Differential OP AMP Circuit Question

[joey5981]

Homework Statement

Find Vin/Vout

Homework Equations

Vout = Rf/R1(Vb-Va) ?

The Attempt at a Solution

I think the answer is the first one.

Answer choices:
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

 

[gneill]

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

 

[NascentOxygen]

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

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

Vin/Vout = -2, ...

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

 

[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.