Common collector amplifier (emitter follower)

Common collector amplifier (emitter follower)

The second most popular transistor amplifier stage is the common collector arrangement. The input signal is applied to the base while output is taken from the emitter. A practical version is shown in (a).

Resistors Rb" Rb2 and Re specify the DC operating conditions as before, and give consistent performance with variation in transistor characteristics.

Because the transistor is always conducting, there is approxi­ mately 0.6 V between emitter and base. The output signal is almost identical to the input signal in both amplitude and phase. Because the emitter closely follows any applied voltage to the base, the circuit is sometimes known as an emitter follower.

The emitter follower has a very low output impedance, typically

less than 100 Q, and an input impedance given by:

Input impedance can be very high, typically 100 kQ, although it is lowered to some extent by the effect of resistors Rbl and Rb2.

With high input impedance, low output impedance and unity gain the emitter follower makes a useful buffer stage.

Emitter followers can exhibit some peculiar effects. It is quite common for an emitter follower to burst into high frequency oscillations. The cure is to insert a low value (less than 100 Q) resistor into the base lead or connect a small (about 22 pF) capacitor from collector to base, local to the transistor, as shown in (b).

Another problem occurs where an emitter follower is capacitively coupled to a load in the manner of (c). If RL is less than Re and the DC bias determined by Rbl and Rb2 is only slightly greater than the signal amplitude, clipping of negative portions of the signal may occur.

This is purely a question of design, the criteria being that

Common base amplifier

The final configuration is shown below. The input signal is applied

to the emitter, while the output signal is taken from the collector. The input impedance is very low, typically 15 Q, whereas the output impedance is high, usually in excess of I MQ.

To a certain extent the output looks like a constant current source. These factors tend to limit the usefulness of a common base amplifier. The circuit, however, gives voltage amplification and is sometimes used with low impedance sources such as microphones, transducers and aerials.

The general arrangement of a common base amplifier is shown.

DC conditions are set by the base resistors Rbl and Rb2• The base is decoupled to AC signals by capacitor C1• The common base circuit is less prone to stabilisation problems, as the leakage current lebo is usually only a few microamperes.