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Old 17th August 2006, 06:14 PM   #1
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Default JFET aleph- design comments please

Hello,
I am planning on building the attached circuit, and I would like some feedback on the design. Please point out anything you see wrong, or that could be better. Thanks.
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Old 17th August 2006, 07:31 PM   #2
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Default Re: JFET aleph- design comments please

Quote:
Originally posted by nobody special
Hello,
I am planning on building the attached circuit, and I would like some feedback on the design. Please point out anything you see wrong, or that could be better. Thanks.

current clumping circ in upper output fet's gate is missing but-who cares;

what I care is that 2,7mA through driving half of input diff stage is somewhat much less than I use (or I dare to use).
all in all - I can't see anything more to comment
go for it
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Old 17th August 2006, 08:12 PM   #3
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Thank you for the critique.
What is a typical bias current for a differential pair of 389's? Where is the sweet spot?
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Old 17th August 2006, 08:18 PM   #4
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If you are using IR parts, I would consider P channel JFETs
in the input and drive the irfp240 parts, which have better
performance in CS than the P channel IRFP9240.
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Old 17th August 2006, 08:24 PM   #5
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two key concerns are:
1. how much current the jfets can handle safely/linearly/etc. and
2. how much current is needed to drive the output power mosfets to meet/exceed your "desired performance" (e.g. distortion, frequency response, etc.)

need to consult the datasheets and characterize your particular devices to bias at "optimum" points.

lots of good hints in nelson pass articles on http://passdiy.com/

good luck,
mlloyd1

Quote:
Originally posted by nobody special
Thank you for the critique.
What is a typical bias current for a differential pair of 389's? Where is the sweet spot?
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Old 17th August 2006, 08:33 PM   #6
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Quote:
Originally posted by Nelson Pass
If you are using IR parts, I would consider P channel JFETs
in the input and drive the irfp240 parts, which have better
performance in CS than the P channel IRFP9240.

in other words- buy few dozens of J176 p-fets and do what BillWW does with his Aleph
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Old 17th August 2006, 08:37 PM   #7
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Quote:
Originally posted by Nelson Pass
If you are using IR parts, I would consider P channel JFETs
in the input and drive the irfp240 parts, which have better
performance in CS than the P channel IRFP9240.
Thank you, Nelson. It's always exciting to get a direct response from you.
Is the problem with the N-channel Jfets due to the kink in the response of the IRF P-channels that you mentioned in your article?
What would you recommend for a dual p-channel jfet, and where could I buy them?
I had considered this, but I have the LSK389's laying around. I don't mind buying something else if it will improve the performance, and I do have a bigger selection of 240's than 9240's, so that would work out.
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Old 17th August 2006, 08:39 PM   #8
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Quote:
Originally posted by mlloyd1
two key concerns are:
1. how much current the jfets can handle safely/linearly/etc. and
2. how much current is needed to drive the output power mosfets to meet/exceed your "desired performance" (e.g. distortion, frequency response, etc.)

need to consult the datasheets and characterize your particular devices to bias at "optimum" points.

lots of good hints in nelson pass articles on http://passdiy.com/

good luck,
mlloyd1

mlloyd1,
Thanks for the summary- good advice. I was just being lazy. I figured someone who had used them before could comment on where they found them to sing the best.
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Old 17th August 2006, 09:10 PM   #9
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Quote:
Originally posted by nobody special
Is the problem with the N-channel Jfets due to the kink in the response of the IRF P-channels that you mentioned in your article?
What would you recommend for a dual p-channel jfet, and where could I buy them?
You can avoid the "kink" by using Harris or Fairchild, not that it's
a big problem, and for P channel JFETs you can try 2SJ74 or
(matched pair) 2SJ109.

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Old 17th August 2006, 09:39 PM   #10
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Quote:
Originally posted by Nelson Pass


You can avoid the "kink" by using Harris or Fairchild, not that it's
a big problem, and for P channel JFETs you can try 2SJ74 or
(matched pair) 2SJ109.

hehe
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at least now when young Kentucky boy is first in Moto GP ,and hearts are blazed.....
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