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.