The Doherty amplifier is a modified class Bradio frequency amplifier invented by William H. Doherty of Bell Telephone Laboratories Inc in 1936. Whereas conventional Class-B amplifiers can clip on high input-signal levels, the Doherty power amplifier can accommodate signals with high peak-to-average power ratios by using two amplifier circuits within the one overall amplifier to accommodate the different signal levels. In this way, the amplifier achieves a high level of linearity while retaining good power efficiency. In Doherty's day, within the Western Electricproduct line, the eponymous electronic device was operated as a linear amplifier with a driver which was modulated. In the 50,000-watt implementation, the driver was a complete 5,000-watt transmitter which could, if necessary, be operated independently of the Doherty amplifier and the Doherty amplifier was used to raise the 5,000-watt level to the required 50,000-watt level. The amplifier was usually configured as a grounded-cathode, carrier–peak amplifier using two vacuum tubes in parallel connection, one as a class B carrier tube and the other as a class B peak tube. The tubes' source and load were split and combined through +90 and −90 degree phase shifting networks. Alternate configurations included a grounded-grid carrier tube and a grounded-cathode peak tube whereby the driver power was effectively passed-through the carrier tube and was added to the resulting output power, but this benefit was more appropriate for the earlier and less efficient triode implementations rather than the later and more efficient tetrode implementations.
Successor developments
As successor to Western Electric Company, Inc. for radio broadcast transmitters, the Doherty concept was considerably refined by Continental Electronics Manufacturing Company of Dallas, Texas. Early Continental Electronics designs, by James O. Weldon and others, retained most of the characteristics of Doherty's amplifier but added medium-level screen-grid modulation of the driver. The ultimate refinement was the high-level screen-grid modulation scheme invented by Joseph B. Sainton. Sainton's 317C series consisted of a class C carrier tube in parallel connection with a class C peak tube. The tubes' source and load were split and combined through +90 and −90 degree phase-shifting networks as in Doherty's amplifier. The unmodulated radio frequency carrier is applied to the control grids of both tubes, and with the same control grid bias points. Carrier modulation is applied to the screen grids of both tubes, but the screen grid bias points of the carrier and peak tubes are different and are established such that the peak tube is cut off when modulation is absent and the amplifier is producing rated unmodulated carrier power and both tubes are conducting and each tube is contributing twice the rated carrier power during 100% modulation as four times the rated carrier power is required to achieve 100% modulation. As both tubes are operated in class C, a significant improvement in efficiency is thereby achieved in the final stage. In addition, as the tetrode carrier and peak tubes require very little drive power, a significant improvement in efficiency within the driver is achieved as well. The commercial version of the Sainton amplifier employed a cathode-follower modulator, not the push-pull modulator disclosed in the patent, and the entire 50,000-watt transmitter was implemented using only nine total tubes of four tube types, all of these being general-purpose, a remarkable achievement, given that the 317C's most significant competitor, RCA's BTA-50G, was implemented using thirty-two total tubes of nine tube types, nearly one-half of these being special-purpose, employed only in the BTA-50G. Nearly 300 CE 317C transmitters were installed in North America alone, easily outdistancing all competitors combined.
