Transistors may be small but they are an important part on an electrical circuit. David Herres explains.

We have looked at diodes – how they work and how to fix them should they fail.

Two semiconductor layers, usually silicon, are doped with minute amounts of impurities. The N layer is exposed to gallium arsenide and the P layer is exposed to Boron. The layers, each with a wire lead, are bonded and that junction is the place of interest.

The N-type silicon has excess negatively charged electrons. The P-type silicon has excess positively charged holes. (A hole is a place where an electron is missing.) The electrons and holes are charge carriers and, when an external electrical bias is applied to the two leads, these charge carriers migrate toward or away from the junction, depending upon the polarity of the external applied voltage. The voltage can be DC or an AC sine or square wave, or a fluctuating signal corresponding to music from a CD or a human voice.

When the charge carriers are repelled by the external bias, they crowd toward the junction and the diode conducts. When they are attracted by the external bias they move away from the junction and the diode does not conduct. That’s how a diode works.

Transistors operate according to the same principles, but it is somewhat more complex because there are three semiconducting layers instead of two. There are three leads, one connected to each layer.

Because one wire is common, these three leads comprise a two-wire input and a two-wire output (Figure 1).

For both types of bipolar transistor, the centre region is called the base. Looking at the figure you will notice that one lead is depicted as a line with an arrow. This is always the emitter, so it is not necessary to label the three leads, the base being self-evident. If the arrow points in toward the base, it is a PNP transistor. If it points out, the transistor is NPN. Both transistors work the same. Only the polarities are reversed. All current flows, inside and outside of the transistor, are also the opposite.

Transistors have two principle applications. They may be configured as analogue amplifiers or digital switching devices.

In the amplifier mode, small changes in the input circuit result in large changes in the output circuit. This does not violate the principle of conserving energy because, for the transistor to work, it must be connected to an outside power source in addition to the signal that is to be amplified.

The input, constant or changing, consists of a bias on one junction, and a larger DC voltage in series with the transistor output makes for bias at the other junction. When both junctions are forward biased at the same time, the transistor conducts and the output is greater than the input. The process is known as amplification.

If the transistor circuit is configured as a common emitter, the base and the emitter are the control circuit. A small current controls the larger output at the collector-emitter circuit. For both PNP and NPN devices, the arrow points in the direction opposite to the flow of electrons. By observing the direction of the arrow, you can ascertain the type of transistor and the terminations.

In addition to the common-emitter setup, common-collector and common-base configurations are possible. These details are revealed by looking at the chassis or the schematic. The only problem you may encounter is that there is no universal standard for colour-coding, marking or positioning the three leads. It would seem that the middle lead of a bipolar transistor should be connected to the base, but don’t count on it. Manufacturers differ in their designs so, to find out what is going on, meter tests are necessary.

There are several ways to test a transistor. The ordinary multimeter in ‘Ohm’ mode is frequently used. But it has limitations compared to the in-circuit tester, service-type transistor tester or laboratory-standard transistor analyser.

Service-type instruments measure forward current gain, a parameter that is known as ‘Beta’. Additionally, it reads base-to-collector leakage current with no voltage at the emitter. Some of these instruments identify emitter, collector, and base leads. The most advanced of these instruments, the laboratory-standard transistor analyser, simulates actual in-circuit conditions. It applies voltage, current and signal inputs and displays the output.

Because of their rugged packaging, transistors are far more reliable than their vacuum tube ancestors. A great many transistors have been in continuous use for over 50 years. If failure occurs, it is usually because excessive heat or voltage has been applied. Static electricity and line surges can destroy these devices. Heat results from high ambient temperature, a nearby component that is overheating, failure of a ventilating fan or poor heat-sinking.

An ohmmeter can be used to test a transistor as well as a diode. The meter’s DC power supply is capable of biasing the semiconductor junction. Either of the two junctions can be biased and the resistance can be read. This ohm value is not a real resistance parameter of the transistor. It is merely indicative as to whether the charge carriers have crowded close to the two sides of the junction so that it will conduct in response to the amount of bias provided by the ohmmeter.

In order to test a transistor using a multimeter, it is necessary first to ascertain the polarity of the probes. Meter manufacturers are not totally consistent. All meters have a red probe and a black probe, but which one is positive? To determine, find a known good diode with the anode marked. Hook up the diode so that your meter indicates that the diode is conducting. The probe that is connected to the cathode will be positive, because it is repelling the holes and forcing them toward the junction. The electrons are similarly repelled by the other probe and, since the two charge carriers have congregated on either side of the junction, the diode will conduct. Now that you know which probe is positive, permanently mark it red so that you will be able to test transistors using your multimeter.

Conceptually, a transistor consists of two diodes having either the two anodes or the two cathodes connected. In this sense, they may be conceived as being connected back to back. This configuration has three terminals, the two free ends and the connection, corresponding to the three leads of a transistor. It is to be emphasised, however, that the two diodes connected in this way comprise a conceptual model that will serve to determine the ohmmeter hookups used to test a transistor. The two-diode construction will never work as a functioning transistor.

Lacking the schematic and the manufacturer’s data sheet, you may not know whether the component is NPN or PNP, and the identity of the three leads may not be apparent. Still, it is possible to take readings with the multimeter in ohms mode, and gather some information. The meter will bias and measure continuity of each pair of leads.

Because each of the three pairs can be either forward- or reverse-biased there are six possible measurements that can be taken and these measurements provide the only information that can be acquired by means of a multimeter.

Since the two imaginary diodes are pointing in opposite directions, the collector and emitter of a good transistor will read open regardless of which way they are biased. (This assumes that avalanche voltage, where a reverse biased device breaks down and conducts, has not been reached.)

If all three pairs, regardless of biasing, read low resistance, the transistor is shorted and should be discarded. If all three pairs, regardless of biasing, read high resistance, the transistor is open and is also defective.

If the transistor does not fall into either of the above categories, then so far it is good. The collector-emitter pair has been identified and the remaining lead is connected to the base. When forward-biased, the base will conduct to either of the other leads. When reverse-biased, it will be open. The biasing for each of these two readings is opposite from the other. For an emitter-base junction to conduct, the anode has to be connected to the base if it is a PNP transistor. Keeping this in mind, if you know whether it is an NPN or PNP device, you can ascertain which lead is the emitter and which is the collector. If you can identify one lead you can identify the other, since the base lead is known. From this, you can decide whether the transistor is PNP or NPN.

You cannot identify the leads, other than the base, unless you know the transistor type, and you cannot determine the transistor type unless you know that one of the non-base leads is either emitter or collector. Even though complete information may not be available, the simple multimeter tests outlined above will provide a good idea whether or not the transistor is good.

A transistor is capable of amplifying a signal only to a certain point, whereupon it becomes saturated. Then, there will be no further amplification even if the biasing is increased. For an audio or any other analogue application, the transistor is operated at biasing levels that will not allow it to become saturated, which would mean a distorted signal. Digital circuits, on the other hand, involve transistors that are driven into the saturation state when they are switched on, i.e. in the high state. In the low state, the transistor in a digital circuit is biased so as not to conduct. The high and low states represent the binary numbers one and zero.

To evaluate a bipolar transistor, the two key metrics are known as Alpha and Beta. Alpha is the collector current divided by the emitter current with base at signal ground. This is known as the dynamic gain of the transistor. Beta is similar, but the transistor is wired differently. It is defined as collector current divided by base current when the emitter is at signal ground. The two relevant formulas are:
•    Alpha = Beta/(1+Beta)
•    Beta = Alpha/(1-Alpha)
Alpha and Beta are given in the manufacturer’s data sheets and are important considerations for determining the correct transistor for a specific application as well as for choosing a replacement when the original is not available.

Amplifiers use transistors in any one of three possible circuit configurations: common-emitter, common-collector and common-base. When high gain is desired, the common-emitter configuration is often used. The output is 180 degrees out of phase with the input but this does not present a problem, as long as it is taken into account. An even number of successive stages will provide an output that is in phase with the input. The common-emitter is operated below saturation when excellent fidelity is required.

The common-base configuration also provides amplification, but output is in phase with input.

The emitter follower is a name used for the common-collector hookup. Here the collector is at signal ground. There is no amplification. The purpose of the circuit is to provide isolation between stages, so it is known as a buffer. Input and output are in phase. In fact the only configuration with an inverted output is the common-emitter.

When electronic equipment malfunctions, it is often the electrician who is asked to make an initial assessment. It is worthwhile to become proficient in these repairs. Information on electronic theory is readily available on the internet and in print and, coupled with some practical experience, you can become adept.

Transistors can be easily changed out. In a printed circuit board or point-to-point wiring, check for burnt components and put your multimeter to work. When soldering and desoldering, be sure to prevent temperature rise so that these sensitive semiconductors do not get damaged. Use smooth-jawed needle nose pliers, held closed by means of a rubber band, to intercept heat so it doesn’t travel along the lead from the solder joint to the transistor.