An experimental 5-kW 'doherty' amplifier

The Doherty system combines two methods for increasing efficiency compared with the class B amplifier: (1) lowering the impedance into which the amplifier works during the positive peaks, and (2) supplying additional power for the positive peaks. A second valve is biassed to supply power only during the positive peaks; this power is fed into the output load, and this lowers the impedance (of a quarter-wave network) into which the first valve works. This network introduces a 90deg phase shift; this is compensated by inserting another quarter-wave network between the grids of the valves. Details of the circuit and the method of adjustment are given. The procedure outlined yields linear operation of the whole transmitter, and not only of the Doherty amplifier; in fact by correct adjustment of the latter it is possible to balance out non-linear operation in an earlier stage. The adjustment is not critical. Application of negative feedback (from a rectifier on the output), to the extent of 12 db reduced the harmonic content from 7% to 2%. Performance graphs are included.