Key PA S-Parameter Metrics
Power amplifier S-parameters are measured at small signal (linear) levels — typically −30 dBm input power. These measurements give gain, port impedances, and stability, but not P1dB, PAE, or IP3 which require power sweeps.
| S-Parameter | PA Typical | What It Tells You |
|---|---|---|
| S21 (gain) | +25 to +35 dB | Small-signal gain, flatness vs frequency |
| S11 (input match) | −8 to −15 dB | Input matching network performance |
| S22 (output match) | −8 to −15 dB | Output network transforms device to 50Ω |
| S12 (reverse isolation) | −10 to −20 dB | Feedback path — impacts stability at all freq |
PA Output Impedance and Matching
GaN and GaAs PAs have very low output impedance (2–20 Ω), requiring a high-Q matching network to transform to 50 Ω. Read S22 on the Smith chart to find the output impedance at the design frequency:
Example: PA S22 on Smith chart at 2.4 GHz shows z=0.3+j0.15 (normalized)
Z_out = 50 × (0.3+j0.15) = 15+j7.5 Ω
Conjugate match: Z_L = Z_out* = 15−j7.5 Ω
Matching network design:
Cancel +j7.5 Ω: series C = 1/(2π·2.4GHz·7.5) = 8.8 pF
Transform 15 Ω → 50 Ω: L-network Q=√(50/15−1)=1.49
Shunt C at 50Ω side: X_shunt=50/1.49=33.6Ω → C=2.0pF
Series L at PA side: X_series=1.49×15=22.4Ω → L=1.5nH
PA Low-Frequency Stability (Critical)
GaN PAs are often unstable at frequencies below 500 MHz: High device gain + feedback capacitance Cgd → K < 1 below 300 MHz Stabilization options: 1. Series gate resistor (4–10 Ω): adds loss at low freq, minimal impact at RF 2. RC stabilization network: parallel R+C at gate input 3. Ferrite bead at drain bias: high impedance at RF, low impedance below 100 MHz
Analysis Workflow in RF View
- Load PA .s2p file → view S21 (gain), S22 (output impedance on Smith chart)
- Select S22 → Smith chart view → read output impedance at f₀
- Auto Matching on Port 2 → synthesizes output matching network
- Load PA as S2P DUT in Circuit Simulator → add matching network elements
- Simulate combined S11, S21, S22 → verify complete circuit performance
- Check S12 level → verify stability margin
RF View PA Analysis: Load PA .s2p file, view S22 on Smith chart, Auto Match the output to 50Ω, and simulate the complete circuit with matching network. Free on Android.