Recall unilateral transducer power gain
For a given device, is fixed, the values of and depend on , . Let's say, we want to investigate the effect of on . Notice that is maximum when
Note that equation of a circle on the complex plane is
(145) can be rearranged in the following form
for given value of , this equation provides the corresponding values of which are in a form of a circle with
This set of circles is called constant circles. Note that
Similarly, a set of constant gain circles can be drawn for the output port.
The constant circles can be drawn on the plane with
Example In a 50 system, a transistor has the following S-parameter at 1.3 GHz.
Plot a few constant unilateral power gain circles.
Because , and have the same set of constant gain circles. Let
The parameters of a set of constant gain circles are listed in the following table
Figure 18: A set of constant unilateral gain circles plotted on a Smith chart