6.8 Broadband Class E

The conventional design of a high-efficiency switched-mode tuned Class-E power amplifier requires a high Q _L-factor to satisfy the necessary harmonic impedance conditions at the output device terminal. However, if a sufficiently small value of the loaded quality factor Q _L is chosen, a high-efficiency broadband operation of the Class-E power amplifier can be realized. For example, a simple network consisting of a series resonant LC circuit tuned to the fundamental frequency and a parallel inductor provides a constant load phase angle of 50 in a frequency range of about 50% [31]. For the first time, such a reactance compensation technique using a single-resonant circuit had been applied to the varactor-tuned Gunn oscillator and parametric amplifier [32]. Moreover, it became possible to increase the tuning range of an oscillator by adding more stages of reactance compensation. For instance, for a resonant circuit having a 50 ? load, an improvement of 4% in the tuning range can theoretically be achieved as a result of applying a double-resonant circuit reactance compensation, whereas, for a resonant circuit operating with 100 ft load, an increase in the tuning range is 17% [33].

Computer-aided design of broadband microwave-transistor amplifiers normally requires considerable time and can be quite tedious. Therefore, an analytical approach, which is capable to define common regularities and express in explicit and simple form the ratios between load network elements, can be useful to substantially shorten and simplify the calculation procedure. Combined with an analytical...