I. Introduction
The class-E oscillator [1]–[7] offers high efficiency at high frequencies because it satisfies the class-E zero-voltage switching and zero-voltage-derivative switching (ZVS/ZVDS) conditions [8]–[14]. The class-E oscillator can be classified into two categories. One is the free-running oscillator and the other is the injection-locked oscillator. The injection-locked techniques allow the class-E oscillators to achieve a high overall efficiency and a high stability of oscillation frequency [5]–[7]. In many applications, the class-E oscillators can perform the same functions as the class-E amplifiers. They are applicable to FM oscillators, AM transmitters, electric ballast, the inverter part of dc/dc converters, and more. The switch-current waveform of the class-E oscillator, however, includes a jump at the switch turn-off instant, which deteriorates the power conversion efficiency and increases the circuit-implementation cost.