Class-C, D, and E RF power amplifiers are all about high efficiency. They are used in large transmitters and industrial induction heaters, where high efficiency reduces the power bill and saves on cooling equipment, and also in the smallest transmitters, such as cell phones, where high efficiency increases battery life. These amplifiers are so nonlinear (the output signal amplitude is not proportional to the input signal amplitude), they might better be called synchronized sine wave generators. They consist of a power supply, at least one switching element (a transistor or vacuum tube), and an LCR circuit. The “R” is the load, RL, often the radiation resistance of an antenna, equivalent to a resistor. The LC network is resonant at the operating frequency. The output sine-wave amplitude, while not a linear function of the input signal amplitude, is proportional to the power supply voltage. Thus, these amplifiers can be amplitude modulated by varying the supply voltage. Of course they can also be frequency modulated by varying the drive frequency (within a restricted bandwidth, determined by the Q of the LC circuit). Finally, they can be used as frequency multipliers by driving them at a subharmonic of the operating frequency.

The class-C amplifier

Figure 9.1 shows a class-C amplifier (a), together with an equivalent circuit (b). The circuit looks no different from the class-B amplifier ofFigure 3.15 or a small-signal class-A amplifier.