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Old 20th May 2003, 09:15 PM   #1
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Default DIY-Pass-OP with only N-Fets ?

Hi,

is it possible to built a DIY OP with N Fets only ?

How would that look like ?



Greetings, Bernhard
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Old 20th May 2003, 10:02 PM   #2
mirlo is online now mirlo  United States
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Default Huh??

What is an "OP"?

-- mirlo
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Old 20th May 2003, 10:27 PM   #3
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op amp
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Old 20th May 2003, 11:52 PM   #4
lohk is offline lohk  Europe
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You will probably like this one (from a forselltech pdf):
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Old 21st May 2003, 01:08 AM   #5
Xray is offline Xray  United States
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Mr. Pass has an article at http://www.passdiy.com/legacy.htm.
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Old 21st May 2003, 01:48 AM   #6
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Default Re: DIY-Pass-OP with only N-Fets ?

Quote:
Originally posted by Bernhard
Is it possible to built a DIY OP with N Fets only?
Quite possible, in the same way they used to with tubes.

An N channel differential pair is biased by a resistor or an N channel current source to the negative rail, and the Drain of the right hand (feedback Gate) device drives either a current source (N channel of course) or a resistor. This Drain also drives the Gate of an N channel output follower whose Source connects to the negative rail through either a resistor or current source. This source is the output of the amplifier.

Now this circuit will naturally have a large positive offset, so if you want to direct couple it, you have to insert a voltage drop (could be a battery, for instance) between the Drain of the 2nd diff pair transistor and the Gate of the output device.

If this isn't clear, I'm certain that one of our artistic readers will render us a schematic.

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Old 21st May 2003, 06:45 AM   #7
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Default Re: Re: DIY-Pass-OP with only N-Fets ?

Quote:
Originally posted by Nelson Pass

Now this circuit will naturally have a large positive offset, so if you want to direct couple it, you have to insert a voltage drop (could be a battery, for instance) between the Drain of the 2nd diff pair transistor and the Gate of the output device.

Wow, you did read my mind.

In the end I need +/- 160V rails and +80V offset to drive a PL519 tube.
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Old 21st May 2003, 03:42 PM   #8
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Quote:
Originally posted by lohk
You will probably like this one (from a forselltech pdf):
This uses not only N-fets, but also P and self conducting (?) and is not single ended.




Quote:
Originally posted by Xray
Mr. Pass has an article at http://www.passdiy.com/legacy.htm.
The page cannot be found
The page you are looking for might have been removed, had its name changed, or is temporarily unavailable.




I like to use IRF460 everywhere want to make it work as a subtractor: Signal - negative Voltage = Signal with positive offset.

Maybe I can render Nelsons wors to a circuit
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Old 21st May 2003, 03:48 PM   #9
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Leave out the final full stop (.) and the link will work

Cheers

Andrea
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Old 21st May 2003, 04:45 PM   #10
Henrik is offline Henrik  Denmark
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Hi Bernhard

I have used this scematic for driving a pushpull outputstage.
Its a kind of an very mini Aleph, a little different than what Nelson suggests.

Happy project!

Regards Henrik.
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