J-FET & MOSFET VAS with current NFB - diyAudio
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Old 9th April 2011, 06:53 PM   #1
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Default J-FET & MOSFET VAS with current NFB

This schematics has no capacitors in the signal path, and demonstrates wide pass band even with standard easy obtainable parts.
Output voltage is rather stable, within 10mV. It is adjusted by the R5* resistor.
40mA idle current through T2 and T3 was choosen for using them without heatsinks. By adding heatsinks, the current can be encreased correspondingly by adjusting (decreasing) the R7.
I put the schematics together on the breadboard, and made some measurements.
Maximum undistorted sine signal is 17V RMS, the clipping is nicely symmetric.
Also shown the 100kHz square wave (output amplitude is scaled to the input), and 1MHz input and output sine waves.
Attached Images
File Type: jpg VAS with current NFB.JPG (30.9 KB, 511 views)
File Type: jpg 1kHz Clipping.jpg (95.2 KB, 482 views)
File Type: jpg 1kHz Maximum Voltage Swing.jpg (101.1 KB, 437 views)
File Type: jpg 100kHz Square.jpg (69.1 KB, 414 views)
File Type: jpg 1MHz Sine.jpg (88.9 KB, 391 views)

Last edited by VladimirK; 9th April 2011 at 07:00 PM.
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Old 9th April 2011, 07:46 PM   #2
lineup is online now lineup  Sweden
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It is not too often we see MOSFET in VAS.
But a pair of IRF610/9610 can deliver a good current with high speed.
Which shows in your images.
JFET at some mA will drive the GATES.
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Old 9th April 2011, 08:09 PM   #3
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Quote:
Originally Posted by lineup View Post
It is not too often we see MOSFET in VAS.
But a pair of IRF610/9610 can deliver a good current with high speed.
Which shows in your images.
JFET at some mA will drive the GATES.
I also plan to transform this circuit into a real power amp - by using special high Idss, high transconductance, low-Ciss j-FETs, like CP650 (or russian KP903A), we can achieve 100-200mA in the first stage, and to use 3A idle current through T2,T3 (these could be K1530 and J201). The only doubt, the current from power rails under signal is not constant, hence some special care is needed to power supply design.
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Old 9th April 2011, 08:11 PM   #4
MiiB is offline MiiB  Denmark
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with a cascode and and lateral output sets it can scale to a beautiful Poweramp...
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Old 9th April 2011, 08:24 PM   #5
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Originally Posted by MiiB View Post
with a cascode and and lateral output sets it can scale to a beautiful Poweramp...
As for using laterals, I don't think it's a good idea, since with such a simle circuit we have to look for (transcondactance at idle current / Ciss) ratio, and hear Toshiba's MOSFETS seem to be more appropriate (or Fairchild's).
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Old 9th April 2011, 08:48 PM   #6
MiiB is offline MiiB  Denmark
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Ohh you misunderstand me

I would insert a cascode to shield the Jfet from higher voltage... and put the laterals after the mosfet stage...with a simple resistor as a bias spreader...

This is similar to my take on Lineups One J-fet amp which i Really like.... but i really like the use of mosfets for a single ended VAS...I think it would make a very good amplifier...
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File Type: pdf J-fet-amp-1.pdf (26.6 KB, 217 views)

Last edited by MiiB; 9th April 2011 at 08:57 PM.
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Old 9th April 2011, 08:57 PM   #7
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Originally Posted by MiiB View Post
I would insert a cascode to shield the Jfet from higher voltage... and put the laterals after the mosfet stage...with a simple resistor as a bias spreader...

This is similar to Lineups One J-fet amp which i Really like..and have made PCB's for.. but i really like the use of mosfets for the VAS...I think it would make a very good amplifier...
This is a good idea, but me personally I shall try to stay with two stages only (Nelson Pass's school ...).
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Old 9th April 2011, 09:06 PM   #8
lineup is online now lineup  Sweden
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Nelson Pass school, of one or two stages and HEXFET output.
But he is just about the only one that can get low dist that way.
And even he must make some clever patented tricks to get it there.
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Old 9th April 2011, 09:21 PM   #9
MiiB is offline MiiB  Denmark
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As much as I like the simple Pass low wattage amplifiers, then he too uses extra stage(s) to get to higher power levels...
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Old 9th April 2011, 09:30 PM   #10
Bigun is offline Bigun  Canada
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I say stick to your guns Vladimirk, build it simple and listen to it. I bet it'll sound great on a single large full range speaker - and will do so without being used above 1W.
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