How do you check RF amplifier stability?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

A two-port is unconditionally stable if no passive source or load causes oscillation (|Γ_in| < 1 and |Γ_out| < 1 for all passive terminations). The Rollett test requires K = (1 − |S₁₁|² − |S₂₂|² + |Δ|²)/(2|S₁₂S₂₁|) > 1 and |Δ| < 1, with Δ = S₁₁S₂₂ − S₁₂S₂₁. The single-parameter test is μ = (1 − |S₁₁|²)/(|S₂₂ − ΔS₁₁*| + |S₁₂S₂₁|) > 1.^[1]^[2]^[3]

What are stability circles?

Stability circles on the Smith chart bound the source and load reflection coefficients where |Γ_out| = 1 or |Γ_in| = 1. A conditionally stable device can still be used if the matching networks keep Γ_S and Γ_L in the stable regions at all frequencies, including far out of band.^[4]

Why must stability be checked across all frequencies?

Transistors have more gain at low frequencies, and matching networks present arbitrary impedances out of band. An LNA stable at 2.4 GHz may oscillate at 200 MHz or 8 GHz. Check K or μ from very low frequency to beyond f_max, and verify in layout simulation including bias networks.

How are amplifiers stabilized?

Series resistance at the gate/base or shunt resistance at the drain/collector (usually with a capacitor so it acts only at low frequency).
Resistive feedback from output to input.
Lossy bias feeds (resistor in series with the RF choke).
Source degeneration, which also helps noise match in LNAs.
Careful grounding: emitter/source via inductance can cause instability.

Frequently asked questions

Is K > 1 alone sufficient?

No; it must be combined with |Δ| < 1 (or an equivalent auxiliary condition). μ > 1 alone suffices.

Can a stable amplifier oscillate on the board?

Yes, through layout feedback, bias network resonances or package effects.

What about large-signal stability?

Small-signal tests do not guarantee stability under drive; parametric oscillation must be checked separately.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Stability and Power-Gain Invariants of Linear Twoports”. J. M. Rollett, IRE Transactions on Circuit Theory, vol. 9, no. 1, pp. 29-32. March 1962.
“A New Criterion for Linear 2-Port Stability Using a Single Geometrically Derived Parameter”. M. L. Edwards and J. H. Sinsky, IEEE Transactions on Microwave Theory and Techniques, vol. 40, no. 12. December 1992.
“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
“Microwave Transistor Amplifiers: Analysis and Design, 2nd ed.”. G. Gonzalez, Prentice Hall. 1997.

Categories: Amplifiers

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