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In Reply to: Re: MOSFETs work fine too... posted by mlloyd1 on December 12, 2001 at 08:16:53:
Quite right. I've always referred to them as depletion and enhancement type MOSFETs. Hitachi are enhancement types. Are the IRFP240 depletion types?Depletion type MOSFETs are routinely used in switchmode power supplies (I think).
I note that complementary pairs are only linear when used in pairs and so a singled ended amplifier would not be linear if only half of the pair were present.
Counterpoint and Forte (among others) used what I suspect are depletion devices. They had to be in matched sets and these amplifiers had no feedback. They also destroy themselves in a cloud of smoke - I know because I got to rebuild around 20 or so amps. I used Hitachi MOSFETs in a redesigned output and the comments from owners was unanimous - the amps never sounded so good, not even when they were new!!
That was a surprise as the first one I rebuilt was for a HiFi shop manager who just wanted it going - we didn’t expect it to sound better than new though I designed the new output stage and apart from having no feedback it was a fairly regular design and similar to the original.
But the upshot is that I have to agree with you regarding MOSFET devices - there are at least two classes of MOSFETs and it is not good science to generalise about them. Toshiba MOSFETs are very different to the Hitatchi, for instance.
Kind Regards,
Robert Karl Stonjek.
Follow Ups:
There are no depletion mode high power devices that I'm aware of, however I remember reading about a company that was planning to do some medium power depletion mode devices. I've heard of lateral, vertical and HEXFETs. The IR devices are HEXFETs.
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