Pros and Cons of MOSFET VAS - diyAudio
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Old 17th May 2014, 01:55 PM   #1
Belaji is offline Belaji  United States
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Default Pros and Cons of MOSFET VAS

I always thought that the ubiquitous three transistor VAS was somewhat artificial. First we add a darlington to get the VAS input Z up, then we get too much current so we add another current limiting circuit. Why not something more direct.

Here is an approach I like, the MOSFET VAS. No base current (high Zin but some more Cin) and the maximum VAS current is set by R9. The MOSFET acts as a variable resistor in the Q6 current source. I'd like to get a variety of opinions on this.

Thanks,

Bela
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Old 17th May 2014, 02:26 PM   #2
gannaji is offline gannaji  India
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Kindly post the .asc file.
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Old 17th May 2014, 04:59 PM   #3
udok is online now udok  Austria
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To get realistic THD results, include the output stage. The output stage causes nonlinear loading.
The Mosfet with cascoce transistor Q6 does limit the max voltage swing which is bad.
The high input impedance of the Mosfet is of no use as the feedback capacitor C3 decreases the impedance to very low levels.
Mosfet normally have high capacitance and are noisier than bipolars.

Greetings,
Udo
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Old 17th May 2014, 09:14 PM   #4
Belaji is offline Belaji  United States
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Testing the input and VAS stages separately can give valuable insights. Pick your favorite output stage.
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Old 17th May 2014, 11:03 PM   #5
tvrgeek is offline tvrgeek  United States
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I suspect there is a pretty good reason most amps have started to look very much the same. Sort of "Blameless" with a few variations. Why is this ubiquitous? Well, because it works pretty darn well. The 99% fix if you will.
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Old 18th May 2014, 12:47 AM   #6
Belaji is offline Belaji  United States
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Sure it works well. But it seems inelegant to me. To me, there is art in electronics design... beauty in simplicity. The typical three transistor VAS seems more like brute force. Beta enhance, now too much current, oh lets put in a fix. I'm not saying this MOSFET approach is better, perhaps more beautiful in purpose.

Last edited by Belaji; 18th May 2014 at 12:51 AM.
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Old 18th May 2014, 01:04 AM   #7
tvrgeek is offline tvrgeek  United States
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If you are in business, performance and cost counts. "Elegant" is for DIY. Go for it.
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Old 18th May 2014, 01:23 AM   #8
Belaji is offline Belaji  United States
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I do both.
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Old 18th May 2014, 02:07 AM   #9
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I have some good result with JFET (2SJ74BL) at VAS. But about MOSFET, what kind to fit?
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Old 18th May 2014, 09:41 AM   #10
AndrewT is offline AndrewT  Scotland
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Looks like M1 is running @ ~ 1.1Vds
Is that enough to get excellent performance?

Have a look at doubling R5 & R6 values.
Try tripling R7 & R8 values.

Do either of these changes compare well or badly to existing values?

Q6 is running hot, what about a To126 device?
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Last edited by AndrewT; 18th May 2014 at 09:46 AM.
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