Chapter 2Class A RF Power Amplifier
2.1 Introduction
The Class A RF power amplifier [1–15] is theoretically a linear amplifier. A linear amplifier is supposed to produce an amplified replica of the input voltage or current waveform. It provides an accurate reproduction of both the envelope and the phase of the input signal. The input signal may contain audio, video, and data information. The transistor in the Class A RF power amplifier is operated as a dependent current source. The conduction angle of the drain or collector current is 
. The efficiency of the Class A RF power amplifier is very low. The maximum drain efficiency with perfect components is 50%. However, the Class A RF power amplifier is a nearly linear circuit with a low degree of nonlinearity. Therefore, it is commonly used as a preamplifier and an output power stage of radio transmitters, especially for amplification of signals with a variable amplitude of the output voltage, for example, in amplitude-modulated (AM) systems. In this chapter, the basic characteristics of the Class A RF power amplifier are analyzed. The amplifier circuits, biasing, current and voltage waveforms, power losses, efficiency, bandwidth, and impedance matching are studied.
2.2 Power MOSFET Characteristics
2.2.1 Square Law for MOSFET Drain Current
A MOSFET is used as a dependent current source in Class A RF power amplifiers. For low ...
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