The Transistor

Objectives

After studying this unit, the student will be able to:

• Discuss the differences between PNP and NPN transistors

• Test transistors with an ohmmeter

• Identify the leads of standard, case-style transistors

• Discuss the operation of a transistor

• Connect a transistor in a circuit

Transistors are made by connecting three pieces of semiconductor material. There are two basic types of transistors: the NPN and the PNP, figure 5-l. The schematic symbols for these tran sistors are shown in figure S-2. These transistors differ in the manner in which they are connected in a circuit. The NPN transistor must have a pos itive voltage connected to the collector and a neg ative voltage connected to the emitter. The PNP must have a positive voltage connected to the emitter and a negative voltage connected to the collector. The base must be connected to the same polarity as the collector to forward bias the tran sistor. Notice that the arrows on the emitters point in the direction of conventional current flow.

An ohmmeter can be used to test a transistor which will appear to the ohmmeter to be two joined diodes, figure S-3. (For an explanation of how to test a transistor, see Procedure 2 in the Appendix.) If the polarity of the output of the ohmmeter leads is known, the transistor can be identified as NPN or PNP. An NPN transistor will appear to an ohmmeter to be two diodes with their anodes connected. If the positive lead of the ohmmeter is connected to the base of the transis tor, a diode junction should be seen between the base-collector and the base-emitter. If the negative lead of the ohmmeter is connected to the base of an NPN transistor, there should be no continuity between the base-collector and the base-emitter junction.

A PNP transistor will appear to an ohmmeter to be two diodes with their cathodes connected. If the negative lead of the ohmmeter is connected to the base of the transistor, a diode junction should be seen between the base-collector and the base emitter. If the positive ohmmeter lead is con nected to the base, there should be no continuity between the base-collector or the base-emitter.

The simplest way to describe the operation of a transistor is to say that it operates like an electric valve. Current will not flow through the collector emitter until current flows through the base-emit ter. The amount of base-emitter current, however, is small when compared to the collector-emitter current, figure S-4. For example, assume that when 1 milliamp of current flows through the base-emitter junction, 100 rnA of current flow through the collector-emitter junction. If this tran sistor is a linear device, an increase or decrease of base current will cause a similar increase or de crease of collector current. Therefore, if the base current is increased to 2 rnA, the collector current will increase to 200 rnA. If the base current is de creased to .5 rnA, the collector current will de crease to SO rnA. Notice that a small change in the amount of base current can cause a large change in the amount of collector current. This permits a small amount of signal current to operate a larger device such as the coil of a control relay.

One of the most common applications of the transistor in industry is that of a switch. When used in this manner, the transistor operates like a digital device instead of an analog device. The term digital refers to a device that has only two states, such as on and off. An analog device can be adjusted to different states. An example of this control can be seen in a simple switch connection. A common wall switch is a digital device. It can be used to turn a light on or off. If the simple toggle switch is replaced with a dimmer control, the light can be turned on, off, or it can be ad justed to any position between on and off. The dimmer is an example of analog control.

If no current flows through the base of the transistor, the transistor acts like an open switch and no current can flow through the collector emitter junction. If enough base current is applied to the transistor to turn it completely on, it acts like a closed switch and permits current to flow through the collector-emitter junction. This is the same action produced by the closing contacts of a relay or motor starter, but, unlike a transistor, a relay or motor starter cannot turn on and off sev eral thousand times a second.

Some case styles of transistors permit the leads to be quickly identified, figures 5-5, 5-6 and 5-7. The TO 5 and TO 18 cases, and the TO 3 case are in this category. The leads of the TO 5 and TO 18 case transistors can be identified by holding the case of the transistor with the leads facing you as shown in figure 5-8A. The metal tab on the case of the transistor is closest to the emitter lead. The base and collector leads are posi tioned as shown.

The leads of a TO 3 case transistor can be identified as shown in figure 5-8B. When the tran sistor is held with the leads facing you and down, the emitter is the left lead and the base is the right lead. The case of the transistor is the collector.