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.

 

[rezeew]

Assuming the circuit is an inverting operational amplifier, the correct answer would be Vin/Vout = -2, Rin = 100. This is because the inverting amplifier equation is Vout = -Rf/Rin * Vin, and in this case, Vb = 0 (since the negative input is grounded) and Va = Vin. Therefore, substituting these values into the equation given in the homework statement, we get Vout = -Rf/R1 * Vin, which matches the inverting amplifier equation. The value of Rf/R1 in this circuit is -2, and since Rin is equal to R1, the value of Rin is 100.