"Kevin McMurtrie" wrote in message
...
In article ,
"Trevor Wilson" wrote:
"RichD" wrote in message
...
Who do MOSFET sound better than bipolar, as an audio amp output
driver?
**Non-sequitur. If you're saying that MOSFET outputs sound better, they
don't.
As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
exhibit VASTLY more THD than BJTs. Only when bias currents are elevated
(around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are
almost
as good as BJTs.
MOSFETs are very tough, have an exceptional ability to deliver high
power,
high frequency audio (and RF), but distortion is very high. They require
lots of Global NFB in order to operate linearly.
That's nonsense.
**Er, nope. Here are the specs for a modern high power BJT:
http://www.toshiba.com/taec/componen...c//66/7890.pdf
Note the hFE vs. Ic. Particularly at elevated temps. It is almost a straight
line, from less than 10mA to several Amps. I call that spectacular
linearity.
Now, I draw your attention to a high power MOSFET:
http://www.magnatec-uk.com/pdf/magnatec/BUZ900.pdf
Note the characteristics of this device. They're pretty good, but as good as
a modern BJT.
Here is another, older, worse example:
http://www.datasheetarchive.com/pdf-...SA-158541.html
Quite ordinary lineariy.
MOSFETs have a lower voltage gain, more capacitive
load, and infinite DC current gain. If you swap your bipolar
transistors for MOSFETs and crank up the bias voltage, yeah, it will run
like crap because the local feedback gain is totally wrong.
**I never suggested anything of the sort. I mentioned ONLY the intrinsic
linearity of the devices. MOSFETs are inferior to BJTs. For now. When
installed in an appropriate topology, it is likely that there will be little
audible, nor measurable difference between a MOSFET amp and a BJT amp.
Device linearity is another story.
It might
even incapacitate every radio within two blocks of your home then
violently explode. The circuits to drive MOSFETs and bipolars are a
little different.
**Again, not in dispute. The intrinsic linearity, is what I refer to.
--
Trevor Wilson
www.rageaudio.com.au