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Old 16th July 2002, 08:31 PM   #1
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Question balanced diff. input stage

I am wondering what the best way to design a balanced input stage for a differential amp is. I need to reinvert the inverted line and combine the noninverted signal with it to cancel out mutual line noise. So, do I slap a transistor on the inverted line such that the base goes to the input, the emmitter goes to negative, and the collector goes to a big resistor and the now reinverted signal out? Then the inverted signal out would connect to the noninverted in. Or is there a better way of doing it, or any special conciderations?

Also, I need both balanced input as well as output. I am guessing it is better to have a single feedback loop, so I'd convert to unbalanced for the input stage, then back to balanced for the output stage by the same method. Does that make sense?

Will
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Old 16th July 2002, 10:09 PM   #2
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Default is there a better way of doing it

Yes
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Old 16th July 2002, 10:12 PM   #3
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Default Which is?

Do you care to share your knowledge, HH?
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Old 16th July 2002, 10:18 PM   #4
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HH, I agree )

Hi Altaic.

If You search www.EDNmag.com for an article dated 11-09-2000, keyword: differential, they have a very interesting article concerning the feedback!!
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Old 16th July 2002, 10:19 PM   #5
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You want a classic differential pair.

Without feedback, it looks a bit like Son of Zen

With feedback, it looks like the examples in DIY Op Amps.

You can see both at www.passdiy.com
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Old 16th July 2002, 11:14 PM   #6
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Default inverting inputs..

Thanks much for the info, guys. The articles were very interesting.

But I am new to this and a little confused.. Balanced inputs consist of non-inverted, inverted, and ground. Both of the signal inputs are AC.. But in the articles you two gave me there was a positive in, and a negative in, both DC. Or are they supposed to be offset a few volts, so each doesn't cross zero?
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Old 17th July 2002, 12:02 AM   #7
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Default a little confused..

Just a little.......
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Old 17th July 2002, 01:31 AM   #8
tiroth is offline tiroth  United States
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+ and - are usually used to refer to non-inverting and inverting, respectively.
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Old 17th July 2002, 07:53 PM   #9
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Default I think I see, now

So the inverted input goes to the negative feedback?

Nifty.
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Old 17th July 2002, 11:04 PM   #10
Freddie is offline Freddie  Sweden
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Altaic,

You might want to have a look at this for an amp with diff.pair with balanced inputs.
much less crazy idea
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