What is the difference between P1dB, Psat, and average power?

P1dB is the output power at which the amplifier gain has compressed by exactly 1 dB from its small-signal value. This is the standard linearity metric because the amplifier is still reasonably linear at P1dB (the distortion products are moderate). Psat is the absolute maximum output power, typically 2 to 4 dB above P1dB, achieved when the amplifier is driven into deep compression. At Psat, the gain has compressed significantly (5 to 10 dB below linear) and distortion is severe. Average power (Pavg) is the mean power of a modulated signal, which must be backed off from Psat by the signal's PAPR to avoid clipping the peaks. For a 5G NR signal with 10 dB PAPR (before CFR) or 7 dB (after CFR), a PA with Psat = plus 50 dBm operates at Pavg = plus 43 dBm average. This back-off from peak capability is the fundamental reason PA efficiency drops for modern modulated signals.

How is power amplifier efficiency measured?

Three efficiency metrics are used. Drain efficiency (DE) is the simplest: DE = P_RF_out / P_DC times 100 percent, measuring what fraction of DC power is converted to RF output. For a PA producing 100 W RF from 200 W DC: DE = 50 percent. Power-added efficiency (PAE) accounts for the RF input drive: PAE = (P_RF_out minus P_RF_in) / P_DC times 100 percent. For a PA with 15 dB gain, P_RF_in is 3.16 percent of P_RF_out, so PAE is approximately 0.97 times DE, a small correction. For low-gain final stages (10 dB gain), the correction is significant: PAE = 0.9 times DE. Average efficiency is the PAE measured with the actual modulated signal (including PAPR and back-off), which is always lower than the peak CW PAE. A Doherty PA might achieve 50 percent DE at P1dB but only 40 to 45 percent average efficiency with a 7 dB PAPR signal, versus 20 to 25 percent for a Class AB PA.

What determines the maximum output power of a transistor?

Maximum output power is fundamentally limited by two parameters: breakdown voltage and maximum current. The RF output power is P_out = V_pk times I_pk / 2 for Class A, where V_pk is limited by the drain-source breakdown voltage minus the knee voltage, and I_pk is the maximum drain current. For a GaN HEMT with V_breakdown = 120 V operating at V_DS = 50 V: V_pk approximately equals 45 V (2 times V_DS minus V_knee). With I_max = 1 A/mm gate width: P_out = 45 times 0.5 / 2 = 11.25 W/mm, which matches the 5 to 10 W/mm practical power density. LDMOS with V_DS = 28 V achieves approximately 2 W/mm. GaAs pHEMT with V_DS = 5 V achieves approximately 1 W/mm. The total output power scales linearly with gate periphery: a 100 mm GaN device delivers approximately 500 to 1000 W peak. Thermal management ultimately limits the maximum device size, as the heat must be extracted through the substrate.

Power Amplifier Metrics

Output Power

/owt-put pow-er/ — P_out

RF signal power delivered to the load, in dBm/W/dBW. Key metrics: P1dB (1 dB compression), P_sat (saturated, ~P1dB + 2-4 dB), P_avg (average = P_sat − PAPR for modulated signals). Efficiency: PAE = (P_out−P_in)/P_DC. Technology limits: GaN 5-10 W/mm, LDMOS 1-2 W/mm, GaAs 1-1.5 W/mm. Total power scales with gate periphery. Thermal management is the ultimate limiter.

P_sat: P1dB+2-4 dB

PAPR back-off: 7-10 dB

Unit: dBm, W

Understanding Output Power

Output power is the defining specification of any power amplifier. It determines the transmitter's range, coverage area, and link budget margin. But specifying "output power" alone is insufficient: the type of power (peak, average, P1dB, Psat) must be clearly defined, as these can differ by 10 dB or more for modern modulated signals. A PA rated at 100 W Psat might deliver only 10 W average power for a 5G NR signal with 10 dB PAPR.

The fundamental tradeoff in PA design is between efficiency and linearity. Maximum efficiency occurs at or near Psat (where the transistor is fully utilized), but this region has severe nonlinearity. Linear operation requires backing off from Psat by the signal's PAPR, dramatically reducing efficiency. Doherty PA architecture with DPD is the standard solution, achieving 40-55% average efficiency versus 15-25% for backed-off Class AB.

Output Power Equations

Power unit conversions:
P(dBm) = 10 log₁₀(P(mW))
P(dBW) = P(dBm) − 30
30 dBm = 1 W, 40 dBm = 10 W
47 dBm = 50 W, 50 dBm = 100 W

Average power:
P_avg = P_sat − PAPR (dB)
P_sat=50 dBm, PAPR=7 dB (w/ CFR):
P_avg = 43 dBm = 20 W

Efficiency:
DE = P_RF,out / P_DC × 100%
PAE = (P_RF,out−P_RF,in) / P_DC
15 dB gain: PAE ≈ 0.97 × DE
10 dB gain: PAE ≈ 0.90 × DE

Max power (Class A):
P_out = V_pk × I_pk / 2
V_pk = V_DS − V_knee

PA Output Power by Application

Application	P_sat	P_avg	PAPR	PAE (avg)	Technology
5G macro BTS	200-320 W	40-80 W	7-8 dB	40-55%	GaN Doherty
5G small cell	5-20 W	1-5 W	7-8 dB	30-40%	GaN-on-Si
Handset PA	2-4 W	0.3-1 W	6-8 dB	35-45%	GaAs/CMOS
WiFi 6E AP	1-4 W	0.2-1 W	10-12 dB	20-30%	GaAs/SiGe
Radar (pulse)	100-10 kW	1-100 W	N/A (pulsed)	50-65%	GaN/TWT

Common Questions

Frequently Asked Questions

P1dB vs. Psat vs. Pavg?

P1dB: gain compressed 1 dB, moderate distortion. Psat: max absolute output, 2-4 dB above P1dB, severe distortion. Pavg: mean modulated power = Psat - PAPR. 5G NR PAPR: 10 dB raw, 7 dB after CFR. Psat=50 dBm (100 W), Pavg=43 dBm (20 W). The back-off is why PA efficiency drops for modern signals.

How is PA efficiency measured?

Drain efficiency: DE = P_RF_out/P_DC. PAE = (P_out-P_in)/P_DC, accounts for input drive. At 15 dB gain: PAE ≈ 0.97×DE (small correction). At 10 dB gain: PAE ≈ 0.90×DE (significant). Average efficiency = PAE with modulated signal including PAPR. Doherty: 40-55% avg vs. Class AB: 15-25% avg for same signal.

What determines max transistor power?

P_out = V_pk×I_pk/2. V_pk limited by breakdown minus knee voltage. GaN: V_DS=50 V, V_pk~45 V, I_max=1 A/mm: 11 W/mm (practical 5-10 W/mm). LDMOS: V_DS=28 V, ~2 W/mm. GaAs: V_DS=5 V, ~1 W/mm. Total scales with gate periphery. 100 mm GaN = 500-1000 W peak. Thermal limits max device size.

Related Terms

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