Class E Switching-Mode High-Efficiency Power Amplifiers - from RF to Microwave
Class E power amplifiers achieve significantly higher efficiency than for conventional Class B or C. Class E operates the transistor as an on/off switch and "shapes the voltage and current waveforms" to prevent simultaneous high voltage and high current in the transistor; that minimizes the power dissipation, especially during the switching transitions. In the published low-order class-E circuit, a transistor performs well at frequencies up to about 70% of its frequency of good class-B operation.
Four (4) hour Tutorial Lecture "RF Power Amplifiers, Class A through S - How They Operate, and When to Use Each"
With at least ten lettered classes of RF power amplifiers, and several combinations of these classes, many engineers are confused about RF power amplifiers. The complexity of the subject is compounded by the fact that RF power transistor acts either as a high-resistance current source or as a low-resistance switch, or - in some amplifiers - as a high-resistance current source during part of the "on" interval and as a low-resistance switch during another part of the "on" interval (mixed-mode operation). The circuit topology does not define unambiguously the transistor operating mode or the amplifier class of operation.