What is the hot S22 measurement of a power amplifier and why does it differ from small signal S22?
Power Amplifier Hot S22 Characterization
The distinction between small-signal S22 and hot S22 is crucial for power amplifier system design. Using small-signal S22 for system-level calculations can lead to: impedance mismatch and power loss between the PA and the next stage, unexpected oscillation due to incorrect stability analysis, and degraded power-added efficiency from sub-optimal load matching.
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
How much does hot S22 differ from small-signal S22?
Significantly. For a GaN PA at P_sat: small-signal S22 might be -15 dB (well matched), while hot S22 might be -3 to -6 dB (poorly matched). The output impedance typically shifts toward lower impedance at high power (the transistor's effective output capacitance increases as the voltage swing compresses). The phase of S22 also changes, sometimes by 30-90 degrees. This is why PA output matching networks designed for small-signal S22 often do not deliver optimal power in the system.
Do I need hot S22 for small-signal amplifiers?
No. For small-signal amplifiers (LNAs, IF amplifiers) that operate well below their compression point, the output impedance does not change significantly with signal level. Small-signal S22 from a standard VNA measurement is sufficient. Hot S22 is only needed when the amplifier operates in compression or near its maximum output power.
Can I simulate hot S22?
Yes, with a nonlinear device model (such as a foundry-provided GaN or GaAs FET model) in a circuit simulator (Keysight ADS, NI AWR). Perform a harmonic balance simulation at the operational drive level and extract the large-signal output reflection coefficient. Large-signal output match simulation is a standard step in PA design. The simulated hot S22 should be verified by measurement to validate the model accuracy.