Simply put, an amplifier is said to be unconditionally stable if there are no passive source or load impedances which can induce an oscillation. Unfortunately, this terminology results in occasional misconceptions among design engineers.
What follows is a brief practical look at “unconditional stability” for small-signal amplifiers from an applications engineering perspective. Power amplifiers have the same issues, but they are addressed differently since few lab setups have the capability of accurately measuring large-signal S-parameters.
Derived from S-parameters, stability indices such as the Rollett Stability K-factor or the geometric stability factors (Mu or Mu Prime) define an unconditionally stable 2-port network when their value is ≥1.0. The key thing to note is that these stability indices are derived from measured S-parameters. Why does this matter?
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