HF Class-E Power Amplifier with Improved Efficiency for Mismatched Loads

The reverse diode in a MOSFET prevents the drain voltage in a class-E power amplifier from swinging negative. This effectively provides automatic turn-on of the MOSFET at the time all charge has been removed from the drain capacitance. This in turn allows the class-E amplifier to maintain high efficiency over a range of load impedances. This paper first extends classic theory to include the effects of the reverse diode. Measurements of a 200-W, 13.56-MHz LDMOS PA confirm improved efficiency in the diode-conduction region. A simple “relocation” network is designed to move load impedances into the regions with the best efficiency. The example PA has efficiencies from 72 to 81 percent for all loads with SWR up to 2:1.