Radio Frequency Integrated Circuit Design - Webs

352 Radio Frequency Integrated Circuit Design
Figure 10.2 Output versus input power.
Figure 10.3 Block diagram of amplifier and matching circuits.
10.4.1 Matching to S22 * Versus Matching to �opt
For low input power where the amplifier is linear, maximum output power is
obtained with �L = S 22 * . However, this value of S 22 will not be the optimum
load for high input power where the amplifier is nonlinear. Nonlinearities result
in gain compression, the appearance of harmonics, and additional phase shift.
The result can be a shift of the operating point and a shift in the optimal load
impedance. For these reasons, for large-signal operation, tuning is done by
determining the optimal load �opt, typically by doing an exhaustive search
called a load pull. The comparison between tuning for small signal and large
signal is shown in Figure 10.4.
As illustrated, the small-signal tuning curve results in higher output power
for small signals, while the large-signal tuning curve results in higher output
power for larger signals. Typically, if operation is at the optimal PAE point (as