Definition
The 1 dB compression point marks where an amplifier's gain has dropped 1 dB below its small-signal (linear) value due to device nonlinearity. It defines the upper boundary of the amplifier's linear operating range.
IP1dB: input power level where gain = G₀ − 1 dB OP1dB: output power at P1dB input OP1dB ≈ IP1dB + G₀ − 1 (in dB) OP1dB ≈ OIP3 − 9.6 dB (rule of thumb for Class A)
Typical P1dB Values by Device
| Device | IP1dB | OP1dB |
|---|---|---|
| LNA (SiGe, 2 GHz) | −10 to −5 dBm | +5 to +12 dBm |
| GaAs driver amp | +15 dBm | +25 dBm |
| GaN PA (2.4 GHz) | +28 dBm | +40–43 dBm |
| CMOS PA (mobile) | +22 dBm | +30–33 dBm |
| Passive diode mixer | +7 dBm | +7 − IL dBm |
Why P1dB Cannot Be Read from S-Parameters
S-parameters are always measured at low (small-signal) power levels where the device behaves linearly. P1dB requires a power sweep — incrementally increasing input power and measuring when S21 drops 1 dB from its low-power value. The VNA or signal generator + spectrum analyzer is set up for this measurement. S-parameters give G₀ (the denominator), but not P1dB itself.
Impact on System Design
Spurious-Free Dynamic Range: SFDR ≈ (2/3)·(IIP3 − Noise Floor) Back-off rule: operate PA at OP1dB − 6 to −10 dB back-off for acceptable linearity in OFDM (LTE, 5G, WiFi) systems. High PAPR signals (12 dB for LTE) require significant back-off.
RF View: While P1dB requires a dedicated power sweep measurement, RF View analyzes the S-parameter (small-signal) characteristics of amplifiers: S21 gain, S11/S22 match, and S12 stability — essential design parameters alongside P1dB.