Conditional vs. Unconditional Amplifier Stability

This article explores the difference between conditional and unconditional stability in amplifiers. It also touches upon the amplifier stability factor, K.

Unconditional stability in an amplifier occurs when the stability factor K is greater than or equal to 1 (K >= 1). In this scenario, no combination of input and output terminations can cause the transistor to become unstable.

Conditional stability, on the other hand, arises when K is less than 1 (K < 1). Here, stability is dependent on the source and load terminations. In these cases, stability circles are necessary to determine which terminations will keep the device stable and which will lead to instability.

How to Identify an Unstable Amplifier

So, how can you tell if an amplifier is unstable? Here are a few telltale signs:

• High Narrowband Gain Peak: An unstable amplifier will exhibit a significant gain peak within a narrow frequency range.
• Poor Performance: Expect to see degraded noise performance, linearity, and port impedance.

RF K-factor Calculator

For more detailed information on calculating the K-factor using complex S-parameters (S11, S22, S12, and S21), refer following calculator page and try following inputs.

• RF K-Factor Calculator

Example RF K-factor Calculation

INPUTS:

• S11 (magnitude, angle) = (0.2552, 156.2°)
• S22 (magnitude, angle) = (0.1544, -95.3°)
• S12 (magnitude, angle) = (0.0994, 41.5°)
• S21 (magnitude, angle) = (5.636, 53.1°)

OUTPUT:

• RF Amplifier K-factor (Stability factor) = 1.0618