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)
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1$\begingroup$ hint: it's usually better to ask for characteristics to look for rather than for actual devices – shopping questions are off-topic, but design help questions are very on-topic here! $\endgroup$– Marcus MüllerCommented Apr 5, 2017 at 12:32
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$\begingroup$ A switching power amp like class E requires a very fast transistor compared to the operating frequency. It also requires deep and sharp/fast drive. Doing that at 144MHz is going to require at least a UHF-grade transistor. Some LDMOS made to work at 7V supply may work well. But going to class C or inverse class F would make your life easier. $\endgroup$– Ryuji AB1WXCommented Oct 11 at 22:21
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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:
- farnell/element14
- mouser
- digikey
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!
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$\begingroup$ Thanks for your extensive answer - very helpful. The CMOS version of the Si570 has a conventional digital CMOS output, a square wave at the frequency of interest. It's natural to think of a switching amp at that point, particularly since I'd have to have some kind of low-pass filter in any case, and I might as well choose component values for a class E amp as for something else. I hadn't thought about the input capacitance of the FET. That's certainly something to pay attention to. $\endgroup$ Commented Apr 6, 2017 at 17:45
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$\begingroup$ Efficiency has two faces here (1) will my battery die before the balloon is recovered, and (2) can my battery provide the necessary current while transmitting. I worry about (2) more than (1). I'll look into a step-up converter, particularly to see how much current would be required to drive it followed by a more conventional amplifier. My interest in class E stems from this article, "Class-E RF power amplifiers", QEX Jan/Feb 2001, Nathan Sokal WA1HQC. Sokal claims that once you design a class E, "it works as expected, without tweaking or fiddling." $\endgroup$ Commented Apr 6, 2017 at 17:57
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$\begingroup$ well "once you designed it properly": yeah, but that's the hard part (as with any device – a well-designed car will run for years and thousands of kilometers without tweaking, but you go and design a reliable car from scratch...) $\endgroup$ Commented Apr 6, 2017 at 20:35