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.