Why does a fixed-supply PA waste so much power with high-PAPR signals?

A Class AB PA achieves peak efficiency only when its output swing reaches the supply rails. For a fixed 28V supply, peak efficiency occurs at about +45 dBm output. But a 64QAM OFDM signal with 8 dB PAPR has an average power of +37 dBm, meaning the PA spends most of its time producing output swings far below the supply voltage. The voltage difference between the supply and the output swing is dropped across the transistor, dissipated as heat. At 8 dB back-off, a Class AB PA that achieves 55% efficiency at peak only delivers about 15 to 20% efficiency at the average power. Envelope tracking reduces the supply voltage during low-envelope periods, reducing the voltage drop and the wasted power.

How does ET compare to Doherty for efficiency?

Both target the same problem (efficiency at back-off) but work differently. Doherty uses load modulation: a main amplifier handles average power, and a peaking amplifier activates only for signal peaks, presenting optimal load impedance at both power levels. ET uses supply modulation: one amplifier with a variable supply voltage. Doherty is simpler (no high-speed supply modulator needed) and works well for base stations with moderate bandwidth (up to 200 MHz). ET achieves higher efficiency at deep back-off (10+ dB) and scales better with signal bandwidth because the supply modulator bandwidth is independent of the PA matching network bandwidth. Most 5G handsets use ET, while most 5G base stations use Doherty. Some advanced base stations combine both (ET-Doherty) for maximum efficiency.

Active Components

Envelope Tracking

Q: How fast must the supply modulator be?

The supply modulator must track the envelope bandwidth, which is 3 to 5 times the RF signal bandwidth. For a 20 MHz LTE signal, the envelope bandwidth is 60 to 100 MHz. For a 100 MHz 5G NR signal, the envelope bandwidth is 300 to 500 MHz. This is extremely challenging for a power supply: it must efficiently convert a battery voltage to a rapidly varying output at hundreds of MHz with currents of several amps. Modern ET modulators use a hybrid architecture: a wideband linear amplifier handles the fast envelope variations, while a high-efficiency switched-mode converter handles the average power. The linear amplifier contributes 10 to 30% of the total output power; the switcher contributes 70 to 90%.

A smartphone PA amplifying a 256QAM OFDM signal at +23 dBm average output with 10 dB PAPR and a fixed 3.7V battery supply achieves about 25% PAE: three quarters of the battery power becomes heat. The problem is that the PA is designed to be efficient at peak power (+33 dBm), but the signal spends 99% of its time well below that level. Envelope tracking fixes this by dynamically adjusting the PA's supply voltage to match the instantaneous signal amplitude. When the signal is at −6 dB from peak, the supply drops from 3.7V to about 1.8V, halving the wasted voltage drop across the transistor. The result: PAE rises from 25% to 40 to 45%, extending battery life by 30 to 40% for the same transmitted power.

Category: Active Components

Efficiency Gain: +15 to 20 pp over fixed supply

Used In: Every modern smartphone PA

Why Fixed Supply Wastes Power

Architecture	PAE at Peak	PAE at 8 dB BO	Modulator BW	Complexity	Typical Use
Fixed supply Class AB	55%	15 to 20%	N/A	Low	Legacy, simple TX
Envelope tracking	50%	40 to 45%	3 to 5× signal BW	High (supply mod)	5G handsets, UE
Doherty	55%	35 to 45%	N/A	Medium (two PAs)	Base stations, macro
ET + Doherty	55%	45 to 50%	3× signal BW	Very high	Advanced BS, mmWave
Average power tracking	55%	25 to 30%	<1 MHz	Low	IoT, low BW

Efficiency recovery principle:
PAE_fixed at back-off BO: PAE_peak / 10^BO/10 (approx.)
At 8 dB BO: 55% / 6.3 = 8.7% (theoretical worst case)

ET supply modulator bandwidth:
BW_mod = 3 to 5 × BW_signal
100 MHz 5G NR signal: modulator needs 300 to 500 MHz bandwidth

Battery life improvement:
Δlife ≈ (PAE_ET − PAE_fixed) / PAE_fixed
From 25% to 42%: 68% less heat, 30 to 40% longer battery

Common Questions

Frequently Asked Questions

Why does fixed supply waste power?

Peak efficiency occurs only at full output swing. High-PAPR signals spend most time well below peak. The voltage difference between supply and output is dropped across the transistor as heat. At 8 dB back-off: only 15 to 20% PAE instead of 55%.

How fast must the supply modulator be?

3 to 5× signal bandwidth. 20 MHz LTE: 60 to 100 MHz modulator. 100 MHz 5G NR: 300 to 500 MHz. Hybrid architecture: wideband linear amp (10 to 30% of power) + switched-mode converter (70 to 90%).

ET vs. Doherty?

Doherty: load modulation, simpler, used in base stations. ET: supply modulation, better at deep back-off, used in handsets. Both target the same problem. Advanced BS combines both (ET-Doherty) for 45 to 50% PAE at back-off.

Related Terms

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Enter signal PAPR, PA peak efficiency, and supply modulator efficiency. Compare ET, Doherty, and fixed-supply architectures to find the optimal efficiency for your waveform.

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