The Operational Amplifier:Circuit Applications

Circuit Applications

The Level Detector

The operational ampliﬁer is often used as a level de- tector or comparator. In this type of circuit, the 741 op amp is used as an inverted ampliﬁer to detect when one voltage becomes greater than another (Figure 65– 10). This circuit does not use above and below ground power supplies. Instead, it is connected to a power supply that has a single positive and negative output.

During normal operation, the noninverting input of the ampliﬁer is connected to a zener diode that produces a constant positive voltage at the noninverting input of the ampliﬁer. This constant positive voltage is used as a reference. As long as the noninverting input is more positive than the inverting input, the output of the ampliﬁer is high.

A light-emitting diode (LED), D1, is used to detect a change in the polarity of the output. As long as the output of the op amp is high, the LED is turned off. When the output of the ampliﬁer is high, the LED has equal voltage applied to its anode and cathode. Since both the anode and cathode are connected to +12 volts, there is no potential difference and, therefore, no cur- rent ﬂow through the LED.

If the voltage at the inverting input becomes more positive than the reference voltage applied to pin #3, the output voltage will fall to about +2.5 volts. The out- put voltage of the op amp will not fall to 0 or ground in this circuit because the op amp is not connected to a voltage that is below ground. To enable the output voltage to fall to 0 volts, pin #4 must be connected to a voltage below ground. When the output drops, a poten- tial of about 9.5 volts (12 – 2.5 = 9.5) is produced across R1 and D1. The lowering of potential causes the LED to turn on, which indicates that the op amp’s output has changed from high to low.

In this type of circuit, the op amp appears to be a digital device in that the output seems to have only two states, high and low. But, the op amp is not a digital device. This circuit only makes it appear to be digital. In Figure 65– 10, there is no negative feedback loop con- nected between the output and the inverting input. Therefore, the ampliﬁer uses its open loop gain, which is about 200,000 for the 741, to amplify the voltage dif- ference between the inverting input and the noninvert- ing input. If the voltage applied to the inverting input becomes 1 millivolt more positive than the reference voltage applied to the noninverting input, the ampliﬁer will try to produce an output that is 200 volts more neg- ative than its high state voltage (0.001 X 200,000 = 200). The output voltage of the ampliﬁer cannot be driven 200 volts more negative, though, because only 12 volts are applied to the circuit. Therefore, the output voltage reaches the lowest voltage it can and goes into satura- tion. This causes the op amp to act like a digital device.

If the zener diode is replaced with a voltage di- vider as shown in Figure 65– 11, the reference voltage can be set to any value by adjusting the variable resis- tor. For example, if the voltage at the noninverting input is set for 3 volts, the output of the op amp will go low when the voltage applied to the inverting input becomes greater than +3 volts. If the voltage at the

noninverting input is set for 8 volts, the output voltage will go low when the voltage applied to the inverting input becomes greater than +8 volts. In this circuit, the output of the op amp can be manipulated through the adjustment of the noninverting input.

In the two circuits just described, the op amp’s out- put shifted from a high level to a low level. There may be occasions, however, when the output must be changed from a low level to a high level. This can be accom- plished by connecting the inverting input to the refer- ence voltage, and the noninverting input to the voltage being sensed (Figure 65– 12). In this circuit, the zener diode is used to supply a positive reference voltage to the inverting input. As long as the voltage at the inverting input is more positive than the voltage at the non- inverting input, the output voltage of the op amp will be low. If the voltage applied to the noninverting input be- comes more positive than the reference voltage, the out- put of the op amp will become high.

Depending on the application, this circuit could cause a small problem. As stated previously, since this circuit does not use an above and below ground power supply, the low output voltage of the op amp is about +2.5 volts. This positive output voltage could cause any other devices connected to the op amp’s output to be on when they should be off. For instance, if the LED shown in Figure 65– 12 is used, it will glow dimly even when the output is in the low state.

One way to correct this problem is to connect the op amp to an above and below ground power supply as shown in Figure 65– 13. In this circuit, the output volt- age of the op amp is negative or below ground as long as the voltage applied to the inverting input is more positive than the voltage applied to the noninverting input. When the output voltage of the op amp is negative with respect to ground, the LED is reverse biased and cannot operate. If the voltage applied to the noninverting input becomes more positive than the voltage applied to the inverting input, the output of the op amp will become positive and the LED will turn on.

Another method of correcting the output voltage problem is shown in Figure 65– 14. In this circuit, the op amp is connected again to a power supply that has a single positive and negative output. A zener diode, D2, is connected in series with the output of the op amp and the LED. The voltage value of diode D2 is greater than the output voltage of the op amp in its low state, but less than the output voltage of the op amp in its high state. For instance, assume that the value of zener diode D2 is 5.1 volts. If the output voltage of the op amp in its low state is 2.5 volts, diode D2 will not conduct. If the output voltage becomes +12 volts when the op amp

switches to its high state, diode D2 will turn on and con- duct current to the LED. The zener diode, D2, keeps the LED completely off until the op amp switches to its high state, providing enough voltage to overcome the reverse voltage drop of the zener diode.

In the preceding circuits, an LED was used to indicate the output state of the ampliﬁer. Keep in mind that the LED is used only as a detector, while the output of the op amp can be used to control almost anything. For example, the output of the op amp can be connected to the base of a transistor as shown in Figure 65– 15. The transistor can then control the coil of a relay which could, in turn, control almost anything.

The Oscillator

The operational ampliﬁer can be used as an oscillator. The simple circuit shown in Figure 65– 16 produces a square wave output. However, this circuit is impractical because it depends on a slight imbalance in the op amp, or random circuit noise, to start the oscillator. A voltage difference of a few millivolts be- tween the two inputs is all that is needed to raise or lower the output of the ampliﬁer. For example, if the inverting input becomes slightly more positive than the noninverting input, the output will go low or become negative. When the output is negative, capacitor CT

charges through resistor RT to the negative value of the output voltage. When the voltage applied to the inverting input becomes slightly more negative than the voltage applied to the noninverting input, the output changes to a high, or positive, value of voltage. When the output is positive, capacitor CT charges through resistor RT toward the positive output voltage.

This circuit would work well if there were no imbalance in the op amp and if the op amp were shielded from all electrical noise. In practical application, however, there is generally enough imbalance in the ampliﬁer or enough electrical noise to send the op amp into saturation, which stops the operation of the circuit.

The problem with this circuit is that a millivolt difference between the two inputs is enough to drive the ampliﬁer’s output from one state to the other. This problem can be corrected by the addition of a hysteresis loop connected to the noninverting input as shown in Figure 65– 17. Resistors R1 and R2 form a voltage divider for the noninverting input. These resistors gen- erally have equal value. To understand the circuit oper- ation, assume that the inverting input is slightly more positive than the noninverting input. This causes the output voltage to be negative. Also assume that the out- put voltage is -12 volts as compared to ground. If re- sistors R1 and R2 have equal value, the noninverting input is driven to -6 volts by the voltage divider. Ca- pacitor CT begins to charge through resistor RT to the value of the output voltage. When capacitor CT has been charged to a value slightly more negative than the -6 volts applied to the noninverting input, the op amp’s output rises to +12 volts above ground. When the output of the op amp changes from -12 volts to +12 volts, the voltage applied to the noninverting input changes from -6 volts to +6 volts. Capacitor CT now begins to charge through resistor RT to the positive voltage of the output. When the voltage applied to the inverting input becomes more positive than the voltage applied to the noninverting input, the output changes to -12 volts. The voltage applied to the noninverting input is driven from +6 volts to -6 volts, and capacitor CT again be- gins to charge toward the negative output voltage of the op amp.

The addition of the hysteresis loop has greatly changed the operation of the circuit. The voltage dif- ferential between the two inputs is now volts instead of millivolts. The output frequency of the oscillator is de- termined by the values of CT and RT. The period of one cycle can be computed by using the formula T = 2RC.

The Pulse Generator

The operational ampliﬁer can be used as a pulse generator. The difference between an oscillator and a pulse generator is the period of time the output is on compared to the period of time it is low or off. For in- stance, an oscillator is generally considered to produce a waveform that has positive and negative pulses of equal voltage and time (Figure 65– 18). The positive value of voltage is the same as the negative value, and the positive and negative cycles are turned on for the same amount of time. This waveform is produced when an oscilloscope is connected to the output of a square wave oscillator.

If the oscilloscope is connected to a pulse generator, however, a waveform similar to the one shown in Figure 65– 19 will be produced. The positive value of volt- age is the same as the negative value, just as it was in Figure 65– 18, but the positive pulse is of a much shorter duration than the negative pulse.

The 741 operational ampliﬁer can easily be changed from a square wave oscillator to a pulse generator (Figure 65– 20). The pulse generator circuit is the same basic circuit as the square wave oscillator with the addition of resistors R3 and R4, and diodes D1 and D2. This circuit permits capacitor CT to charge at a

different rate when the output is high, or positive, than when the output is low, or negative. For instance, assume that the voltage of the op amp’s output is -12 volts. When the output voltage is negative, diode D1 is reverse biased and no current can ﬂow through resistor R3. Therefore, capacitor CT must charge through resistor R4 and diode D2, which is forward biased. When the voltage applied to the inverting input becomes more negative than the voltage applied to the noninverting input, the output voltage of the op amp rises to +12 volts. When the output voltage is +12 volts, diode D2 is reverse biased and diode D1 is forward biased. Therefore, capacitor CT begins charging to- ward the +12 volts through resistor R3 and diode D1. The amount of time the output of the op amp is low is determined by the value of CT and R4, and the amount of time the output remains high is determined by the value of CT and R3. The ratio of the amount of time the output voltage is high to the amount of time it is low can be determined by the ratio of resistor R3 to resistor R4. A typical 741 operational ampliﬁer is shown in Figure 65– 21.

Review Questions

1. When the voltage connected to the inverting input is more positive than the voltage connected to the noninverting input, will the output be positive or negative?

2. What is the input impedance of a 741 operational ampliﬁer?

3. What is the average open loop gain of the 741 operational ampliﬁer?

4. What is the average output impedance of the 741 operational ampliﬁer?

5. Operational ampliﬁers are commonly used in what three connections?

6. When the operational ampliﬁer is connected as a voltage follower, it has a gain of 1 (one). If the input voltage is not ampliﬁed, what is?

7. Name two effects of negative feedback.

8. Refer to Figure 65– 8. If resistor R1 is 200 ohms and resistor R2 is 10 kilohms, what is the gain of the ampliﬁer?

9. Refer to Figure 65– 9. If resistor R1 is 470 ohms and resistor R2 is 47 kilohms, what is the gain of the ampliﬁer?

10. What is the purpose of the hysteresis loop when the op amp is used as an oscillator?