How to Read PA Test Curves: S-Parameters, Efficiency, P1dB, and AM-AM/AM-PM
Time：2026-04-21 Views：12
Beyond numeric specifications, a power amplifier’s datasheet contains many graphs. These curves are the “window” into the PA’s real behavior – showing gain vs. power, efficiency vs. power, amplitude/phase transfer characteristics, and more. For RF engineers, learning to read these curves is more valuable than only looking at maximum power and efficiency numbers.
1.S-Parameter Curves
S-parameters are the foundation of RF systems. For a PA, the most important are:
S21 (forward transmission coefficient): shows small-signal gain vs. frequency. It visually indicates the operating band and gain flatness. Ideally, S21 is flat within the passband and drops sharply outside.
S11 (input reflection coefficient): often expressed as return loss or VSWR. Lower (more negative) S11 means better input matching and less signal reflection. Typical requirement is S11 < -10dB (VSWR < 2:1).
S-parameters are measured with a network analyzer under small-signal conditions and do not reveal nonlinear behavior at high power.
2.Pout vs. Pin, Gain vs. Pin
These are the classic PA transfer curves. The horizontal axis is input power (Pin); the vertical axis can be output power (Pout) or gain (Gain).
·Pout vs. Pin: linear at low power (slope = small-signal gain), then bends into saturation. The point where the actual curve is 1dB below the linear extension is the P1dB compression point.
·Gain vs. Pin: flat at low power (small-signal gain), then drops. The point where gain drops 1dB from the flat region corresponds to the same P1dB.
From these curves you can also read saturated output power (Psat) and the linear operating range.
3. Efficiency Curves (Efficiency vs. Pout)
Efficiency is typically drain efficiency (DE) or power-added efficiency (PAE). The efficiency curve plots output power on the horizontal axis and efficiency on the vertical.
·For Class A/AB amplifiers, efficiency rises with output power, reaches a maximum near P1dB, then slightly drops or saturates.
·For Doherty or ET amplifiers, the efficiency curve often shows a “hump” in the back-off region, indicating high efficiency at average power levels.
When interpreting efficiency curves, evaluate based on the system’s average output power (not peak power) to determine real-world energy savings.
4. AM-AM and AM-PM Curves
These are critical for quantifying PA nonlinearity:
·AM-AM: output amplitude vs. input amplitude (normalized). Ideally a 45° straight line; in reality it bends at high input (gain compression).
·AM-PM: output phase vs. input amplitude. An ideal PA has zero AM-PM; real PAs may exhibit 10°-30° phase shift at high power.
AM-AM and AM-PM curves are the basis for digital pre-distortion (DPD) modeling. Ampbuc’s DPD solutions compensate for both amplitude and phase distortion.
1.S-Parameter Curves
S-parameters are the foundation of RF systems. For a PA, the most important are:
S21 (forward transmission coefficient): shows small-signal gain vs. frequency. It visually indicates the operating band and gain flatness. Ideally, S21 is flat within the passband and drops sharply outside.
S11 (input reflection coefficient): often expressed as return loss or VSWR. Lower (more negative) S11 means better input matching and less signal reflection. Typical requirement is S11 < -10dB (VSWR < 2:1).
S-parameters are measured with a network analyzer under small-signal conditions and do not reveal nonlinear behavior at high power.
2.Pout vs. Pin, Gain vs. Pin
These are the classic PA transfer curves. The horizontal axis is input power (Pin); the vertical axis can be output power (Pout) or gain (Gain).
·Pout vs. Pin: linear at low power (slope = small-signal gain), then bends into saturation. The point where the actual curve is 1dB below the linear extension is the P1dB compression point.
·Gain vs. Pin: flat at low power (small-signal gain), then drops. The point where gain drops 1dB from the flat region corresponds to the same P1dB.
From these curves you can also read saturated output power (Psat) and the linear operating range.
3. Efficiency Curves (Efficiency vs. Pout)
Efficiency is typically drain efficiency (DE) or power-added efficiency (PAE). The efficiency curve plots output power on the horizontal axis and efficiency on the vertical.
·For Class A/AB amplifiers, efficiency rises with output power, reaches a maximum near P1dB, then slightly drops or saturates.
·For Doherty or ET amplifiers, the efficiency curve often shows a “hump” in the back-off region, indicating high efficiency at average power levels.
When interpreting efficiency curves, evaluate based on the system’s average output power (not peak power) to determine real-world energy savings.
4. AM-AM and AM-PM Curves
These are critical for quantifying PA nonlinearity:
·AM-AM: output amplitude vs. input amplitude (normalized). Ideally a 45° straight line; in reality it bends at high input (gain compression).
·AM-PM: output phase vs. input amplitude. An ideal PA has zero AM-PM; real PAs may exhibit 10°-30° phase shift at high power.
AM-AM and AM-PM curves are the basis for digital pre-distortion (DPD) modeling. Ampbuc’s DPD solutions compensate for both amplitude and phase distortion.
