Conditional vs. Unconditional Amplifier Stability

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

Amplifier stability factor

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.

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
RF AM to PM Conversion Calculator

RF AM to PM Conversion Calculator

Calculate RF AM to PM conversion using our online calculator. Understand the impact of AM to PM conversion on RF amplifier non-linearity.

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RF Amplifier Drain Efficiency Calculator

RF Amplifier Drain Efficiency Calculator

Calculate the drain efficiency of an RF amplifier using this online tool. Enter output power (Pout) and DC power (Pdc) to determine efficiency.

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