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Altaic 16th July 2002 08:31 PM

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?


HarryHaller 16th July 2002 10:09 PM

is there a better way of doing it

Altaic 16th July 2002 10:12 PM

Which is?
Do you care to share your knowledge, HH?

Hoffmeyer 16th July 2002 10:18 PM

HH, I agree :o)

Hi Altaic.

If You search for an article dated 11-09-2000, keyword: differential, they have a very interesting article concerning the feedback!!

Nelson Pass 16th July 2002 10:19 PM

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

Altaic 16th July 2002 11:14 PM

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?

Ren Hoek 17th July 2002 12:02 AM

a little confused..
Just a little.......:confused:

tiroth 17th July 2002 01:31 AM

+ and - are usually used to refer to non-inverting and inverting, respectively.

Altaic 17th July 2002 07:53 PM

I think I see, now
So the inverted input goes to the negative feedback?


Freddie 17th July 2002 11:04 PM


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

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