A resonant Class E power amplifier for low resistance load

In some applications such as radio transmitters with loop antennas, h.f. drivers for RFID transmitting coils, wireless h.f. power supplies for biomedical implants and endoscope capsules etc. resonant power amplifiers operate with high-reactance and low-resistance load. The paper analyses an off-nominal Class E ZVS amplifier to identify conditions for its high-efficiency operation with low-resistance load. Power loss of the transistor switch in Class E amplifiers with normalized transistor turn-on time D= 0.5 operating in nominal and off-nominal conditions are compared. Analytical closed-form estimations for power losses in the transistor switch in the off-nominal amplifier are given. Theoretical results experimentally validated have demonstrated that by designing the off-nominal Class E amplifier for a high enough dc supply voltage the power losses in the transistor switch can be significantly reduced. This makes the circuit useful in those applications that require the amplifier to operate with high output current and/or low-resistance load. A laboratory model of the off-nominal amplifier was designed and built to operate at the frequency 140 kHz with output power 50 W, dc supply voltage 24 V, load resistance 2 n and D= 0.5. Its measured efficiency was 96 %.