# Homework Help: Differential Amplifier with negative feedback

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1. Nov 6, 2017

### DragonChase29

1. The problem statement, all variables and given/known data

2. Relevant equations
• β=R2/(R1+R2)

3. The attempt at a solution
I am really confused as to how to find A. I know the feedback of the circuits around the R2 and R1, but I dont understand how the open loop gain can be found. Can someone assist me in deriving it? First question is what gain are they asking for: common mode or differential? I assume differential, but I am honestly lost.

2. Nov 7, 2017

### LvW

Do you know the meaning of the various given quantities like Vov, VA, ro, RD ?
Note that the gain is Adiff=0.5*gm*ro||RD.

Last edited: Nov 7, 2017
3. Nov 7, 2017

### DragonChase29

Yes; my concern is whether the open loop gain(A) is just this equation.

4. Nov 7, 2017

### LvW

I have a question to you:
The first of the "relevant" equations (gain) - did YOU write down the formula or were it given as part of the original text?
My question is because I do not think that we need this equation.
So I don`t think we need ro. By the way - what is RD?

In any case: We need the transconductance gm. From where do we get this value?

Last edited: Nov 7, 2017