Transistor Circuits and Biasing

Transistor symbols

The standard transistor symbol is as shown here. The direction of the arrow on the emitter indicates the polarity of the transistor (either NPN or PNP) by representing the conventional current direction in this connection.

In some diagrams the surrounding circle is not included.

The terminal names are often abbreviated to "e", "b" and "c" to save space. On most diagrams they are not shown at all - the engineer being expected to identify them from the symbol.


The term "biasing" is used a lot when discussing transistor circuits. It simply means applying the relevant voltages and/or currents to make the transistor work the way we want it to.

The term "forward biased" means applying a voltage/current to a junction (such as a diode or an emitter-base junction of a transistor) with the appropriate polarity for it to conduct. "Reverse biased" means applying the voltage/current the other way around, so that it doesn't conduct.

From the description of the transistor operation above, you will appreciate that the base-emitter junction is like a diode and needs to be forward biased to allow current to flow through the collector-emitter junction.

To bias the base-emitter junction we need to supply sufficient voltage to overcome the forward voltage drop of the material used (0.2V to 0.3V for germanium and 0.6V to 0.7V for silicon). Once the junction is forward biased we should not try to force the voltage higher as this will damage the junction. We need to vary the current flowing through the base-emitter junction, in order to vary the collector-emitter current.

For most radio applications we will want to adjust the base current so that the transistor is part way between fully on (maximum collector current) and fully off (zero collector current). With the transistor biased in this way, we can apply an input signal that will vary the base current either side of the static level, and the collector current will be able to vary either side of its static level (obviously much more than the base current due to the amplification) without approaching the full on or full off state.

In some applications, transistors are used as switches. The base current is either high enough to turn the transistor fully on, or completely absent to turn the transistor fully off. This mode of operation is not used in radio circuits however. The only radio application I can think of is switching on the radio section of a digital clock/radio when the appropriate time is reached - but this technology is a bit too modern for us!

Transistor circuit configurations

There are three basic transistor circuit configurations known as "common emitter", "common base" and "common collector". The name indicates which terminal is connected to a common rail and is therefore the common terminal for signal input or output.

The different configurations have different input and output impedances, voltage and current gains, and phase shift, as summarised below.



Medium Low High
Medium High Low
High Less Than
High High Slightly Less
Than Unity

Common emitter

The most commonly used arrangement is common emitter.

Figure (a) above shows the most basic way of connecting a transistor in common emitter mode. RL is the collector load resistor, and would typically be a few k-ohms in value. R1 is the base bias resistor, the value of which would be set to give about half the supply voltage on the collector. The value of R1 would probably be several hundred k-ohms or a few M-ohms.

When an AC signal is applied to the base via the capacitor, the base current will be varied. This in turn varies the collector by a greater amount, due to the amplification effect of the transistor. This collector current variation is converted to a voltage variation on the collector by RL. We therefore have a simple amplifier stage.

If this circuit was assembled and the value of R1 was chosen by trial-and-error to give the correct voltage on the collector, the circuit would work fine. However if the transistor was replaced with another one of the same type, the voltage at the collector would probably be different due to the natural variation in the current gain and other characteristics of the devices. These characteristics also vary to some extent with temperature. The circuit is therefore fine for a one-off design at a relatively constant temperature, but is not sufficiently repeatable for production.

Figure (b) above shows an enhancement to the circuit. Instead of connecting R1 to the supply, it is connected to the collector. Now if the transistor fitted had a higher current gain, the voltage at the collector would tend to reduce due to the increased current through RL giving a higher voltage drop across RL. A lower voltage is therefore applied to R1, which reduces the base current. This in turn reduces the collector current.

The circuit is therefore self-compensating to some extent. The voltage at the collector of the transistor would still vary with changes in the transistor gain, but nowhere near as much as it would with the circuit in fig (a).

This type of circuit would generally be used for small signal audio amplifier stages, where the smaller resulting signal could appear on the collector without clipping, even if the DC voltage at the collector was some way away from the intended value. When dealing with larger signals that could result in a signal on the collector that is close to the full supply voltage, the circuit in fig (b) would not be adequate.

Fig (c) shows a further enhanced circuit. The voltage on the base of the transistor is set by R1 and R2. The values would be calculated so that Re drops about 10% of the supply voltage and the collector is about 60% of the supply voltage. Actual values obviously depend on the application.

If the transistor used had a higher current gain, a higher current would tend to flow through RL, the transistor and Re. This would increase the voltage dropped across Re. Since the base is held at a steady voltage, the increasing voltage on the emitter would reduce the voltage across the base-emitter junction. This would reduce the base current, resulting in a reduction in the collector current.

When an AC signal is applied to the base via the capacitor, it is amplified as before. However not all of the current variation through the collector-emitter results in voltage variation on the collector. Some of it results in voltage variations on the emitter, due to Re. This in turn affects the base current in exactly the same manner as described above. The result is negative feedback, which reduces the gain of the stage and also reduces the distortion.

Whether this is a good thing or a bad thing depends on the application. For a high quality audio stage it is probably a good thing, because the stage gain is predictable and the distortion is reduced. However for a portable radio the designers generally could not afford to lose any gain, and quality was often a secondary consideration.

To eliminate this negative feedback and realise the full gain from the stage, a suitable value capacitor is connected in parallel with Re. The capacitor would be large enough to hold the voltage at the emitter steady at the lowest frequency to be amplified.

This circuit is able to compensate for variations in transistor current gain and temperature much better than the circuit in fig (b), but at the expense of some additional components.

These waveforms show why we need good compensation when dealing with larger signals. With a smaller signal (a) it does not matter that much if the collector is not at the mid supply point because it can still be amplified properly (b). However with a larger signal (c) it is necessary for the collector voltage to remain close to the mid point otherwise the top or bottom of the signal will be clipped (d) resulting in distortion.

Common collector and common base

These circuit configurations are not used as frequently as the common emitter configuration detailed above, however they do turn up occasionally so it is worth spending a couple of minutes discussing them.

The common base circuit is shown in (a). This type of circuit is used in the front-end of VHF tuners, because it offers the best performance (gain and stability) at high frequencies.

The two central resistors provide the base bias, the resistor and inductors to the right form the collector load, and the resistor and inductor to the left provide the emitter bias. Thus the DC conditions are set up in the same way as fig (c) for the common emitter arrangement. The AC conditions are different however. The base is grounded to AC signals by a capacitor, and the input signal is fed into the emitter.

The common collector circuit is shown in fig (b). This arrangement provides less than unity voltage gain but high current gain. It is therefore useful for delivering current into a load, if the voltage amplification has been done in preceding stages. The voltage on the emitter will be equal to the voltage on the base less the base-emitter forward drop. This arrangement's only appearance in radio circuits is in transformerless class B amplifier stages, which we will discuss later.

Transistor polarity

As mentioned before, there are two different types of transistor - NPN and PNP. The circuits for the two are similar, however the polarity of the power supply for PNP types is the opposite way around to that for NPN types. With NPN types the emitter is negative, whereas with PNP types it is positive. To anyone familiar with valve circuits or more modern transistor electronics, the power supply to PNP transistors seems to be upside down.

This diagram shows the same common-emitter amplifier circuit using an NPN transistor (a) and a PNP transistor (b). The polarity of the battery and electrolytic capacitors are reversed, but otherwise the circuits are identical.

When carrying out voltage measurements with an analogue meter, you will need to connect the negative probe to the ground rail for the NPN circuit and the positive probe to the ground rail for the PNP circuit. A digital meter will just read positive or negative as appropriate.

Nearly all early transistor radios use germanium PNP transistors, and the circuit diagrams in the following sections all use this type.

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Last updated 14th April 2006.