The prince of FETs

Hans Summers <Hans.Summers@...>

I agree with all Jim's comments about the use of FET's salvaged from PSU's
etc in power amplifiers for bands 40m or below. I always like to use
"recycled" components wherever possible.

But when it comes to higher frequency amps, the IRF510 is in my opinion the
best choice available. A month or two I was considering a highly efficient
class E amp for 30m, for a /p rig. I wondered why the IRF510 was so popular
in QRP designs. Was it just fashion? Low cost (relatively)? So I read *many*
power FET datasheets to see if there was any better alternative, at ANY
price. Gate capacitance is an issue at higher frequencies, it must be as
small as possible. Similarly fast switching times are increasingly important
at higher frequencies. For best efficiency one wants low ON resistance. And
of course high power handling capability is essential.

Unfortunately, the choice of power FET necessarily involves some compromise
of these parameters. Larger device size means lower ON resistance and higher
power dissipation, but at the expense of higher gate capacitance and thereby
also slower speed. And so on. After all my searching, I was unable to find a
device that I considered offered a more optimum compromise of these
characteristics, for an efficient QRP HF amp, than the IRF510.

It therefore appears that this is THE best FET for QRP rigs, not just a
fashion or low cost, availability etc. All the more remarkable if there is
any truth in the story that I heard, about this FET originally being
designed for use switching car indicator lights. Or is this just urban
legend? Either way, long live the prince of FET's!

72/3 de Hans G0UPL

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