R.F. Class B Amplifiers

Author: J.B. Hoag

In the radio frequency amplifier stages of a transmitter, large amounts of power must be handled by the tubes and circuits. The question of the efficiency of the units becomes of considerable importance. The fact that Class B (and Class C) amplifiers are characterized by greater efficiency than Class A makes their use desirable. A double-ended Class B circuit may be used with two tubes as in Fig. 23 I (but with air-core transformers and with the addition of tuning condensers in the grid and plate circuits). However, it is also possible to use a one-tube or single-ended Class B unit for radio frequency amplification, as in Fig. 23 M, without suffering the large distortion which might be expected from the rectifier action indicated in Fig. 23 J.

Fig. 23 M. If the C-bias voltage is small, this elementary r.f. amplifier is operating as a Class A unit. If it is larger; Class B: If very large: Class C

This is possible because of the resonant property of the tuned plate circuit, which responds to only one of the many harmonics of the complex plate current wave-form. A tank or tuned plate circuit of fairly high Q (10) is required for effective exclusion of all but one frequency (or narrow band of frequencies). The selectivity or sharpness of the resonance curve (or Q) must not be too great for m.c.w. (modulated carrier wave) amplifiers, or the higher-pitched audio notes will be excluded as well as the undesirable harmonics.