I'm building a small APRS radio for a high altitude balloon, and I'm thinking of using an Si570 (3.3 V CMOS) at 144 MHz to drive a small class E amplifier. I'm looking for about 1W output power. I'm having trouble finding a FET that can operate at this frequency from 3.3 V power. Failing that, I could use two LiPoly batteries in series to power it with about 7 V. Does anyone know of a good device that I might use? -Rod (KD0FFJ)
First of all: Considering a class-E is a rather complex (not in parts count, but in sensitivity of the design to choosing the right Ls and Cs) thing, I'd go for: It won't kill you to use a step-up converter to first get a higher DC supply. These things are really simple to build, thanks to suppliers like Texas Instruments having wizards on their homepage where you just enter in-, output voltage and max current to source, and then are greeted with different optimizations (cost, size, stability) of possible designs including BOMs and schematics and further design helps.
Regarding choice of a FET: It all depends!
I don't know what's going to drive your FET's gate, but chances are you'd want one with a small gate charge so that you don't need insane driver strengths to switch it fast enough. Low Rdson is probably a must – you'll need a good switching behaviour to get your class-E amp running at all, and also, you're doing this because you want efficiency, I guess, instead of simply using "old-fashioned" amplifier architectures.
When looking for FETs, I usually consult my three main distributor's tables. They all have their web site weaknesses :( so I cross-compare a lot:
Now that I've told you to step up the voltage as needed, you're getting yet another design freedom – so you'd probably just want to use an already proven, evolved class-E design and use a FET fitting that (if in doubt, post that design as a new question here and ask for FET characteristics to look for, not for FET recommendations), and build an efficient supply for that.
All in all, at 1W on 144 MHz I'm not 100% class-E is really the optimum architecture. Then again, I'm not a switched-amp guru, so I trust your research and choice!