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Subject: Re: seeking help about deigning a diffrential amplifier

Date: 03/06/02 at 8:49 PM
Posted by: Russ Kincaid
E-mail: russlk@yahoo.com
Message Posted:

In Reply to: seeking help about deigning a diffrential amplifier posted by vimal dubey on 03/06/02 at 12:46 PM:

I am not going to design it for you, but here are some pointers: you have 5 transistors to work with, use one in the common emitter circuit to provide a constant current and reduce the common mode gain. You want to avoid voltage swing for wide bandwidth, but with all NPN transistors you are limited. Run the input stages at low current for low noise. But the gain is reduced at low current, so you have to trade-off noise and gain.
You want the output to be able to swing + and _, so you need a way to couple from the input stage collector which is necessarily positive to the output stage base which is necessarily negative. You could use a voltage divider, but a zener would give less loss. A PNP stage would be ideal, but you don't have one. I suggest using one of the transistors as an emitter follower, you will have to figure out where to put it.

Good luck.

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