Untuned Power Amplifiers

The voltage amplifiers discussed in Chaps. 5 and 6 are designed to increase a voltage signal from a low level to one which is adequate for operating some low-power circuit. Such amplifiers are generally operated in class A since the amplification is to be accomplished without distortion.

Power Amp Tube Schematic
Pig. 9-1. The schematic and equivalent circuits of a simple series-fed power amplifier.

A power amplifier serves to supply an appreciable amount of power to some power-absorbing circuit, although in general it must be accomplished under very low grid driving-power demands. Power amplifiers may be operated as class A, B, or C or at any point between these limits, the choice of operating conditions being determined by the ultimate purpose of the amplifier. If the amplifier is to reproduce the audio spectrum without distortion, then the amplifier must be operated in class A if a single tube is used. If two tubes are used in a push-pull circuit, then the amplifier may be operated in class AB or class B. If the amplifier is to reproduce the input wave shape over a very narrow range of frequencies, tuned class B or tuned class C amplifiers may be used. Only a-f amplifiers will be considered in this chapter.

9-1. Class A Triode Power Amplifiers. The basic schematic diagram of a typical series-fed power amplifier and its equivalent circuit are given in Fig. 9-1. It is observed that this circuit is identical with that of Fig. 5-1 for the simple voltage amplifier.

If it is assumed that the dynamic curve is linear over the entire range of operation, then the plate current is given by

and the power supplied to the load is

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