BJT, MOSFET or JFET? Which sound better in class-A - diyAudio
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Old 6th November 2011, 04:38 PM   #1
neazoi is offline neazoi  Greece
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Default BJT, MOSFET or JFET? Which sound better in class-A

This is a very general question which depends on many factors of course but in general which one should sound better in a single ended class-A stage, BJT, MOSFET or JFET?
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Old 6th November 2011, 10:10 PM   #2
CBS240 is offline CBS240  United States
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MMMmmmm J-fets..

Unfortunately power J-fets are a thing of the past.
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Old 6th November 2011, 11:49 PM   #3
AKSA is offline AKSA  Australia
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Try the new SemiSouth power JFETs. Aside from a stiff gate current, they are reputed to be very good by Nelson Pass.

Hugh
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Old 7th November 2011, 07:09 AM   #4
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I've seen more MOSFETS in single-ended class A amps than BJTs.

Must be because they require less circuitry as far as pre-drivers, temperature compensation e.a. go, therefore they fit in better with the minimalist philosophy of single-ended class A.
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Old 7th November 2011, 08:23 AM   #5
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Quote:
Originally Posted by AKSA View Post
Try the new SemiSouth power JFETs. Aside from a stiff gate current, they are reputed to be very good by Nelson Pass.
That would be the SJEP120R100A and SJEP120R063ASiC JFETs for audio

SiC Transistors | Semisouth Laboratories, Inc.

It would be nice if they published SPICE models...
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Old 7th November 2011, 08:42 AM   #6
neazoi is offline neazoi  Greece
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I can see a tension towards audio morfets of new audio jfets. What about bjt.
constructions like these have remarkable sound from my listening impression.
Le monstre
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Old 7th November 2011, 10:33 AM   #7
AndrewT is offline AndrewT  Scotland
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MosFETs in ClassAB do not seem to respond to optimal bias voltage/current in reducing crossover distortion. This makes ClassAB for BJT & FETs somewhat different.

Most knowledgeable commentators draw attention to the FACT that mosFETs seem to get better the more the output bias is increased. I recall Borbely mentioned 500mA of output bias as minimum recommendation for every mosFET amplifier.

It is only a small leap of faith to move from >=500mA of bias to full ClassA and then there can be no crossover distortion to attenuate.

To me, that seems to explain why mosFETs predominate in small ClassA amplifiers.
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Old 7th November 2011, 11:08 AM   #8
AKSA is offline AKSA  Australia
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Default Mosfet switching transitions in Class AB

Andrew,

You may not need that much. Here is the (simulated) crossover event depicted by the source resistor currents on a mosfet AB output stage. The pmos (R18) is rather more linear than the nmos (R17), but you can see the transition is well controlled with no sign of ringing or instability. This is into 8R//100nF, a difficult load, at 1KHz.

Quiescent current is just over 150mA.

I feel that modern mosfets do not lose out too much to bipolars these days.

Hugh
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File Type: gif Switchingoutputs160mA.gif (18.4 KB, 534 views)
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Last edited by AKSA; 7th November 2011 at 11:12 AM.
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Old 7th November 2011, 11:23 AM   #9
AndrewT is offline AndrewT  Scotland
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Hugh,
I do not think that your example is a good guide to what modern FETs can achieve in terms of reducing crossover distortion and doing so at lower bias currents.
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Old 7th November 2011, 11:58 AM   #10
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If properly designed they won't sound different at all, not even a tiny bit.
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