Linear vs Nonlinear Amplifier Operation
Linear region (small signal): Vout = G × Vin → S21 = constant, gain = G₀ As input power increases: Device enters nonlinear region → gain decreases Output power vs input power deviates from ideal linear P1dB: input power where gain = G₀ − 1 dB P_sat: saturation power (gain → 0, maximum output power) Typically: P_sat ≈ P1dB + 3 to +6 dB
AM-AM Distortion
AM-AM (amplitude-to-amplitude) conversion: Input envelope A_in → Output envelope A_out(A_in) Ideal (linear): A_out = G × A_in (straight line) Real PA: A_out < G × A_in above P1dB (gain compression) AM-AM model: A_out = G × A_in / (1 + (A_in/A_sat)²)^0.5 For OFDM signals (LTE, 5G, WiFi): Each subcarrier experiences different gain depending on instantaneous envelope → Constellation rotation and compression → EVM degradation
Back-Off Strategy
| Modulation | PAPR | Required Back-off from P1dB |
|---|---|---|
| CW (single carrier) | 0 dB | 0–3 dB |
| QPSK | 3.5 dB | 3.5–5 dB |
| 16-QAM | 6.5 dB | 6.5–8 dB |
| 64-QAM (LTE) | 8.5 dB | 8.5–10 dB |
| 256-QAM | 10.5 dB | 10.5–12 dB |
Note: S-Parameters Don't Capture Compression
S21 from a VNA at −30 dBm input gives only the small-signal (linear) gain. P1dB, gain compression curves, and AM-AM characteristics require power sweep measurements with a signal generator and power detector. For P1dB measurement: sweep input power from linear region to saturation, record where gain drops 1 dB.
RF View: RF View analyzes small-signal S21 gain from VNA .s2p files. For complete PA characterization, combine RF View's S-parameter analysis with bench P1dB and EVM measurements. Load PA .s2p for gain, match, and stability analysis. Free on Android.