The Transistor Amplifier.

Transistors are used in a great variety of circuits. Fortunately, we can divide the ways in which they are used into two fairly simple classes: amplifiers and switches.

Transistors switches form the basis of all modern electronic digital computers. This particular lab doesn't deal with digital electronics. Here we will look at an example of using a bipolar transistor in an amplifier.

Figure 6 illustrates a typical single-transistor amplifier circuit. This arrangement is often called the common emitter amplifier because the input voltage to the transistor appears between the base & emitter, and the output voltage appears between the collector & emitter — i.e. the emitter terminal is shared by (or 'common to') the input and output.

Note. , , and  are the voltages between each of the transistor base, collector, and emitter terminals and the 'ground' (zero volts). They aren't the same thing as  or  which are the voltages from base-to-emitter and collector-to-emitter! The diagram also shows the input and output signal AC voltages,  and . Thesearen't equal to  and  because the 0·1F capacitors block any d.c. connection between these potentials. (If you're puzzled by all this, ask a demonstrator.)

In order to build a working amplifier you have to choose suitable values for resistors, , , , and . For now, assume that  (i.e. it is a piece of wire). We will want to choose a value for  later, but for now we'll worry about everything else.

Anyone who has been confused by reading an electronics textbook will suspect that choosing the 'right' values for the resistors is quite complicated. However, it is possible to select satisfactory values using some simple rules. It is worth bearing in mind again that electronics is a practical subject which shares some things with cookery! (Transistors can get hot, too...) In particular, there are situations (and this is one) where there isn't always a single 'correct' solution for the resistor values you need. It is possible to make a working amplifier using a wide range of resistor values. For a theorist or mathematician this can be depressing — there isn't one 'right' answer. For the rest of us it's good news as it means there are a wide range of values which are 'OK'. It also means that some simple approximations aren't likely to lead to serious problems.

Experience with bipolar transistors has taught engineers that — 9 times out of 10 — a good start is to make just three assumptions and use them as 'rules' unless we know better:—

1. The base-emitter voltage will always be about 0·6 Volts (or 0·6 for a PNP transistor).
2. The current gain (the  value) will be a few hundred.
3. The large  value means that , so we can assume that 

If you look at your transistor's characteristic curves you should see that, although  does depend upon , over most of the measured range it is around 0·6 Volts or so. The  of your transistor will probably be somewhere in the 200 — 600 range. So these approximations are a moderately good place to start in the absence of any better information.

The resistors in the amplifier circuit will determine the steady bias voltages and currents, , , etc. The capacitors in the circuit are used to control the effects of a.c. signals. Start off by ignoring the capacitors as they don't affect the way the actual transistor operates. We can therefore work out all the resistor values, etc, without bothering about them.

There are various ways to decide what values to choose for the bias resistors. They all give roughly similar results, and the following simple argument is about as good as any other.

For the circuit to work as an amplifier we need to make the collector voltage, , move up and down in response to any input signal variations. These changes in collector voltage are coupled out through the capacitor to provide the output voltage signals, . This means that — in the absence of any input signal — the transistor should have a 'moderate' set of applied bias voltages/currents to give  'room' to move up and down under the influence of any input.

The circuit is driven by a +15V power line and the collector-emitter voltage is applied via the two series resistors,  & . In the absence of any good reason for making some other choice we might just as well assume that the available voltage should be shared equally between , , and the transistor. We therefore want about 5 volts across , 5 volts across , and 5 volts between the collector and emitter. This means that the amplifier should have,  V,  V, and  V.