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Old 20th March 2011, 01:56 AM   #11
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I would tend to think that FETs with the large die areas would be decent for class A or AB amplifiers these days because the junction to case thermal resistance is so low. A 1/2 degree C per Watt is common place these days for even TO-220 packages. The lower that figure, the larger the die area and the easier it is go get heat out of the silicon.

Yeah, you might have to design a decent driver circuit of course. That should be the fun part !

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Old 20th March 2011, 03:08 AM   #12
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Originally Posted by abraxalito View Post
Well if we're really in price-no-object territory, how about this Cree one?

CREE | SiC MOSFET
Like those SemiSouth SiC JFETs, this looks like another device that's going to have to run at higher than usual voltages (attached). You could run 'em as source followers operating into an OPT at 600Vdc.

The SiC JFETs looked to be very linear. Not too sure these are so much better than Si MOSFETs operating at more convenient voltages.
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Old 8th May 2017, 11:20 PM   #13
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Old thread but still very pertinent...

How does one go about choosing the best output device?

I do subscribe to the argument that the workings of the brain are not well understood and there is not a good translation of what `sounds good' into something that is readily measured quantitatively. On the other hand there is an element of physics here, so we should at least be able to agree on the type of things that are important and how some parameters may be important:
Maximum operating voltage, current and power have some importance but many of the current generation of large devices laugh in the face of what we might subject them to.
High transconductance, lower thermal resistance and low capacitance seem to be important. But what else - do we have to go back to empirical observations or can we gather more insight from datasheets?
Nelson cited the use of the IXFN140n20P, but there are ~50 of these types of IXSYS SOT packages , some have lower Rds, some have higher Gfs and some crazy 1200V pucks that have relatively modest Ciss (albeit at $60 a pop).
Any rules of thumb (or even guidelines)?
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Old 12th May 2017, 12:31 AM   #14
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Originally Posted by BeardyWan View Post
Old thread but still very pertinent...

How does one go about choosing the best output device?
It doesn't matter very much. Transistors are high gain devices, and so the performance has more to do with the topology, so long as your transistors are capable of operating at the signal frequencies, and less with the active devices themselves. It's not like vacuum tube design that involves low gain devices where performance is more dependent on active device characteristics.

Topology is more critical: BJTs for the OPT-less, SEPP output. NPN/PNP complimentary pairs are a good deal more complimentary than N-Channel/P-Channel "complementary" pairs. I also prefer Sziklai pairs or Q-comp finals over the CFP topology. The SEPP topology originated for BJTs, and it's with BJTs that it performs best. If you're using MOSFETs, then stick to one type: N-Channel or P-Channel, but not both in the same circuit. You can use MOSFETs in either the Circletron topoloogy, or use an OPT for push-pull output, or Class A single ended. The latter could include passive loading by means of a choke or active load.
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