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Stability criteria

The primary concern in the design of a high-frequency amplifier is to ensure that the active device does NOT oscillate at any frequency, especially at the frequency of operation.

A two-port active device is unconditionally stable if both its input and output impedances have positive real part for ANY passive source and load terminations. In S-parameter terminology, it is to say

It can be proved, will be shown in below, that the necessary and sufficient conditions for unconditionally stable operation are

  1. for text_wrap_inline3021 and text_wrap_inline3023 in most of the practical situation, these conditions are natural for a transistor, otherwise the transistor could have been oscillating under the measuring termination.
  2. text_wrap_inline3025 and text_wrap_inline3027.
  3. The stability factor K, also known as Rollet stability factor, is greater than unity

Note that a unilateral device, text_wrap_inline2965, is always unconditionally stable.