Combiner Outphasing
Understanding Combiner Outphasing
Modern wireless signals (LTE, 5G NR, Wi-Fi 6E) use complex modulation schemes (64/256-QAM OFDM) that produce high peak-to-average power ratios (PAPR) of 7 to 12 dB. A conventional linear PA must operate with enough back-off to handle these peaks without clipping, meaning it spends most of its time at low efficiency. At 8 dB back-off, a Class AB PA achieves only 10 to 15% efficiency, wasting 85 to 90% of the DC power as heat. Outphasing addresses this by allowing both PAs to operate at or near saturation at all times, regardless of the output power level.
The key insight is that any amplitude-modulated signal can be represented as the sum of two constant-envelope signals with appropriate phase offsets. At peak output, the two signals are in phase and add constructively. At minimum output, they are nearly 180° out of phase and cancel. The combiner topology determines what happens to the "cancelled" power: in an isolated combiner, it dissipates in the isolation resistor (wasteful); in a Chireix combiner, the reactive load modulation reflects power back toward the PA where it recirculates, maintaining efficiency. The Chireix approach adds compensating shunt reactances (one inductive, one capacitive) to cancel the reactive component of the load at a chosen back-off level, creating a second efficiency peak in addition to the peak-power peak.
Outphasing Signal Decomposition
S(t) = A(t) cos(ωt + φ(t))
Component Signals:
S1(t) = cos(ωt + φ(t) + θ(t))
S2(t) = cos(ωt + φ(t) − θ(t))
where θ(t) = arccos(A(t) / Amax)
Recombined Output:
Sout = S1 + S2 = 2cos(θ) cos(ωt + φ) = (A(t)/Amax) × 2cos(ωt + φ)
Both S1 and S2 have constant envelope = 1 (normalized), so each PA runs at saturation. The outphasing angle θ varies from 0° (full output) to 90° (zero output). At 6 dB back-off: A/Amax = 0.5, θ = 60°.
High-Efficiency PA Architecture Comparison
| Architecture | η at Peak | η at 6 dB BO | Bandwidth | Complexity | Best Application |
|---|---|---|---|---|---|
| Class AB (linear) | 50 to 60% | 15 to 25% | Wide | Low | Low-power, wide BW |
| Doherty | 55 to 65% | 45 to 55% | 10 to 20% | Moderate | Cellular base station |
| Envelope tracking | 60 to 70% | 55 to 65% | Wide | High (modulator) | 5G small cell, handset |
| Outphasing (Chireix) | 65 to 75% | 50 to 70% | Moderate | High (SCS, matching) | High-power, high PAPR |
| Outphasing (isolated) | 50 to 60% | 20 to 30% | Wide | Moderate | Wideband, not power-critical |
Frequently Asked Questions
How does outphasing achieve linear amplification from nonlinear PAs?
The SCS decomposes the input into two constant-envelope signals with equal amplitude but opposite phase modulation: θ(t) = arccos(A(t)/Amax). Since both have constant envelope, they're amplified by saturated PAs at maximum efficiency. The combiner sums them: S1+S2 = 2cos(θ)cos(ωt+φ), reconstructing the original amplitude via constructive/destructive interference.
What is the difference between isolated and non-isolated combiners?
Isolated (Wilkinson/hybrid) combiners maintain constant 50 Ω PA load but waste power in the isolation resistor (75% loss at 6 dB back-off). Non-isolated Chireix combiners allow load modulation between PAs, with compensating reactances maintaining high efficiency (50 to 70%) across 6 to 10 dB dynamic range by recirculating cancelled power rather than dissipating it.
How does outphasing compare to Doherty and envelope tracking?
Doherty: 50 to 55% at 6 dB back-off, simple implementation. ET: 55 to 65%, needs wideband supply modulator. Outphasing (Chireix): 50 to 70% but requires precise path matching (<0.1 dB, <0.5°) and high-speed SCS. Modern implementations add DPD for −50 dBc ACLR linearity.