Transistor power amplifiers with switched mode of operation

A CONVENTIONAL circuit for class B power amplification with transistors is shown in Fig. 1(A). The two transistors (p-n-p) are connected in the common emitter arrangement and in the absence of signal are quiescent at a point on the output characteristics such as P in Fig. 1(B). When a sinusoidal voltage signal is applied as base drive, the transistors operate on a load line such as PQ in alternate half-cycles, and a sinusoidal output is obtained. At any intermediate point on the load line PQ the active transistor sustains a collector-emitter voltage equal to the difference between the battery voltage and the instantaneous output voltage. The product of this collector-emitter voltage and the current flowing gives the instantaneous power loss in the transistor (excluding the small dissipation associated with the base drive). Simple calculations based on these considerations show that the maximum possible efficiency of this arrangement is 78 per cent and that maximum power dissipation occurs in the transistor when operating with an output that is 64 per cent of the possible full output voltage.