Differential Op Amp Gain Problem

  • Thread starter caprice24
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  • #1
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Hi,

I have a problem I am trying to work out on simple Op Amps gain, and I am having trouble trying to prove what the gain is. I know what the gain is supposed to be (see below), but I am stumped as to how to actually go about proving it. When I try to setup the Kirchoff node equations I get a really ugly polynomial.

I can put the design into a spice deck and prove it, but I want to understand the design from an analytical point of view.

Gain = 2*R2/R1 * (1 + R2/Rg)(V2-V1) (GAIN FOR THE OP AMP in the attached jpg file)

Now looking at the above equation it is obvious if you eliminate resistor Rg, the equation becomes a simple differential amplifier gain, which I can easily solve and prove. However when you add the positive feedback from the output, you get a different Beta for the loop gain that is hard to figure out.

Nick
 

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Answers and Replies

  • #2
anorlunda
Staff Emeritus
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That makes no sense at all. R2 is in parallel with RG. So whether or not there is positive feedback, can not depend on the value of RG.
 

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