Compression Curve
Reading the Pout-versus-Pin Transfer Characteristic
A compression curve is captured by sweeping a single-tone input across a wide drive range, usually 30 to 40 dB, while a calibrated power meter or vector network analyzer records the fundamental output power at each step. The result is two regions joined by a knee. In the linear region the amplifier adds a constant gain G to every input level, so output and input track one to one on a dB scale. As the transistor approaches its voltage and current limits, the incremental gain falls, the curve flattens, and the trace separates from the extrapolated linear line. The vertical distance between the measured curve and that extrapolation is the instantaneous compression in dB.
The most widely quoted marker is the 1 dB compression point, where the gain has dropped exactly 1 dB from its small-signal value. It is reported either input-referred (P1dB) or output-referred (OP1dB), the two separated by the small-signal gain less the 1 dB of compression. Continuing to drive the stage harder eventually yields no further increase in fundamental output; this is the saturated output power, Psat, which for a Class A or AB device typically sits 2 to 4 dB above OP1dB. Beyond P1dB the energy that no longer appears in the fundamental is converted to harmonics and intermodulation products, so the compression curve is the amplitude half of the broader AM/AM and AM/PM nonlinear description.
Because the curve tells you exactly how close the amplifier is to its limits at any drive level, system designers use it to set operating back-off. A signal with high peak-to-average ratio, such as a 256-QAM OFDM waveform, must be backed off well below P1dB so that its peaks still land in the linear region; reading the curve lets the designer trade output power against adjacent-channel distortion quantitatively rather than by rule of thumb.
Quantifying Compression and Back-Off
C(Pin) = [Gss + Pin(dBm)] − Pout,meas(dBm) dB
1 dB compression point (definition):
Pout(OP1dB) = Gss + Pin − 1 dB → gain has dropped 1 dB
Output-to-input referral:
OP1dB ≈ P1dB + Gss − 1 dB
Output back-off from saturation:
OBO = Psat − Pavg dB, with peak headroom ≈ PAPR
Where Gss = small-signal gain (dB), Pin/Pout in dBm, Psat = saturated output power, PAPR = peak-to-average power ratio. Example: Gss = 20 dB, P1dB = 10 dBm → OP1dB ≈ 29 dBm; a 7 dB PAPR signal needs roughly 7 dB output back-off to keep peaks below saturation.
Points and Regions on the Curve
| Region / Point | Definition | Curve behavior | Typical value (GaN PA) | Why it matters |
|---|---|---|---|---|
| Linear region | Gain constant at Gss | Slope = 1 dB / dB | Up to ~6 dB below P1dB | Distortion-free operation |
| Compression knee | Gain begins to droop | Slope < 1, curve bends | 2 to 4 dB before P1dB | Onset of nonlinearity |
| P1dB / OP1dB | Gain down 1 dB | 1 dB below extrapolation | OP1dB ~38 to 42 dBm | Standard linearity spec |
| Psat | Max fundamental output | Curve plateaus | 2 to 4 dB above OP1dB | Hard output ceiling |
| P3dB | Gain down 3 dB | Deep into saturation | ~1 to 2 dB below Psat | Switching / limiter use |
Frequently Asked Questions
How do you find the 1 dB compression point on a compression curve?
Extend the small-signal portion of the Pout-versus-Pin trace as a straight 1 dB per dB line, then read the level where the measured curve has dropped exactly 1 dB below that extrapolation. Fit the linear region 10 to 20 dB below the knee for an accurate slope. P1dB is input-referred and OP1dB output-referred; because the gain is down 1 dB at compression, the two differ by (gain minus 1), so a 29 dBm OP1dB amplifier with 20 dB small-signal gain has a 10 dBm input-referred P1dB.
What is the difference between the AM/AM curve and the compression curve?
Same physics, different axes. The compression curve plots output power versus input power in dBm, so the linear slope is 1 and compression appears as the curve bending below the 1:1 line. The AM/AM curve plots gain or output amplitude versus drive, appearing flat then drooping. AM/AM is the form fed to digital predistortion and behavioral models; the compression curve is the form measured on the bench with a power meter or VNA.
Why does gain drop off at the top of the compression curve?
At high drive the transistor's output voltage swing is clipped by the supply rail and its current swing hits the device maximum, so the fundamental output rises more slowly than the input and the slope falls below 1 dB per dB. Energy spills into harmonics and intermod products. When the fundamental stops rising entirely you reach saturated output power, Psat, typically 2 to 4 dB above OP1dB for a Class A or AB amplifier.