Class E AM Transmitter

Class E amplifiers are very efficient amps and are generally built with MOSFET transistors. The principle is to drive the MOSFET's gate input with square waves to quickly put the device into it's low ohmic region and to do this when the voltage across the drain of the MOSFET is at or near zero volts. This greatly reduces the heat dissipated by the MOSFET and increases efficiency. A choke value for the drain is chosen so that it resonates at the operating frequency, in combination with the parasitic capacitance of the drain and the output filter. The "fly wheel" effect of the resonant tank causes the drain voltage to drop to zero before the MOSFET is switched back on. Efficiencies of 70% or more can be achieved this way. This article describes a simple Class E transmitter and is shown built for 40 meters. It uses a 74HC02 NOR gate as a crystal oscillator. The amplifier delivers about 2 watts output with a 9 volt supply and about 4 watts with a 12 volt supply.

http://www.classeradio.com/