The Doherty amplifier has been developed by William H. Doherty at the Bell Telephone Laboratories in 1936.
The Doherty principle is based on the variation of the load line through an active load-pull principle. Two amplifiers drive while the same load resistance RL. While the main amplifier always operates, the peak amplifier is switched on only at high powers. Peak amplifier changes in the ON state, the load impedance of the main amplifier so that it always operates at maximum output power, and thus high efficiency.
The increase in efficiency depends on the point at which the peak amplifier turns on and the type of control of the tip amplifier. Usually this works in class-C operation and shuts down about 6 dB below the maximum power of the overall amplifier .