The class E transmitter I'm currently building uses 2 FETs in parallel and a drain voltage of 48 vdc. At 100 watts carrier, that should equal 2.32 amps drain current (assuming 90% efficiency).

Optimal load impedance for the FETs is .5 x (Edrain/Idrain). In my case it equals just over 10 ohms. The 50 ohm antenna and the loading cap match the output to that 10 ohm value.

My question is regarding the well known 3-diode keep alive schematic. I'm going to modulate this transmitter with a modified Heising circuit, and I want to use the keep alive circuit to prevent any negative peaks. On the AMWindow website, the schematic is applied to tube transmitters with the resistor's value in the circuit equal to plate load impedance. In a class E transmitter, would the value of that resistor be optimal drain impedance (10 ohms)?

In my 8-FET 75 meter rig, I think I used either 50 or 100 ohms in this same application. I have a modulation monitor and I've never gone over 100% negative, even with hitting over 125% positive.

Jon

http://www.amwindow.org/tech/htm/3diodeka.htm