How does a Doherty amplifier work?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

The Doherty amplifier, invented by William H. Doherty at Bell Labs in 1936, combines a main (carrier) amplifier biased in class AB or B and a peaking amplifier biased in class C. At low power only the main amplifier operates, into a high load impedance that brings it to voltage saturation early, giving peak efficiency at back-off. As drive rises, the peaking amplifier turns on and, through a quarter-wave impedance inverter, actively lowers the main amplifier’s load. A symmetric two-way Doherty has an efficiency peak at about 6 dB back-off.^[1]^[2]

What does the quarter-wave inverter do?

The λ/4 line of impedance Z_T transforms the load the main amplifier sees as Z_main = Z_T²/Z_combined. As the peaking amplifier injects current, the combined-node impedance seen through the inverter rises, so the main amplifier’s presented impedance falls, keeping it at maximum voltage swing while delivering more current.^[2]

How are asymmetric and multi-way Dohertys used?

Unequal device sizes or supply voltages move the efficiency peak to deeper back-off (e.g., 8–10 dB) to match higher-PAPR signals. Three-way designs add further peaks. Doherty is the dominant architecture for cellular base-station PAs, combined with DPD.

What are the limitations?

Bandwidth limited by the λ/4 inverter and offset lines; wideband variants are an active research area.
Class-C peaking device gives incomplete current and gain compression; uneven drive or adaptive bias compensates.
Nonlinear transition region requires DPD for linear standards.

Frequently asked questions

When was the Doherty invented?

Described by W. H. Doherty in the Proceedings of the IRE, September 1936.

Why 6 dB back-off?

With equal main and peaking devices, the main amplifier saturates at half the peak voltage, a quarter of peak power.

Is Doherty used in phones?

Increasingly in some designs, but envelope tracking is more common in handsets.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“A New High Efficiency Power Amplifier for Modulated Waves”. W. H. Doherty, Proceedings of the IRE, vol. 24, no. 9, pp. 1163-1182. September 1936.
“RF Power Amplifiers for Wireless Communications, 2nd ed.”. S. C. Cripps, Artech House. 2006.

Categories: Amplifiers

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