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. 

Darlington transistor

Circuit diagram of a Darlington pair using NPN transistors

In electronics, the Darlington transistor (often called a Darlington pair) is a compound structure consisting of two bipolar transistors (either integrated or separated devices) connected in such a way that the current amplified by the first transistor is amplified further by the second one. This configuration gives a much higher current gain (written β, h_fe, or h_FE) than each transistor taken separately and, in the case of integrated devices, can take less space than two individual transistors because they can use a shared collector. Integrated Darlington pairs come packaged singly in transistor-like packages or as an array of devices (usually eight) in an integrated circuit.

The Darlington configuration was invented by Bell Laboratories engineer Sidney Darlington in 1953. He patented the idea of having two or three transistors on a single chip, sharing a collector.

A similar configuration but with transistors of opposite type (NPN and PNP) is the Sziklai pair, sometimes called the "complementary Darlington."

Behavior

A Darlington pair behaves like a single transistor with a high current gain (approximately the product of the gains of the two transistors). In fact, integrated devices have three leads (B, C and E), broadly equivalent to those of a standard transistor.

A general relation between the compound current gain and the individual gains is given by:

If β₁ and β₂ are high enough (hundreds), this relation can be approximated with:

A typical modern device has a current gain of 1000 or more, so that only a small base current is needed to make the pair switch on. However, this high current gain comes with several drawbacks.

One drawback is an approximate doubling of base-emitter voltage. Since there are two junctions between the base and emitter of the Darlington transistor, the equivalent base-emitter voltage is the sum of both base-emitter voltages:

For silicon-based technology, where each V_BEi is about 0.65 V when the device is operating in the active or saturated region, the necessary base-emitter voltage of the pair is 1.3 V.

Another drawback of the Darlington pair is its increased saturation voltage. The output transistor is not allowed to saturate (i.e. its base-collector junction must remain reverse-biased) because its collector-emitter voltage is now equal to the sum of its own base-emitter voltage and the collector-emitter voltage of the first transistor, both positive quantities in normal operation. (In symbols, V_CE2 = V_BE2 + V_CE1, so V_C2 > V_B2 always.) Thus the saturation voltage of a Darlington transistor is one V_BE (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon. For equal collector currents, this drawback translates to an increase in the dissipated power for the Darlington transistor over a single transistor.

Another problem is a reduction in switching speed, because the first transistor cannot actively inhibit the base current of the second one, making the device slow to switch off. To alleviate this, the second transistor often has a resistor of a few hundred ohms connected between its base and emitter terminals.

This resistor provides a low impedance discharge path for the charge accumulated on the base-emitter junction, allowing a faster transistor turn-off.

The Darlington pair has more phase shift at high frequencies than a single transistor and hence can more easily become unstable with negative feedback (i.e., systems that use this configuration can have poor phase margin due to the extra transistor delay).

Darlington pairs are available as integrated packages or can be made from two discrete transistors; Q₁ (the left-hand transistor in the diagram) can be a low power type, but normally Q₂ (on the right) will need to be high power. The maximum collector current I_C(max) of the pair is that of Q₂. A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at I_C=10A.

A Darlington pair can be sensitive enough to respond to the current passed by skin contact even at safe voltages. Thus it can form the input stage of a touch-sensitive switch.