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From: fudd@fluke.UUCP (Mark Freeman)
Newsgroups: net.audio
Subject: Re: Tubes/bipolar/FETs
Message-ID: <1073@vax2.fluke.UUCP>
Date: Fri, 29-Jun-84 11:14:11 EDT
Article-I.D.: vax2.1073
Posted: Fri Jun 29 11:14:11 1984
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This is a reply to some comments by D.A. Seifert:

	power-FETs are supposed to have some of the 'good' qualities
	of tubes. One of these is 'soft-clipping'.  

	Another spec that came out about the time power-FETs did is
	TIM (and its siblings).  FETs tend to be fast and have good
	bandwidth/low TIM.

	Are there any other theories of why FETs sound better than
	bipolars?  Anyone think bipolars sound better than FETs?

The advantages of FET's over bipolars that I am aware of are:

1) They are positive temperature coefficient devices.  This is mainly
an aid to designers.

2) They are faster devices.  This may help increase slew rate.

3) They are closer to linear devices.  This is probably a more
important factor in reducing TIM.  When slew-rate limiting was
"discovered" by the audio industry, high loop gain was identified as
the culprit.  FET's are square-law devices (and V-FET's are are almost
linear devices), and so allow less feedback for the same level of
distortion.  Some designers have gone to the extreme of "no feedback"
(well, each stage has its own feedback), to prevent the evil TIM from
appearing.

When monolithic op-amps with FET inputs appeared, they were noted for 
their fast slew rate.  But not for the reasons you might expect.  
They were fast because the FET's had lower gain than the bipolars they
replaced.  This allowed the miller capacitor in the voltage gain
stage to be reduced, and delayed the onset of saturation of the first
stage (the cause of slew rate limiting).

An excellent article about this subject, from someone at National, I
believe, appeared around 1975.  I can dig it up, if you really want to
get into this subject.