3.2.1 Introduction

Linear Class A power and small-signal amplifiers can be designed by S parameters. Nonlinear, Class B and C power amplifiers cannot reliably exploit these parameters, but instead must depend mainly on large-signal input/output parameter design. These values can be found in the power transistor s data sheet in rectangular notation (such as 1.1 j3.2) for its series input and output impedances at a number of frequencies, and at a specific V _CC and P _OUT. The series input and output impedances can be made available as a Smith chart representation, or in a much easier to read tabular format on the device s data sheet. If desired, the series impedance values can be converted to a shunt impedance (as presented earlier in this chapter), if this information is not so provided. When provided, they may be in the form of a separate graph of input and output parallel equivalent resistance versus frequency, and a graph of input and output parallel equivalent capacitance versus frequency, at a set V _CC and P _OUT.

Designing Class C power amplifiers with small-signal S parameters will result in a circuit that is not optimized and will not function as intended. This is because any transistor s input resistance, capacitance, gain, and output resistance will be significantly different when the device is run as a large-signal power amplifier as opposed to a small-signal, Class A amplifier. However, sometimes S parameters...