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Old 24th August 2008, 03:53 PM   #1
rkn is offline rkn  Germany
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Default Fully balanced Opamp

Hello,

I'm not sure where to post this, but this project is inspired by Nelson Pass' article about Opamps so I guess it is the right place here.
My idea is to build a discrete opamp with balanced inputs and outputs. My design follows mainly Nelson Pass' article but without the feedback loop. This can be attached externally if wanted. I'm going to make a seperate PCB for the discrete Opamp. Below you can see my design. I used JFETs inputs and a bipolar stage on every side.
Maybe the values I've selected are not good so comments regarding the design are welcome!
one point where I'm not sure is wether I should use two resistors in series with the jeft inputs or not. In Nelson Pass article you have these resistors but if you look on other discrete Opamp designs you don't have them.
I connected the three constant current sources to one reference. i hope it is allowed to do that. I've then that in some designs of power amplifiers but only for two current sources not for three like in his design.

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Greetings,

Raphael
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Old 24th August 2008, 08:33 PM   #2
rkn is offline rkn  Germany
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Sorry, I forgot to refer to GRollins and his reverse engineering of the XVR1! As you can see the design shows some similarities to his design. I apologize for forgetting him!

Raphael
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Old 24th August 2008, 10:28 PM   #3
rkn is offline rkn  Germany
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Ok, added some further stuff and changed some values. The "biggest" change is, that there is now a current mirror inserted into the dif pair.

Greetings,

Raphael
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Old 25th August 2008, 06:16 AM   #4
jam is offline jam  United States
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Schematic?
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Old 25th August 2008, 07:48 AM   #5
rkn is offline rkn  Germany
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Schematic? You shoul dbe able to see the first version in my first post.
For the new version with the added stuff I#ll post the schematic later this day when I'm back at home.

Raphael
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Old 25th August 2008, 09:50 PM   #6
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Before worrying about parts values, you're going to need to make at least one change. Each output will need a resistor to ground, feedback to the front end, or both. As it stands now, you'll have uncontrollable DC offset and horrendous amounts of gain (on the order of 60-80 dB).

Grey
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Old 25th August 2008, 10:49 PM   #7
rkn is offline rkn  Germany
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Well, the idea was to place these feedback resisistors on the mainboard where I want to plug in my OPamp. But yesterday I found out that I have some space left on the OPamp PCB so that there is no big problem to place two additional feedback resistors.

Raphael
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Old 25th August 2008, 11:00 PM   #8
Tim__x is offline Tim__x  Canada
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You need a common-mode feedback loop.
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Old 26th August 2008, 01:04 AM   #9
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It wouldn't be any problem to run the circuit with zero feedback. There are other ways to attend to the DC offset. But if feedback was the original intention, then that's easily enough arranged.

Grey
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Old 26th August 2008, 02:39 PM   #10
rkn is offline rkn  Germany
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Ok, here is the new version. I added feedback resistors like Nelson Pass demonstrates it in his paper and I removed the 1M00 to gound at the inputs because I think they are not needed when I add feedback like now.
Comments?

Raphael

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