5.3 Amplifier Design Stages
211
Input stability circles allow us to identify the region of the Smith chart which contains
terminations which, if connected at the input of the transistor, will make the magnitude of the
output reflection coefficient S22, greater than one.
The output stability circles help us identify the region of the Smith chart which contains
terminations which, if connected at the output of the transistor will make the magnitude of the
input reflection coefficient S11 greater than one.
Since oscillations are possible when either input or output reflection coefficients are greater
than one, these
circles greatly help us avoid picking "unfriendly" terminations which may make the
transistor oscillate.
Microwave Office allows us to plot stability circles very easily as we will see in video 5.3
and 5.4 and clearly identify whether the inside or the outside of the circles contain the
terminations which may cause potential instability. For instance in Figure 5.3-5, the dashed line
on the Input Stability circle plot (SCIR1) clearly indicates that the inside of the circle contains
the troublesome terminations.
Figure 5.3-5 Input stability circle
Similarly for the output stability circle (SCIR2), the set of terminations which may cause
potential instability is on the inside of the circle. This is again indicated by the dashed lines and
clearly marked in Figure 5.3-6.
Conquer Radio Frequency
211 www.cadence.com/go/awr
