What is the difference between conditional and unconditional stability in an RF amplifier?
Stability Classification
The stability circle on the source Smith Chart defines the boundary between source impedances that keep the amplifier stable and those that cause oscillation. Similarly, the load stability circle defines safe load impedances. The circles are calculated from the S-parameters and provide a visual tool for understanding the stability margin.
If the stability circle lies entirely outside the Smith Chart, all passive impedances are safe and the device is unconditionally stable at that frequency. If the stability circle intersects or lies within the Smith Chart, some passive impedances cause instability, and the amplifier is conditionally stable.
A common design error is to declare an amplifier stable because it does not oscillate on the bench with 50Ω terminations, without checking stability circles. An antenna with poor match (VSWR > 3:1) or a disconnected cable (open circuit) can present impedances that enter the unstable region. Production amplifiers must be unconditionally stable to prevent field failures.
Frequently Asked Questions
How do I stabilize a conditionally stable device?
Add resistance to reduce gain at the unstable frequencies. Options: (1) series feedback (source/emitter resistor + inductor for frequency selectivity), (2) shunt feedback (resistor from drain/collector to gate/base), (3) resistive loading on the input or output. Each method trades gain for stability. Choose the method that minimizes impact on the desired in-band performance.
Can a stability circle analysis replace S-parameter measurement?
No. The stability analysis uses S-parameters, so it is only as good as the S-parameter data. At frequencies where S-parameter data is unavailable (very low frequencies, beyond the measurement range), stability must be inferred from device physics or measured with appropriate test equipment.
What about stability with active loads?
K > 1 guarantees stability with passive loads only. If the amplifier drives an active load (like a negative resistance oscillator or another amplifier with S11 > 1), additional analysis is needed. Active loads can present impedances outside the Smith Chart, which the standard K factor does not address.