Class E-A New Class of High-Efficiency Tuned Single-Ended Switching Power Amplifiers

The previous literature on tuned power amplifiers has not made clear the fundamental differences between amplifiers in which the output device acts 1) as a current source, or 2) as a switch. Previous circuits have often operated in contradiction to their design assumptions, resulting in the need for “cut-and-try” design. The new class of amplifiers described here is based on a load network synthesized to have a transient response which maximizes power efficiency even if the active device switching times are substantial fractions of the ac cycle. The new class of amplifiers, named “Class E,”1 is defined and is illustrated by a detailed description and a set of design equations for one simple member of the class. For that circuit the authors measured 96 percent transistor efficiency at 3.9 MHz at 26-W output from a pair of Motorola 2N3735 TO-5 transistors. Advantages of Class E are unusually high efficiency, a priori designability, large reduction in second-breakdown stress, low sensitivity to active-device characteristics, and potential for high-efficiency operation at higher frequencies than previously published Class-D circuits. Harmonic output and power gain are comparable to those of conventional amplifiers.