Class E/F amp

Here's what almost kept me out of the USA: electronic parts and gear to make a class E/F amplifier. I got it built, finally, and after a few tweaks, it looks pretty good. Here's the schematic for the main amplifier:

L3 and L4 represent the output transformer (with the coupling constant set by the Spice directive K1 L3 L4 1). L8 and L9 represent leakage inductance due to the imperfect nature of the transformer at RF. They are why I had to move the capacitance to the primary side of the transformer - with the capacitance on the secondary, large oscillations occur on the drains of the MOSFETs. I haven't tuned the drain supply inductors L1 and L2 for drain current flatness yet - at 4MHz, it's already pretty flat. I'm using IRLML0060 for the MOSFETs, as they will handle the drain voltage swing resulting from a 12-15V supply, but LTSpiceIV doesn't have them in the library - so I just use IRLML0040 in the simulation.

Here's what it looks like in dead-bug style construction:

 I have a little 78L05 to supply 5V to all the logic chips: 74VHC04 for the 4MHz crystal oscillator, 74AC00 for the keyer, 74AC04 for the driver. The keyer supplies complementary outputs to the driver, or not, depending on the state of the keyer input (the yellow jumper in the picture). With the yellow jumper disconnected, the keyer outputs are both high, so with the inverted driver outputs low, both MOSFETs are off and no current flows in the output. When it's grounded, the 4MHz complementary outputs alternately switch the MOSFETs on and off. The output LC tank circuit does the rest.

Powered up (using separate supplies for the logic and output stages, so I can vary the output stage voltage down to 1V or less, just to make sure it doesn't blow up):

The supply on the left is for the logic (40mA draw when it's all running), and the supply on the right is for the output. 12.06V*1.09A = 13.145W. Let's hope the poor little SOT23 MOSFETs aren't dissipating all of it! (They're only rated at 1.25W each, at 25C ambient.)

Probing the output voltage at the 50 Ohm dummy load:

(sorry it's a bit blurry - hard to hold the camera-phone steady enough)
The output voltage is 24.4V RMS (using a x10 probe), so at 50 ohms, that's 11.907W out. Drain efficiency is then 11.907/13.145 = 90.5%. Not bad for a first attempt. The MOSFETs could be dissipating (13.145-11.907)/2 = 0.62W each. That's within their specs, and the heatsinking on them isn't too bad - the source leads are soldered directly to the big copper groundplane, and the drain leads are soldered to big copper wires (2x16ga., one for the transformer, and one for the supply inductor).

I've tried ramping the voltage up higher too - at 15V supply, I get 20W out - not bad for a couple of transistors that are so small you can barely see them in the circuit photo! Four times the output power of my FT-817, but only on one frequency. I could use this little thing to send CW around the world (once I get a harmonic filter for it - you can see some of the nastiness on the output trace that would need to be eliminated  before sending the signal to an antenna).

-mark.