Recall available power gain equation
(166) can be rearranged as an equation of circle on plane
For each value of , we have a corresponding
circle with
Note that the centers of circles are always on the line drawn between
and the origin of the
plane. The radius of
circle is getting smaller for a larger
.
In case of an unconditionally stable device, when
,
reaches its maximum, i.e.,
.
In case of a potentially unstable device, when , i.e.,
, the
circle equals to output stability circle.