How does the Doherty amplifier achieve high efficiency at back-off?

The key mechanism is active load modulation through the quarter-wave impedance inverter. At low power levels (below the back-off point, typically 6 dB below peak), only the carrier amplifier is active. The quarter-wave line transforms the 50-ohm combining node impedance to 100 ohms at the carrier amplifier's output. This higher impedance means the carrier amplifier saturates at half the voltage swing it would need into 50 ohms, achieving maximum efficiency at only one-quarter of peak power (6 dB back-off). As input power increases beyond the back-off point, the peaking amplifier begins conducting. Its current injection at the combining node effectively raises the impedance seen at that node, which the quarter-wave line transforms into a lower impedance at the carrier output, from 100 ohms progressively down to 50 ohms at peak power. Both amplifiers are saturated at peak power, delivering maximum combined efficiency. This dual-efficiency-peak behavior gives approximately 45 to 55 percent average efficiency for signals with 6 to 8 dB PAPR.

Why is Doherty the dominant base station PA architecture?

Modern cellular signals (LTE, 5G NR) have PAPR of 8 to 12 dB, meaning the PA spends most of its time at 6 to 10 dB below peak power. A conventional Class AB PA has 50 percent efficiency at saturation but only 10 to 15 percent at 8 dB back-off. The Doherty architecture provides 45 to 55 percent efficiency at 6 to 8 dB back-off, reducing DC power consumption by 3 to 4 times. For a macro base station with 4x 60-watt PA channels, this translates to hundreds of watts of DC power savings per sector, directly reducing electricity costs and cooling requirements. Combined with digital pre-distortion (DPD), which linearizes the inherently nonlinear Doherty response, the architecture achieves both high efficiency and acceptable linearity (ACLR less than minus 45 dBc, EVM less than 3 percent). GaN HEMT technology further enhances Doherty PA performance with higher breakdown voltage (65 to 100 V), higher power density (8 to 10 W/mm), and better thermal conductivity than LDMOS.

What are extended Doherty and N-way Doherty architectures?

The classical 2-way symmetrical Doherty provides an efficiency peak at 6 dB back-off. For signals with higher PAPR (10 to 12 dB), the efficiency improvement is less dramatic because the signal spends significant time below the 6 dB back-off point. Asymmetric Doherty uses a larger peaking amplifier (2x or 3x the carrier size) to shift the back-off efficiency peak to 8 or 10 dB, better matching 5G NR signal statistics. Three-way (N-way) Doherty adds a second peaking amplifier, creating two efficiency peaks: one at 9.5 dB back-off and another at 6 dB, providing more uniform efficiency across a wider power range. Digital Doherty replaces the analog quarter-wave combiner with a digital signal processing approach, using separate DACs and upconverters for each amplifier path, enabling dynamic load modulation that adapts to the signal statistics in real time. These advanced architectures achieve 55 to 65 percent average efficiency for 5G NR wideband signals.

Power Amplifier Architecture

Doherty Amplifier

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A high-efficiency PA using a carrier amplifier and peaking amplifier connected through a λ/4 impedance inverter. Active load modulation maintains efficiency at back-off: carrier sees 2R_opt at low power (saturates at 6 dB back-off), peaking turns on above this point, pulling carrier impedance to R_opt at full power. Efficiency: 45-55% at 6-8 dB back-off vs. 10-15% for Class AB. Dominant architecture for 4G/5G base stations. Combined with DPD for linearity.

Efficiency: 45-55% @ 6-8 dB BO

Technology: GaN HEMT

Invented: 1936

Understanding the Doherty Amplifier

William Doherty invented this architecture in 1936 at Bell Labs for AM broadcast transmitters. It was largely forgotten until the 2000s, when modern cellular signals (W-CDMA, LTE) with high PAPR made back-off efficiency the dominant concern in base station design. Today, virtually every macro base station in the world uses a Doherty PA, and the architecture is the single most important innovation in RF power amplifier efficiency over the past two decades.

The core insight is that an amplifier achieves maximum efficiency only at saturation, where the output voltage swing fills the available supply voltage range. At back-off, the swing is smaller, and efficiency drops proportionally. Doherty's solution: present a higher load impedance to the carrier amplifier at back-off, so it saturates (and reaches peak efficiency) at a lower power level. The λ/4 impedance inverter performs this impedance transformation, and the peaking amplifier's current injection dynamically modulates the effective load impedance as power increases.

Doherty Efficiency Equations

Doherty efficiency:
η = η_max × P_in/P_sat (main amp)

Back-off efficiency:
η(6dB BO) = η_max/2 (ideal Class-B Doherty)
vs η_max/4 for conventional Class-B

Impedance inverter:
Z_inv = R_opt = V_DD²/(2P_max)
At 6dB BO: Z_main = 2R_opt (load modulation)

PA Architecture Efficiency Comparison

Parameter	Conventional	2-way Doherty	3-way Doherty	Notes
η @P_sat	78%	78%	78%	Same peak
η @6dB BO	20%	50%	50%	Doherty 2.5×
η @9dB BO	14%	35%	50%	3-way extends
Linearity	Best	Good+DPD	Fair+DPD	Needs DPD
BW	Wide	10–20%	5–15%	BW limited

Common Questions

Frequently Asked Questions

How does Doherty achieve high back-off efficiency?

Active load modulation via λ/4 impedance inverter. At low power: carrier sees 2R_opt, saturates at 6 dB back-off (peak η). As input grows: peaking amplifier turns on, injects current at combining node, pulling carrier impedance from 2R_opt to R_opt. Both saturated at peak power. Result: dual efficiency peaks at 6 dB back-off and at peak, maintaining 45-55% average for 6-8 dB PAPR signals.

Why is Doherty dominant for base stations?

LTE/5G PAPR = 8-12 dB. Class AB at 8 dB back-off: 10-15% η. Doherty at same: 40-50% η. For 4x60 W sectors: saves 1500+ W DC per site. Combined with DPD for linearity (ACLR < -45 dBc, EVM < 3%). GaN HEMT enables high breakdown (65-100 V), power density (8-10 W/mm). Virtually every macro BTS worldwide uses Doherty GaN PAs.

What are extended Doherty variants?

Asymmetric: larger peaking amp (2-3x carrier) shifts back-off peak to 8-10 dB for 5G signals. 3-way: adds second peaking amp, dual efficiency peaks at 9.5 and 6 dB back-off. Digital Doherty: separate DAC/upconverter per path, real-time adaptive load modulation. These achieve 55-65% average efficiency for wideband 5G NR signals.

Related Terms

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