What's the highest frequency people have tested IRF series MOSFETs?

[Kage]

Not sure where I saw the schematic but there was one someone made showing it was possible
to get around 5W(?) out on the low side of the FM broadcast band. Can't remember if it was the
IRF510 but it was in that ballpark. I'm sure the efficiency would be laughable but possibly still
useful.

I'd think the gate capacitance would become so high that they would be useless in the lower VHF
range but I've never actually tried to see the limits.

Seems there are hams who use them in the 6m band so if they can do 50MHz that begs the question
where will the maximum usable frequency of amplification become basically unity gain (if I should
call it that), or useful amount of amplification before it's useless?

[redhat]

In commercial designs, 13-15 MHz is about the limit for power devices in the 500W class.  Smaller fets you should be able to run quite a bit higher, although 40-50 MHz is likely the practical limit.

+-RH

[Charlie_Dont_Surf]

In fact there are two commercially available QRP transceivers, the uBITX and the sBITX from HFSignals.com, that use the IRF510 in the finals and these transceivers can be used at 10 meters.

The IRF510 is the lightest load (in terms of Ciss and Qg) of the IRF and IRL series of transistors, closely followed by the IRL510, and they are very cheap because they are very old. I believe that they were released in the late 1980s. The IRF510 typical Ciss (180 pF) approaches that of many lower frequency RF transistors I have seen so it would be interesting to see how high they can be pushed.

Difficulties:

• The IRF510 is not characterized for HF or VHF operation so you would have to either make some assumptions (read: guesses) for small-signal parameters for the matching design at those frequencies, measure them yourself or just tweak your amp forever until you are happy.
• The transistor package (TO-220) is quite high inductance by modern standards for 100 MHz.
• Finally, because the transistor is not intended nor characterized for operation above maybe a few MHz (probably less), that means that the manufacturer is not keeping an eye on  parameters that matter for RF performance over the manufacturing lifetime so certainly not looking at things like transconductance at RF, Cgd, Cgs, etc., thus the batch lot-to-batch lot variation could be much more than you think. You might build one amp and it will be fine then build another one a year later and can't get it to work well at all.

[Kage]

I did find these schematics from WSQT FM pirate designs on the homebrewpirateradio groups.io ...

5 watt IRF510 driver
https://i.imgur.com/2hnudUy.jpeg
23 watt IRF510x2 PA
https://i.imgur.com/oeATltP.jpeg

Took me a while to remember where I saw those schematics back in the day as they always peeked
my curiosity.

I have a lot of junked PSU and controller boards with other interesting mosfets including smaller ones
compared to the IRF510 which has me itching to dig through my scrap and look up their gate capacitance
and other parameters to see if I can find some that work up into VHF even though they were absolutely
never intended to

Maybe worth a shot to experiment since I already have a 1 watt driver to force into their gates and
some free time to dork around at the workbench.

[Charlie_Dont_Surf]

5 watt IRF510 driver
https://i.imgur.com/2hnudUy.jpeg
23 watt IRF510x2 PA
https://i.imgur.com/oeATltP.jpeg

Since these schematics indicate gate bias at 3.5 to 4.0 V, this means that the transistors will be just barely off without RF drive and these amps are probably Class B or maybe Class C. (You can certainly guarantee Class C by reducing the bias voltage.) Since I favor efficiency, Class C would probably be the recommended low-complexity amplifier class for a constant envelope signal such as FM. Class C efficiency "should be" (quotation marks added for sarcasm) in the 60% range (67% theoretically), if  the RF gods are taking a liking to you. Though Class B should be higher gain than C.

That being the case, the 40-50% efficiency is likely due to the numerous losses that the IRF510 will have at 100 MHz. I have no idea if this is true but I'm also visualizing that the author did this "deadbug" on a copper groundplane PC board and that could be the other major culprit. While this sort of thing can be quite compact, there will likely be more parasitic L and C with such a construction method and those two items are a major limitation.

In any case, it's interesting that someone got usable output at ~100 MHz.