# Homework Help: Differential OP AMP Circuit Question

1. Nov 5, 2014

### joey5981

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

2. Relevant equations
Vout = Rf/R1(Vb-Va) ???

3. The attempt at a solution
I think the answer is the first one.

Vin/Vout = -2, Rin = 100

Vin/Vout = -2, Rin = 200

Vin/Vout = -2.5, Rin = 100

Vin/Vout = -2.5, Rin = 200

Vin/Vout = -3, Rin = 100

Vin/Vout = 2, Rin = 100

Vin/Vout = 2, Rin = 200

Vin/Vout = 2.5, Rin = 100

Vin/Vout = 2.5, Rin = 200

None of the above

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2. Nov 5, 2014

### Staff: Mentor

Can you show us how you arrived at your conclusion? We won't confirm a guess...

3. Nov 5, 2014

### Staff: Mentor

Hi joey5981. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

So you believe the 300 ohms resistor (and its 100 ohm companion) do nothing here? How can you prove this?

The gain was given as this? Or did you mis-type what the book showed as Vout/Vin?

Last edited by a moderator: May 7, 2017
4. Nov 5, 2014

### rude man

Due to the unequal 200 and 300 ohm resistors this is not a differential amplifier in the accepted sense. In other words, it has a huge common-mode gain.

If you accidentally labeled the 300 ohm resistors so it should have been 200 ohms, then you should come up with a computation as gneill asks for.