Practical Transistor Audio Amplifiers for the Home Constructor

A transistor has to be supplied with the correct power if it is to
operate efficiently and provide maximum gain with minimum distortion.
Since the parameters of a transistor vary with temperature and tune the
power supply must normally be designed to be self compensating, that is
to say, the circuit must be stabilized. The need for stabilization is
increased by the fact that two transistors of a given type can differ
very consider- ably in their parameters. A transistor may be operated
in any of three modes these being known as common base, common emitter
and common collector. In each case the electrode referred to is common
to both the input and the output. Since the common emitter greatest
power gain and does not involve difficult coupling problems it is used
most frequently. The common base mode is sometimes used, as far as A.F.
amplifiers are concerned, when a high voltage gain is required or when
matching a very low to a high impedance. The common collector mode is
used mainly as a match between a high output impedance and a low input
impedance; it may therefore be used as a matching component between two
common emitter stages.”