I'm thinking about using Steve's 8 FET Class E design that's posted for Class C instead. I like the idea of simplicity and no tuning involved. I do realize that there's a loss of power and efficiency by doing this. Also, the Class E design matches the low FET output impedance to something significantly lower than 50 ohms (by using the Class E output section). So, my 1:1 output transformer wouldn't be a great match for 50 ohms and power output would suffer.
Attached is a picture of what I've got so far. I've mounted the heat sinks (1 for each set of 4 FETs). The L-bracket in the center will hold the N-connector for the analog RF drive. The one on the right will be the output N-connector.
The plastic piece will hold the 16 ferrite cores for the output transformer, and the connectors on the right are for DC input. I just placed an order for all the cores I'll need (including the ones for the gate transformers).
I will use 48 vdc for carrier and use a modified Heising circuit just like the one I've built for my lower power solid state rig. This time though I'll use a really big audio amp.
Thoughts?
Jon
KA1TDQ
Jon having spent the winter doing most of what you describe allow me some thoughts on your planned project.
QIX Steves design is excellent and it will work well in Class D. If you use C2M0280120D FETs they switch so fast there is only a small improvement in class E efficiency (less than 5%). However you will still need some serious output filtering for harmonic reduction such as a 6 pole low pass filter, thats 3 inductors, 3 caps, some design work for impedance and probably a bunch of fiddling to get it right. Or use a series tuned L and variable cap and tune it then its done, and if set just right gives class E. And FETs are basically switches which work best with square wave drive not analog. A simple VFOscillator, Flip Flop divider and 74HC14 driver is all you need. I can post a schematic and board layout if you want it.
If you use two FT200-2 cores wound 1:1 for OP transformers they will run cooler and give you the added ability to add or remove a turn or two on the secondaries for the best impedance match.
I have a Peavey 300 Watt Industrial PA amp with a honking big OP transformer with 25 and 70 volt taps. It makes a great series modulator up to about 200W carrier then runs out of steam and clips. If you do this it will work BUT your 45V power supply needs to be well regulated as any trace of ripple will show up on your carrier as HUM. A linear regulator will get pretty hot and be bulky. A switcher will be much better, but hey guess what that is what a PW modulator is, a tricked out switching regulator with feed forward from the +45 to regulate out the ripple. So just build a PW modulator. Way more power than a PA amp, smaller, more efficient, etc, etc.
I guess what Im saying is just build it as Steve designed it with minor modifications because you will end up there anyway. Good luck. 73s Nigel VE3ELQ
