Transistor Amplifiers

An electronic system requires amplifiers in order to produce, from low-level signals often of the order of microvolts, signals of sufficient level to drive an output amplifier stage; the output stage in turn produces signal power to drive a transducer of one sort or another. As an example, a television receiver amplifies the weak aerial signal of some 50 to 300 μV and ultimately uses the result to provide sound and picture information. In one case the speaker system associated with the sound channel may require several watts of signal power to provide sufficient sound level and in the other case over 100 V of signal is needed to provide the picture information via the cathode-ray tube. Multiple-stage amplifiers with gains in excess of 105 are therefore normal and the problems associated with such amplifiers are the maintenance of stability and constant gain of the amplifier. In general in the explanations that follow concerning amplifiers npn transistors are considered but identical circuits may be used with pnp types the only difference being that the supply potentials are reversed from those of npn transistors.