What is the difference between input P1dB and output P1dB?

Input P1dB (IP1dB) is the input power that causes 1 dB of gain compression. Output P1dB (OP1dB) is the corresponding output power, which is the input P1dB plus the compressed gain (small-signal gain minus 1 dB). For an amplifier with 20 dB small-signal gain and IP1dB of -5 dBm: OP1dB = -5 + (20-1) = +14 dBm. Receiver datasheets typically specify IP1dB because the concern is how much input power the receiver can handle. PA datasheets specify OP1dB because the concern is how much power can be delivered to the antenna.

How much back-off from P1dB does my signal need?

The required back-off depends on the signal's peak-to-average power ratio (PAPR). CW and FM signals have 0 dB PAPR and can operate at or near P1dB. QPSK has about 3 dB PAPR and needs 3 dB back-off. 64QAM OFDM has about 8 dB PAPR and needs 8 dB back-off from P1dB to avoid clipping the peaks and generating out-of-band emissions. With DPD, the PA can operate 2 to 3 dB closer to P1dB because the predistorter compensates for the compression. For a PA with OP1dB of +40 dBm amplifying a 64QAM OFDM signal: average output power is +40 minus 8 = +32 dBm without DPD, or +34 to +35 dBm with DPD.

What is the relationship between P1dB and IP3?

For most amplifiers, the output IP3 (OIP3) is approximately 10 dB above the output P1dB: OIP3 is roughly OP1dB + 10 dB. This relationship arises from the mathematical properties of a weakly nonlinear transfer function (third-order polynomial). Input-referred: IIP3 is approximately IP1dB + 10 dB. This rule of thumb holds well for amplifiers operating in mild compression but can vary by 8 to 12 dB for devices with different compression characteristics (GaN amplifiers with asymmetric compression can have OIP3 12 to 15 dB above OP1dB). Always use measured data when available.

RF Design

Compression Point

P1dB

An amplifier with 20 dB of small-signal gain should produce +30 dBm output from a +10 dBm input. But transistors are not ideal: as the input power increases, the output approaches a ceiling set by the DC supply voltage and current. At some point, the gain drops by 1 dB: the output is +29 dBm instead of +30 dBm. That input level is the 1 dB compression point (P1dB). Above it, the amplifier clips the signal peaks, generating harmonics and intermodulation products that spill into adjacent channels. For a PA driving a 64QAM OFDM signal with 8 dB PAPR, the average operating power must be 8 dB below P1dB, or the signal peaks will be crushed and EVM will fail.

Category: RF Design

Definition: Gain = G_ss − 1 dB

Rule: OIP3 ≈ OP1dB + 10 dB

How Much Room Do You Have?

P1dB relationship:
OP1dB = IP1dB + (G_ss − 1) dB
OIP3 ≈ OP1dB + 10 dB (typical)

Required back-off from OP1dB:
P_avg = OP1dB − PAPR (without DPD)
P_avg = OP1dB − PAPR + 2 to 3 dB (with DPD)

Back-off Requirements by Signal Type

Signal Type	PAPR	Back-off Needed	PA Efficiency at Back-off	Example
CW / FM	0 dB	0 dB	50 to 70% (Class AB)	Radar, FM broadcast
GMSK	0 dB	0 dB	50 to 80% (Class C)	GSM
QPSK	3.5 dB	3 to 4 dB	25 to 35%	DVB-S, LTE edge
16QAM SC	4.5 dB	4 to 5 dB	20 to 30%	Microwave backhaul
64QAM OFDM	8 to 9 dB	7 to 8 dB	8 to 15%	WiFi, LTE
256QAM OFDM	10 to 12 dB	9 to 11 dB	5 to 10%	5G NR, WiFi 6

Common Questions

Frequently Asked Questions

Input vs. output P1dB?

IP1dB: input power at 1 dB compression (used for receiver specs). OP1dB: output power at 1 dB compression (used for PA specs). OP1dB = IP1dB + G_ss − 1. An amp with 20 dB gain and IP1dB = −5 dBm has OP1dB = +14 dBm.

How much back-off?

Back-off = PAPR of the signal. CW: 0 dB. 64QAM OFDM: 8 dB. 256QAM OFDM: 10+ dB. DPD recovers 2 to 3 dB. A PA with OP1dB = +40 dBm and 64QAM signal: average output = +32 dBm without DPD, +35 dBm with DPD.

How do P1dB and OIP3 relate?

OIP3 ≈ OP1dB + 10 dB for most amplifiers. GaN devices may show 12 to 15 dB due to asymmetric compression. Always verify with measured data when available.

Related Terms

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PA Sizing

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