What is the stability factor K and why does it matter when selecting an amplifier for my design?
Amplifier Stability Analysis
Oscillation occurs when the round-trip gain around a feedback loop exceeds unity. In an amplifier, the S12 parameter (reverse transmission) creates a feedback path from output to input. If the source and load impedances create sufficient positive feedback through S12, the amplifier oscillates. The stability factor K quantifies the margin against this condition.
| Parameter | LNA | Driver | Power Amplifier |
|---|---|---|---|
| Noise Figure | 0.3-2.0 dB | 3-8 dB | 5-15 dB (not specified) |
| Gain | 10-25 dB | 10-20 dB | 8-15 dB |
| P1dB | -10 to +10 dBm | +15 to +25 dBm | +30 to +50 dBm |
| OIP3 | +5 to +25 dBm | +25 to +40 dBm | +40 to +55 dBm |
| DC Power | 10-100 mW | 0.5-5 W | 5-500 W |
Frequently Asked Questions
What if K < 1 at some frequencies?
Add stabilization: resistive loading at the input or output (reduces gain but increases K), or negative feedback (series or shunt feedback resistors). Place the stabilization elements to affect the unstable frequencies without degrading in-band performance. Common approach: series resistor in the gate/base bias feed for low-frequency stabilization.
Is unconditional stability always needed?
For production amplifiers and gain blocks: yes. For laboratory prototypes with controlled impedance environment: conditional stability may be acceptable if the source and load impedances stay within the stable region. But conditional stability is risky and not recommended for deployed systems.
Does stability change with temperature and bias?
Yes. S-parameters change with temperature and bias point, and K changes accordingly. A marginally stable amplifier at room temperature may oscillate at cold temperature (where gain increases) or at different bias voltages. Stability must be verified across the full operating temperature and bias range.