Back-Off Efficiency
Understanding Back-Off Efficiency
Modern wireless signals (LTE, 5G NR, Wi-Fi 6/7) use OFDM modulation with many subcarriers that occasionally add constructively, creating instantaneous peaks 7 to 12 dB above the average power level. The PA must be linear enough to amplify these peaks without clipping, which means its peak capability (Psat or P1dB) must be 7 to 12 dB above the average operating power. This back-off is necessary but devastating to efficiency: the PA spends most of its time at a low-efficiency operating point, wasting 80-95% of its DC power as heat.
The efficiency at any back-off level depends on the PA class. A Class A amplifier's efficiency drops linearly with output power: at 6 dB back-off, efficiency is only 12.5% (versus 50% at peak). Class B drops as the square root: 25% at 6 dB back-off (versus 78.5% at peak). Class AB falls between these. The key insight driving modern PA architectures is that the efficiency profile vs. back-off matters more than the peak efficiency for determining the electricity cost and thermal design of a base station. A PA with 40% average efficiency consumes half the DC power and produces half the heat of one with 20% average efficiency, even if both have the same peak efficiency.
Efficiency vs. Back-Off Formulas
ηA = ηpeak × (Pout / Psat) = 50% × 10-OBO/10
At 6 dB OBO: η = 50% × 0.25 = 12.5%
Class B Efficiency at Back-Off:
ηB = ηpeak × √(Pout / Psat) = 78.5% × 10-OBO/20
At 6 dB OBO: η = 78.5% × 0.5 = 39.3%
Average Efficiency (with signal PDF):
ηavg = ∫ Pout(x) × p(x) dx / ∫ PDC(x) × p(x) dx
p(x) = probability density of signal amplitude x
Doherty at 6 dB Back-Off:
ηDoherty ≈ ηpeak (maintained by load modulation)
PA Architecture Efficiency Comparison
| Architecture | Peak η | η at 6 dB OBO | η at 10 dB OBO | Avg η (5G NR) |
|---|---|---|---|---|
| Class AB | 55-65% | 15-20% | 6-10% | 8-12% |
| 2-Way Doherty | 60-70% | 50-60% | 20-30% | 35-45% |
| 3-Way Doherty | 55-65% | 45-55% | 40-50% | 40-50% |
| Envelope Tracking | 55-65% | 45-55% | 35-45% | 40-55% |
| Outphasing (LINC) | 60-70% | 40-50% | 30-40% | 30-40% |
Frequently Asked Questions
Why does PA efficiency drop at back-off?
In Class AB, DC power consumption is roughly constant (fixed bias and supply voltage) regardless of output level. At peak output, efficiency approaches the theoretical maximum (50-78%). At 6 dB back-off, RF output drops by 4x but DC stays constant, so efficiency drops by approximately 4x. A Class AB PA with 60% peak efficiency drops to about 15% at 6 dB back-off. For 5G NR with 8 dB PAPR, average efficiency is only 8-12%. This is why base station electricity costs are dominated by PA inefficiency.
How does a Doherty amplifier improve back-off efficiency?
A Doherty uses a carrier amplifier (always on) and a peaking amplifier (on only at peaks). At average power, only the carrier amplifier is active, and load modulation from the Doherty combining network keeps it near saturation efficiency. At peaks, both amplifiers contribute. A 2-way Doherty maintains near-peak efficiency at 6 dB back-off. A 3-way extends to 9-12 dB. Modern GaN Doherty PAs achieve 40-50% average efficiency with 5G NR versus 8-12% for conventional Class AB.
What is envelope tracking and how does it compare to Doherty?
Envelope tracking dynamically adjusts the PA supply voltage to follow the signal envelope, keeping the transistor near saturation at every power level. It requires a wideband supply modulator (3-5x signal bandwidth). For 100 MHz 5G NR, the modulator must track at 300-500 MHz. ET achieves 40-55% average efficiency. Doherty dominates base station PAs (higher power, lower bandwidth) while ET is used in handset PAs (lower power, moderate bandwidth).