Class C Amplifiers

Author: J.B. Hoag

The principle of operation of Class C amplifiers is indicated in Fig. 23 N, where the grid bias is set at approximately twice the cutoff value.

Fig. 23 N. The principle of Class C amplifiers. Plate current flows only during part of one-half cycle

The plate current flows only during a portion of the positive half-cycle of the input signal voltage. The input signal voltage must be sufficiently great to drive the grid positive up to saturation. Then the plate efficiency will be high (70 to 75 per cent), the power output will be large, although the power amplification is low. In this type of amplifier, the plate current is proportional to the plate voltage and hence the output power is proportional to the square of the plate voltage. Class C amplifiers are very efficient because the plate current flows only during part of one half-cycle, but their distortion is very bad if they are used without tuned circuits in the amplification of audio signals. The amplifier is generally used in the amplification of a narrow band of frequencies, say the modulated carrier frequency of a transmitting station, in which case the anti-resonant plate load passes only the fundamental frequency and suppresses the harmonics which arise because of the tube distortion. Class C amplifiers require power in their grid circuit, the more so as the grid goes more positive. This represents a minor loss of energy. Because of their high efficiency, Class C amplifiers are used where a very large power output is the primary consideration, as in the last stages of a transmitter.