Why can't a power amplifier be both highly efficient and perfectly linear?

Efficiency requires the transistor to spend as little time as possible in its linear (resistive) region, because voltage across and current through the device simultaneously means power dissipated as heat. Maximum efficiency occurs when the transistor acts as a switch: fully on (high current, near-zero voltage drop) or fully off (zero current, full voltage). But switching creates a square wave, which is rich in harmonics and distorts the signal envelope. A Class A amplifier keeps the transistor in its linear region at all times for minimal distortion but wastes 50% of DC power as heat at peak output and much more at backed-off levels. A Class E amplifier switches the transistor and achieves 80 to 90% efficiency but generates severe harmonic and envelope distortion that must be corrected with digital predistortion.

When should I choose GaN over LDMOS or GaAs?

GaN operates at higher voltage (28 to 50V vs 28V for LDMOS and 5 to 12V for GaAs), which means higher impedance at the die, easier matching networks, and wider bandwidth. GaN also has higher power density (8 to 10 W/mm vs 1 to 2 W/mm for LDMOS), enabling smaller die for the same power. Use GaN for wideband applications (5G massive MIMO, military radar, electronic warfare), frequencies above 4 GHz (where LDMOS gain drops off), and applications needing high efficiency with DPD. LDMOS remains cost-effective for narrowband applications below 4 GHz (cellular macro base stations, broadcast). GaAs is used for handset PAs where low voltage operation (3.5V battery) is required.

What is PAE and why does it matter more than drain efficiency?

Power Added Efficiency (PAE) accounts for the input drive power: PAE = (Pout - Pin) / PDC times 100 percent. Drain efficiency only considers Pout/PDC and ignores the power consumed by the driver stage. For a high-gain PA (20 dB), PAE and drain efficiency are nearly equal because Pin is 1% of Pout. But for a low-gain PA at high frequency (10 dB gain at 28 GHz), Pin is 10% of Pout and the driver stage consumes significant DC power. In this case, drain efficiency might be 50% but PAE is only 45%. PAE is the correct metric for system-level efficiency because it accounts for the total DC power consumption of the entire transmit chain.

Active Components

Power Amplifier

A 5G massive MIMO base station contains 64 power amplifiers, each delivering 5 watts at 3.5 GHz. At 50% efficiency, each PA wastes 5 watts as heat. Across all 64 channels, that is 320 watts of thermal dissipation in an enclosure the size of a suitcase. Increase efficiency to 60% and the heat drops to 213 watts, eliminating the need for active cooling. That 10 percentage points of efficiency improvement, achieved through Doherty architecture and digital predistortion, is the difference between a passively cooled radio and one requiring fans, heat pipes, and liquid cooling. This is why PA efficiency is not a secondary specification; it determines the size, weight, cost, and reliability of the entire radio system.

Category: Active Components

Key Trade-off: Efficiency vs. Linearity

Dominant Tech: GaN HEMT, LDMOS

Operating Classes: The Fundamental Trade-off

Class	Conduction Angle	Max Drain Eff.	Linearity	Gain Compression	Application
A	360°	50%	Best	Gradual	Driver stages, lab amps
AB	180 to 360°	50 to 78%	Good	Moderate	Most cellular, WiFi PAs
B	180°	78.5%	Moderate	Sharp crossover	Push-pull audio, some RF
C	<180°	85 to 90%	Poor	Severe	FM transmitters, CW
E	Switch mode	90 to 95%	None (switch)	N/A	With DPD, envelope tracking
F / F⁻¹	Switch + harmonic	90 to 100% (theory)	None	N/A	Research, advanced Doherty

Power Added Efficiency:
PAE = (P_out − P_in) / P_DC × 100%

Drain efficiency:
η_D = P_out / P_DC × 100%

Thermal example: 64-element MIMO, 5 W per PA:
At 50% PAE: P_DC = 10 W/PA, P_heat = 5 W/PA, Total = 320 W
At 60% PAE: P_DC = 8.3 W/PA, P_heat = 3.3 W/PA, Total = 213 W
107 W thermal savings = passive cooling vs. forced air

Common Questions

Frequently Asked Questions

Why can't a PA be efficient and linear?

Efficiency requires switching (minimal voltage-current overlap). Switching creates harmonics and envelope distortion. Class A stays linear but wastes ≥50% as heat. Class E switches for 90%+ efficiency but needs DPD to restore linearity. The Doherty architecture achieves a practical middle ground.

GaN vs. LDMOS vs. GaAs?

GaN: high voltage (28 to 50V), high power density, wideband, best above 4 GHz. LDMOS: cost-effective below 4 GHz for narrowband cellular. GaAs: low voltage (3.5V) for handset PAs running from battery. GaN is displacing LDMOS in 5G due to bandwidth and efficiency advantages.

PAE vs. drain efficiency?

PAE subtracts input drive power: (P_out − P_in)/P_DC. At 20 dB gain, PAE ≈ drain eff. At 10 dB gain (mmWave), PAE is 5% lower than drain eff because the driver consumes significant power. PAE is the correct system metric.

Related Terms

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PA Design

PA Efficiency & Thermal Calculator

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