Technical Digests---A New High-Efficiency Power Amplifier for ModulatedWaves

* Digest of a paper presented before the Annual Convention of the Institute of Radio Engineers, May 11-13, 1936, at Cleveland, Ohio, and to be published later in the Proceedings. Chambers, Jones, Fyler, Williamson, Leach, and Hutcheson, "The WLW 500Kilowatt Broadcast Transmitter," Proc. I. R. E., Vol. 22, p. 1151, October, 1934. Chireix, "High Power Outphasing Modulation," Proc. I. R. E., Vol. 23, p. 1370, November, 1935. 1 2 469 470 BELL SYSTEM TECHNICAL JOURNAL nearly to the cut-off point so that the plate-current pulse width is approximately a half cycle. Under these conditions the plate efficiency is proportional to the amplitude of the radio-frequency plate voltage. It is possible to obtain large outputs from tubes with radio-frequency plate voltage amplitudes of 0.85 to 0.9 of the applied d-c potential, i.e., with the plate voltage swinging down to a minimum value as low as 10 to 15 per cent of the d-c potential. The corresponding plate efficiency for the tube and its tuned circuit is approximately 67 per cent. This condition, however, prevails only at the peak output of the amplifier, and since the amplitude of the plate voltage wave, in a transmitter capable of 100 per cent modulation, is only half as great for the unmodulated condition as for the peaks of modulation, the efficiency with zero modulation in the conventional amplifier does not exceed half this peak value, or about 33 per cent. Even during complete modulation the effective efficiency over the whole audio cycle is only 50 per cent, and for the average percentage modulation of broadcast programs the all-day efficiency is only slightly greater than the efficiency for unmodulated carrier.