Doherty amplifier

The Doherty amplifier is a high-efficiency linear radio-frequency power amplifier originally developed for high-power amplitude-modulated broadcasting. It was introduced by William H. Doherty of Bell Telephone Laboratories in 1936 as a means of maintaining high efficiency over the modulation cycle, particularly in transmitters operating at tens to hundreds of kilowatts, where power consumption and operating cost were significant concerns.

In the classical Doherty configuration, one amplifier tube (carrier) supplies the required output power under unmodulated carrier conditions, while a second tube (peaking) conducts only during positive modulation peaks. Through a coupling network, the load impedance presented to the carrier amplifier is dynamically reduced as the peaking amplifier contributes power. This allows both tubes to operate at high efficiency throughout the modulation cycle and to share the power required for positive modulation peaks. The coupling network functions as a quarter-wave impedance inverter, and the carrier and peaking amplifiers are driven with radio-frequency signals that are ninety degrees out of phase.

Early Western Electric Doherty systems used a modulated driver transmitter, typically 5 kW in 50 kW broadcast installations, with the Doherty amplifier providing the final power stage.

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