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.
