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

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

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