two stepped-up voltages is connected between ground and one of the two anodes of the rectifier diodes. At the two rectifier anodes (points 4 and 5), there is still no change in the shape of the stepped-up voltage-the scope picture still shows a complete sine wave.
Figure 4-51. – Complete power supply (without regulator).

However, when you look at the scope pattern for point 6 (the voltage at the rectifier cathodes), you see the waveshape for pulsating direct current. This pulsating dc is fed through the first choke (L1) and filter capacitor (C1) which remove a large part of the ripple, or "hum," as shown by the waveform for point 7. Finally the dc voltage is fed through the second choke (L2) and filter capacitor (C2), which remove nearly all of the remaining ripple. (See the waveform for point 8, which shows almost no visible ripple.) You now have almost pure dc.

No matter what power supplies you use in the future, they all do the same thing – they change ac voltage into dc voltage.

Component Problems

The following paragraphs will give you an indication of troubles that occur with many different electronic circuit components.
TRANSFORMER AND CHOKE TROUBLES. – As you should know by now, the transformer and the choke are quite similar in construction. Likewise, the basic troubles that they may develop are comparable.

• A winding can open.
• Two or more turns of one winding can short together.
• A winding can short to the casing, which is usually grounded.
• Two windings(primary and secondary) can short together.
• This trouble is possible, of course, only in transformers.

When you have decided which of these four possible troubles could be causing the symptoms, you have definite steps to take. If you surmise that there is an open winding, or windings shorted together or to ground, an ohmmeter continuity check will locate the trouble. If the turns of a winding are shorted together, you may not be able to detect a difference in winding resistance. Therefore, you need to connect a good transformer in the place of the old one and see if the symptoms are eliminated. Keep in mind that transformers are difficult to replace. Make absolutely sure that the trouble is not elsewhere in the circuit before you change the transformer.
Occasionally, the shorts will only appear when the operating voltages are applied to the transformer. In this case you might find the trouble with a megger-an instrument which applies a high voltage as it reads resistance.

CAPACITOR AND RESISTOR TROUBLES. – Just two things can happen to a capacitor:

• It may open up, removing the capacitor completely from the circuit.
• It may develop an internal short circuit. This means that it begins to pass current as though it were a resistor or a direct short.
• 

You may check a capacitor suspected of being open by disconnecting it from the circuit and checking it with a capacitor analyzer. You can check a capacitor suspected of being leaky with an ohmmeter; if it reads less than 500 kilohms, it is more than likely bad. However, capacitor troubles are difficult to find since they may appear intermittently or only under operating voltages. Therefore, the best check for a faulty capacitor is to replace it with one known to be good. If this restores proper operation, the fault was in the capacitor.

Resistor troubles are the simplest. However, like the others, they must be considered.

• A resistor can open.
• A resistor can increase in value.
• A resistor can decrease in value.

You already know how to check possible resistor troubles. Just use an ohmmeter after making sure no parallel circuit is connected across the resistor you wish to measure. When you know a parallel circuit is connected across the resistor or when you are in doubt disconnect one end of the resistor before measuring it. The ohmmeter check will usually be adequate. However, never forget that occasionally intermittent troubles may develop in resistors as well as in any other electronic parts.
Although you may observe problems that have not been covered specifically in this chapter, you should have gained enough knowledge to localize and repair any problem that may occur.

Q.41 What is the most important thing to remember when troubleshooting?

Q.42 What is the main reason for grounding the return side of the transformer to the chassis?

Q.43 What are two types of checks used in troubleshooting power supplies?

Back
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Up
Next

SUMMARY of FILTERS

This chapter has presented you with a basic description of the theory and operation of a basic power supply and its components. The following summary is provided to enhance your understanding of power supplies.
POWER SUPPLIES are electronic circuits designed to convert ac to dc at any desired level. Almost all power supplies are composed of four sections: transformer, rectifier, filter, and regulator.

The POWER TRANSFORMER is the input transformer for the power supply.

The RECTIFIER is the section of the power supply that contains the secondary windings of the power transformer and the rectifier circuit. The rectifier uses the ability of a diode to conduct during one half cycle of ac to convert ac to dc.
HALF-WAVE RECTIFIERS give an output on only one half cycle of the input ac. For this reason, the pulses of dc are separated by a period of one half cycle of zero potential voltage.

FULL-WAVE RECTIFIERS conduct on both halves of the input ac cycles. As a result, the dc pulses are not separated from each other. A characteristic of full-wave rectifiers is the use of a center-tapped, high-voltage secondary. Because of the center tap, the output of the rectifier is limited to one-half of the input voltage of the high-voltage secondary.

BRIDGE RECTIFIERS are full-wave rectifiers that do not use a center-tapped, high-voltage secondary. Because of this, their dc output voltage is equal to the input voltage from the high-voltage secondary of the power transformer. Bridge rectifiers use four diodes connected in a bridge network. Diodes conduct in diagonal pairs to give a full-wave pulsating dc output.

FILTER CIRCUITS are designed to smooth, or filter, the ripple voltage present on the pulsating dc output of the rectifier. This is done by an electrical device that has the ability to store energy and to release the stored energy.
CAPACITANCE FILTERS are nothing more than large capacitors placed across the output of the rectifier section. Because of the large size of the capacitors, fast charge paths, and slow discharge paths, the capacitor will charge to average value, which will keep the pulsating dc output from reaching zero volts.

INDUCTOR FILTERS use an inductor called a choke to filter the pulsating dc input. Because of the impedance offered to circuit current, the output of the filter is at a lower amplitude than the input.

PI-TYPE FILTERS use both capacitive and inductive filters connected in a pi-type configuration. By combining filtering devices, the ability of the pi filter to remove ripple voltage is superior to that of either the capacitance or inductance filter.

VOLTAGE REGULATORS are circuits designed to maintain the output of power supplies at a constant amplitude despite variations of the ac source voltage or changes of the resistance of the load. This is done by creating a voltage divider of a resistive element in the regulator and the resistance of the load. Regulation is achieved by varying the resistance of the resistive element in the regulator.
A SERIES REGULATOR uses a variable resistance in series with the load. Regulation is achieved by varying this resistance either to increase or to decrease the voltage drop across the resistive element of the regulator. Characteristically, the resistance of the variable resistance moves in the same direction as the load. When the resistance of the load increases, the variable resistance of the regulator increases; when load resistance decreases, the variable resistance of the regulator decreases.

SHUNT REGULATORS use a variable resistance placed in parallel with  the load. Regulation is achieved by keeping the resistance of the load constant. Characteristically the resistance of the shunt moves in the opposite direction of the resistance of the load.

The CURRENT LIMITER is a short-circuit protection device that automatically limits the current to a safe value. This is done when the current-limiting transistor senses an increase in load current. At this time the current-limiting transistor decreases the voltage on the base of the pass transistor in the regulator, causing a decrease in its conduction. Therefore, current cannot rise above a safe value.
TROUBLESHOOTING is a method of detecting and repairing problems in electronic equipment. Two methods commonly used are the VISUAL CHECK and SIGNAL TRACING. The visual check allows the technician to make a quick check of component problems, such as shorts, discolored or leaky transformers, loose or broken connections, damaged resistors or capacitors, smoking parts, or sparking. The signal tracing method is used when the technician cannot readily see the problem and needs to use test equipment. Component failure is also important in troubleshooting. In transformers and chokes, a winding can open, or two or more windings can short, either to themselves or to the case that is usually grounded. In a capacitor only two things can occur: either it can short and act as a resistor, or it can open, removing it from the circuit. A resistor can open, increase in value, or decrease in value.

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ANSWERS TO QUESTIONS Q1. THROUGH Q43.

A1. Transformer, rectifier, filter, regulator.

A2. To change ac to pulsating dc.

A3. To change pulsating dc to pure dc.

A4. To maintain a constant voltage to the load.

A5. The half-wave rectifier.

A6. 15.9 volts.

A7. It isolates the chassis from the power line.

A8. The fact that the full-wave rectifier uses the full output, both half cycles, of the transformer.

A9. 120 hertz. A10. 63.7 volts.

A11. Peak voltage is half that of the half-wave rectifier.

A12. The bridge rectifier can produce twice the voltage with the same size transformer.

A13. It will decrease. Capacitance is inversely proportional to:

A14. The capacitor filter.

A15. Parallel.

A16. At a high frequency.

A17. A filter circuit increases the average output voltage.

A18. Value of capacitance and load resistance.

A19. Good.

A20. Yes.

A21. The CEMF of the inductor.

A22. From 1 to 20 henries.

A23. Decrease.

A24. Expense.

A25. When ripple must be held at an absolute minimum.

A26. LC capacitor-input filter.

A27. Cost and size of the inductor.

A28. Regulators.

A29. Variation.

A30. Series and shunt.

A31. An increase.

A32. In parallel.

A33. Bias.

A34. Increases.

A35. Increases.

A36. Decreases.

A37. An increase.

A38. Two.

A39. Trippler.

A40. In parallel.

A41. Safety precautions.

A42. To eliminate shock hazard.

A43. Visual and signal tracing.

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