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Power Amplifier Efficiency Calculator (PAE & Drain Efficiency)

Calculate RF power amplifier PAE, drain efficiency, gain, and heat dissipation from Pout, Pin, and DC bias. Essential for PA design. Free, instant results.

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Formula

PAE=(Pout−Pin)/Pdc×100PAE = (Pout − Pin) / Pdc × 100%
PAE— Power-added efficiency (%)
Pout— RF output power (mW)
Pin— RF input power (mW)
Pdc— DC supply power (Vdc × Idc) (mW)
η_D— Drain efficiency (Pout/Pdc) (%)

How It Works

Power amplifier efficiency measures DC-to-RF power conversion — wireless infrastructure engineers, transmitter designers, and battery-powered device developers use efficiency metrics to minimize heat dissipation and maximize operating time. Drain efficiency eta_D = P_RF_out / P_DC ranges from 25% (Class A) to 90% (Class E/F) depending on amplifier topology, per Cripps' 'RF Power Amplifiers for Wireless Communications' (2nd ed.).

Power-added efficiency PAE = (P_RF_out - P_RF_in) / P_DC accounts for driver power, which becomes significant in high-gain systems. For a 20 W amplifier with 15 dB gain consuming 40 W DC: eta_D = 20/40 = 50%, but P_RF_in = 20/31.6 = 0.63 W, so PAE = (20-0.63)/40 = 48.4%. PAE converges to drain efficiency at high gain.

Class definitions per Krauss's 'Solid State Radio Engineering': Class A (conduction angle 360 degrees, theoretical max 50%) operates linearly with constant bias current. Class AB (180-360 degrees, 50-78%) reduces quiescent current for efficiency. Class B (180 degrees, 78.5% max) eliminates quiescent current. Class C (< 180 degrees, up to 90%) is highly efficient but nonlinear. Class D/E/F switching amplifiers achieve 90%+ efficiency through zero-voltage or zero-current switching. Modern 5G base stations use Doherty architecture achieving 50-55% PAE at 6 dB output back-off.

Worked Example

Problem

Design thermal management for a 100 W cellular base station power amplifier with 45% drain efficiency and 15 dB gain.

Efficiency analysis:

  1. DC power consumption: P_DC = P_RF_out / eta_D = 100 / 0.45 = 222 W
  2. Input RF power: P_RF_in = 100 W / 10^(15/10) = 100/31.6 = 3.16 W
  3. Power-added efficiency: PAE = (100 - 3.16) / 222 = 43.6%
  4. Heat dissipation: P_heat = P_DC - P_RF_out = 222 - 100 = 122 W

Thermal design per MIL-HDBK-217F:

  1. Junction-to-case thermal resistance: Rth_jc = 0.5 C/W (typical LDMOS)
  2. Maximum junction temperature: T_j_max = 175 C (GaN) or 200 C (LDMOS)
  3. Ambient temperature: T_amb = 55 C (outdoor cabinet)
  4. Maximum case-to-ambient thermal resistance:
Rth_ca = (T_j_max - T_amb) / P_heat - Rth_jc Rth_ca = (175 - 55) / 122 - 0.5 = 0.48 C/W
  1. Heatsink requirement: 0.48 C/W with forced-air cooling
- Natural convection heatsinks: typically 1-3 C/W minimum - Solution: fan-cooled heatsink or cold plate with liquid cooling

Efficiency improvement options:

  1. Doherty PA: 52% efficiency at 8 dB OBO — saves 31 W at same output
  2. Envelope tracking: 55% average efficiency — saves 40 W
  3. Digital predistortion (DPD) allows operation closer to saturation: +3% efficiency

Solution

fan-cooled heatsink or cold plate with liquid cooling

Efficiency improvement options:

  1. Doherty PA: 52% efficiency at 8 dB OBO — saves 31 W at same output
  2. Envelope tracking: 55% average efficiency — saves 40 W
  3. Digital predistortion (DPD) allows operation closer to saturation: +3% efficiency

Practical Tips

  • ✓Specify PAE at rated output AND at 8-10 dB back-off for linear applications (cellular, WiFi) — saturated efficiency is misleading for signals with high PAPR
  • ✓Budget 30-50% efficiency for linear PAs in production systems; 60-70% for constant-envelope (FM, FSK) or switching amplifiers; claims above 70% linear efficiency require advanced techniques (Doherty, ET, outphasing)
  • ✓For battery applications, consider average efficiency over the power probability distribution — a PA with 50% peak efficiency but 20% efficiency at typical output levels wastes more power than 40%/35% design

Common Mistakes

  • ✗Measuring efficiency only at saturation — practical signals (OFDM, LTE) have 8-12 dB peak-to-average ratio (PAPR); efficiency at 8 dB back-off is 3-4x worse than saturated efficiency. Always specify efficiency at operating back-off point
  • ✗Neglecting thermal runaway risk — GaAs and GaN devices have positive temperature coefficient of drain current; inadequate heatsinking causes thermal runaway and catastrophic failure within seconds at high power
  • ✗Ignoring driver stage power — a 10 W driver for a 100 W PA operating at 10% efficiency consumes 100 W DC, equaling the final stage dissipation; include all stages in system efficiency calculation
  • ✗Using wrong supply voltage for efficiency comparison — efficiency increases with lower supply voltage due to reduced I^2*R_on losses; compare amplifiers at same supply voltage and output power

Frequently Asked Questions

Depends on amplifier class and linearity requirements per Cripps: Class A linear: 25-35% practical (50% theoretical max). Class AB linear: 35-50% typical (78% theoretical). Class B (push-pull): 50-65% achievable. Class C (FM/radar): 65-80%. Class D/E/F (switching): 80-95%. Doherty (cellular base station): 45-55% at 8 dB OBO. Envelope tracking (handsets): 40-50% average over signal distribution. Industry benchmarks: cellular base station expects > 45% PAE at rated power; mobile handset expects > 40% average efficiency across power range.
Efficiency decreases with frequency due to: (1) Higher parasitic capacitance requires more reactive power circulation; (2) Lower transistor gain requires more driver stages; (3) Matching network losses increase with Q factor. Typical degradation: 45% at 2 GHz drops to 35% at 6 GHz for same topology. GaN technology maintains higher efficiency at microwave frequencies than GaAs or LDMOS due to higher operating voltage (lower I^2*R loss) and smaller parasitics. Above 30 GHz, 25-35% PA efficiency is state-of-art.
Key factors per Cripps' analysis: (1) Amplifier class — determines theoretical maximum based on conduction angle. (2) Device technology — GaN > LDMOS > GaAs > Si for power density and efficiency at same frequency. (3) Load impedance — optimal load for efficiency differs from load for linearity; compromise required. (4) Supply voltage — higher voltage reduces I^2*R_on loss but increases device stress. (5) Operating point — backed-off operation dramatically reduces efficiency. (6) Matching network Q — higher Q means more loss. (7) Signal PAPR — efficiency averaged over amplitude distribution, not just at peaks.
Drain efficiency eta_D = P_RF_out / P_DC measures DC-to-RF conversion of the PA stage alone. Power-added efficiency PAE = (P_RF_out - P_RF_in) / P_DC subtracts input RF power, accounting for driver requirements. At high gain (> 15 dB), PAE approximately equals drain efficiency. At low gain (10 dB), PAE is approximately 10% lower than drain efficiency. For system efficiency, use PAE because it reflects real power consumption including driver. For device characterization, drain efficiency isolates the output stage performance.
5G/LTE base stations use multiple techniques: (1) Doherty architecture — auxiliary amplifier engages at high power, improving efficiency at back-off from 25% to 45-55%. (2) Digital predistortion (DPD) — linearizes PA allowing operation closer to saturation, +3-5% efficiency gain. (3) Envelope tracking (ET) — modulates supply voltage to follow signal envelope, achieving 50-60% efficiency for mobile handsets. (4) GaN transistors — higher voltage operation (28-48V versus 12V LDMOS) reduces current and I^2*R losses. (5) Carrier aggregation management — power multiple carriers from efficient shared PA rather than separate PAs per carrier.

Methodology & References

References

  • RF Power Amplifiers for Wireless Communications, 2nd ed. — Steve C. Cripps (2006), Chapter 2 — Efficiency definitions
  • Microwave Engineering, 4th ed. — David M. Pozar (2011), Chapter 12 — Power amplifier gain and PAE

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