TABLE OF CONTENTS This chapter will introduce what will be termed conventional, or reduced conduction angle, high efficiency amplifier modes. These are the familiar Class AB, Class B, and Class C configurations.
The concept of making a more highly efficient RF amplifier by biasing the active device to a low quiescent current and allowing the RF drive signal to swing the device into conduction is very old, dating back to the earliest days of vacuum tubes. For this reason, it is often considered to be an elementary subject, not worthy of extensive discussion. In fact, we will see that there are many issues that come out of some elementary analysis, based on ideal device models, that are of great significance in the context of modern wireless communications systems.
Straightforward but more detailed analysis than is often presented in elementary textbooks will show that merely reducing the conduction angle of an RF power device is necessary, but frequently not sufficient to obtain a useful improvement in efficiency. In general, it is necessary to increase the drive level substantially from the Class A condition and to provide suitable impedance terminations at harmonics of the signal frequency. Most older textbooks assume that all higher harmonics will be shorted at the output of the PA device. This simplifies the analysis and was a much easier condition to realize in the days of tube amplifiers. This has led to some confusion regarding the kinds of matching topologies which should be used for today s transistor counterparts, and...
