Radio Frequency Integrated Circuit Design - Webs

70 Radio Frequency Integrated Circuit Design
Figure 4.8 The eight possible impedance-matching networks with two reactive components.
4. Stability. Since transistor gain is higher at lower frequencies, there may
be a low-frequency stability problem. In such a case, sometimes a highpass
network (series capacitor, parallel inductor) at the input may be
more stable.
5. Harmonic filtering can be done with a lowpass matching network
(series L, parallel C ). This may be important, for example, for power
amplifiers.
Example 4.2 General Matching Example
Match Z = 150 − 50j to 50� using the techniques just developed.
Solution
We first normalize the impedance to 50�. Thus, the impedance that we want
to match is 3 − 1j. We plot this on the Smith chart as point A, as shown in
Figure 4.10. Now we can see from Figure 4.9 that in this region we have two
possible matching networks. We choose arbitrarily to use a parallel capacitor
and then a series inductor. Adding a parallel capacitor moves the impedance
around a constant conductance circle to point B, which places the impedance