A Study of Class B and C Amplifier Tank Circuits

An investigation was made of the relations between the constants L, C, and R of a power amplifier output tank circuit. Experimental data were taken on an amplifier excited at a frequency of 480 cycles per second. An indication of the harmonic content of the output was obtained by means of a cathode ray oscillograph. The results of the experimental data show that the output power and voltage and the efficiency depend only on the output circuit impedance and are independent of the L/C ratio. However, for a given output, the second harmonic in the output voltage increases with the L/C ratio. It is shown both experimentally and theoretically that the per cent of second harmonic in the output is a function of the volt-ampere-to-watt ratio existing in the circuit. With a very large value of this ratio, the tank circuit will have a large flywheel effect so that the current amplitude will not follow the modulation closely at the higher modulating frequencies. A formula for calculating this effect is developed. It is demonstrated that, for class B and C amplifiers the tube cannot be considered as a generator having a fixed internal resistance and generated voltage with any degree of exactness.