PA S-Parameter Characteristics
A power amplifier's S-parameters are measured at small signal (low input power, typically −30 dBm). This linear characterization gives gain, port matches, and stability information — but not P1dB or efficiency, which require power sweeps.
| Parameter | PA Typical Value | Note |
|---|---|---|
| S21 (gain) | +25 to +35 dB | Small-signal linear gain |
| S11 (input match) | −8 to −15 dB | Input matching network applied |
| S22 (output match) | −8 to −15 dB | Output matching from device to 50 Ω |
| S12 (isolation) | −10 to −20 dB | Feedback path; impacts stability |
Step 1: Assess Gain and Bandwidth
Load PA .s2p → S21 dB Read gain at f₀: e.g., S21 = 28 dB at 2.4 GHz Check −3 dB bandwidth: e.g., 1.8–3.2 GHz (wideband MMIC PA) Check gain flatness: variation across operating band <1 dB (class AB)
Step 2: Output Match – S22 on Smith Chart
Switch to Smith chart → select S22 GaN PA output impedance at 2.4 GHz: z = 0.3 + j0.2 (normalized) Z = 15 + j10 Ω (device is low impedance, high current) Design output matching network to transform 15+j10 → 50 Ω: Use RF View Auto Matching on Port 2 (output) L-network result: shunt C = 4.7 pF, series L = 3.3 nH
Step 3: Stability at All Frequencies
Check K-factor from DC to max operating frequency: GaN PAs are often conditionally stable at low frequencies (<500 MHz) due to high S12 caused by device feedback capacitance Cgd Signs of low-frequency instability (K < 1 below 500 MHz): → Add series gate resistor (4–10 Ω) for stabilization → RF choke inductance at drain can help if resonance-controlled → Place 50 Ω resistor at drain for low-frequency stability network
Step 4: Full Circuit Simulation in RF View
- Load PA S2P as DUT block in Circuit Simulator
- Add input matching network elements (series L, shunt C) on left
- Add output matching network elements on right
- Simulate: view combined S11 (input), S21 (gain through match), S22 (output match)
- Transfer results to S-Parameter Plot for comparison with raw PA S2P
RF View PA Design: Load PA .s2p file, use Auto Matching to design output network, simulate complete circuit (PA + matching), and run Monte Carlo to verify yield. All free on Android.