GaAs pHEMT vs GaN HEMT S-Parameter Differences
| Property | GaAs pHEMT | GaN HEMT |
|---|---|---|
| Noise Figure (at 2 GHz) | 0.3–0.8 dB | 1–2 dB |
| Gain (S21 at 2 GHz) | +18 to +25 dB (LNA) | +15 to +25 dB (PA) |
| Output power | Low (10–100 mW) | High (1–100 W) |
| Supply voltage | 2–5V (low power) | 28–48V |
| Frequency range | DC to 100+ GHz | DC to 30+ GHz |
| Application | LNA, driver, low-power PA | High-power PA, transmit |
GaAs LNA Noise and Gain Circles (from S-Parameters)
Available Gain (MAG) at 2 GHz: typically +22 to +28 dB MSG (Maximum Stable Gain): S21·S12 boundary Noise matching differs from gain matching: Γ_opt (noise-optimal reflection): read from noise parameter file (.nsp) Γ_in* (gain-optimal match): from S11 Typically Γ_opt ≠ Γ_in*, giving NF-gain tradeoff: Best NF: match to Γ_opt → S11 may be only −6 to −10 dB Best gain: match to Γ_in* → NF degraded by 0.5–1.5 dB
Stability Check for GaAs LNA
GaAs pHEMTs are typically stable at RF frequencies (K > 1 at 1–10 GHz) but may need stabilization at: 1. Very low frequencies (<100 MHz): gate-drain feedback path active 2. Frequencies above gain rolloff: unusual modes Check S12 level: if S12 > −20 dB AND S21 > +20 dB → review K-factor Add gate series resistor (2–5 Ω) for out-of-band stabilization
Analysis in RF View
- Load GaAs LNA .s2p → S21: verify gain level and flatness
- S11 Smith chart: note input impedance at design frequency
- Auto Match Port 1 → synthesizes input matching for maximum gain (or noise match)
- S22 → design output matching to 50 Ω
- Check S12 at all frequencies for potential instability
RF View GaAs Analysis: Load GaAs LNA .s2p, view gain (S21), input impedance (S11 Smith chart), and design optimal matching network. Compare multiple GaAs LNA options on same chart. Free on Android.