Why is a 3-way Doherty needed instead of a standard 2-way?

A standard symmetric 2-way Doherty amplifier uses one carrier and one peaking amplifier. It provides maximum efficiency at saturation and at 6 dB back-off from saturation. However, modern 5G NR signals and wideband OFDM waveforms have Peak-to-Average Power Ratios (PAPR) of 8 to 10 dB. If a 2-way Doherty operates at 9 dB back-off to accommodate the peaks without clipping, its average efficiency drops significantly because it operates below its second efficiency peak. The 3-way Doherty adds a second peaking amplifier, dividing the active load modulation into two stages. This extends the high-efficiency region down to 9.5 dB back-off (when sized symmetrically 1:1:1), perfectly aligning the amplifier's maximum efficiency point with the average power level of a modern 5G signal.

How does the active load modulation work in a 3-way configuration?

The system operates in three regions. In the low-power region (below 9.5 dB back-off), only the carrier amplifier (Class AB) operates. It sees a high load impedance (typically 3 times Ropt), allowing its voltage to swing to maximum early, achieving high efficiency. In the medium-power region (9.5 dB to 3.5 dB back-off), the first peaking amplifier (Class C) turns on and pumps current into the combining node. Through the quarter-wave transformer, this lowers the apparent impedance seen by the carrier amplifier, allowing the carrier to deliver more current without clipping. In the high-power region (3.5 dB back-off to peak power), the second peaking amplifier turns on, further lowering the impedance seen by the first two amplifiers. At peak power, all three amplifiers operate into their optimal impedance (Ropt), delivering maximum combined power.

What are the primary challenges of designing a 3-way Doherty?

The primary challenge is complexity. A 3-way Doherty requires a three-way input power splitter (often unequal to optimize the turn-on points of the peaking amplifiers) and a complex three-way output combining network with multiple quarter-wave inversion lines. This drastically increases the PCB footprint and reduces the operational bandwidth compared to a 2-way design. Furthermore, tracking the phase and amplitude alignment of three separate amplifiers across frequency and temperature is extremely difficult. The turn-on synchronization of the two Class C peaking amplifiers must be perfect to prevent efficiency dips or severe AM-AM and AM-PM distortion. Digital Predistortion (DPD) is mandatory to linearize a 3-way Doherty, and the DPD algorithms must be robust enough to handle the complex, multi-inflection distortion profile generated by three interacting active devices.

Active Components

3-Way Doherty

A 5G base station transmits an OFDM signal with a Peak-to-Average Power Ratio (PAPR) of 9 dB. A standard Class AB amplifier operating at 9 dB back-off achieves less than 20% efficiency, turning most of its DC power into heat. A standard 2-way Doherty improves this, hitting peak efficiency at 6 dB back-off, but still struggles at 9 dB. The 3-way Doherty solves the 5G power problem by using one carrier amplifier and two sequential peaking amplifiers. The carrier handles the average signal; peaking amplifier #1 activates for moderate peaks; peaking amplifier #2 activates only for the rare, highest peaks. Through active load modulation, the 3-way architecture maintains peak efficiency down to 9.5 dB back-off, keeping the massive MIMO arrays cool while faithfully reproducing modern high-PAPR waveforms.

Category: Active Components

Configuration: 1 Carrier, 2 Peaking

Efficiency Peak: 9.5 dB back-off (1:1:1 ratio)

Doherty Architecture Comparison

Architecture	Size Ratio	Efficiency Peaks	Target Application	DPD Complexity
Class AB (Reference)	N/A	0 dB (Pmax)	CW, FM, GMSK	Low
Symmetric 2-way	1:1	0 dB, 6.0 dB	WCDMA, LTE (low PAPR)	Medium
Asymmetric 2-way	1:2	0 dB, 9.5 dB	LTE, 5G (moderate PAPR)	High
Symmetric 3-way	1:1:1	0 dB, 3.5 dB, 9.5 dB	5G NR, WiFi 6	Very High
Asymmetric 3-way	1:1.5:2	0 dB, 4.5 dB, 12 dB	DVB-T, Wideband OFDM	Extreme

Back-off extension formula (Symmetric N-way):
OBO = 20·log₁₀(N) dB
2-way: 20·log₁₀(2) = 6.0 dB
3-way: 20·log₁₀(3) = 9.5 dB

Carrier impedance modulation (3-way, 1:1:1):
Low power (carrier only): Z_c = 3·R_opt
Mid power (carrier + peak 1): Z_c drops from 3·R_opt to 1.5·R_opt
High power (all active): Z_c drops to R_opt

Common Questions

Frequently Asked Questions

Why 3-way instead of 2-way?

2-way Doherty hits peak efficiency at 6 dB back-off. 5G signals have 8-10 dB PAPR. A 2-way PA operating at 9 dB back-off drops off its efficiency curve. A 3-way (1:1:1) Doherty pushes the efficiency peak out to 9.5 dB back-off, perfectly aligning with the average power level of 5G waveforms.

How do the three amplifiers interact?

In low power (>9.5 dB back-off), only the Class AB carrier runs into a high impedance (3×Ropt), clipping early for high efficiency. At 9.5 dB back-off, Peaking #1 (Class C) turns on, lowering the carrier's apparent impedance so it can output more current. At 3.5 dB back-off, Peaking #2 turns on, lowering impedances further until all three reach Ropt at peak power.

What are the design challenges?

Complexity and bandwidth. You need a 3-way input splitter, a multi-stage output combining network with quarter-wave lines, and precise phase alignment across three separate devices. This drastically reduces bandwidth compared to a 2-way. Furthermore, linearizing a 3-way PA requires advanced DPD algorithms to handle the double-inflection distortion profile created by the two Peaking amplifier turn-on points.

Related Terms

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PA Architecture

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