What is the Voltage Gain of Vo/Vi in an Op Amp Circuit?

[StripesUK]

Homework Statement

Find the voltage gain of Vo/Vi. I'm pretty confident I have this down now, but I'd be greatful if someone could confirm I have this sussed.

Homework Equations

For the inverting op amp:
$Vo= \frac{-R2}{R1}*Vi$
For the differential op amp:
$Vo= \frac{-R2}{R1}(V--V+)$

The Attempt at a Solution

$Vx= \frac{-R2}{R1}*Vi$
$Vx= \frac{-68}{12}*Vi=-5.667Vi$

$Vo= \frac{-R2}{R1}(Vx-Vi)$
$Vo= \frac{-220}{12}((-5.667Vi)-Vi)$
$Vo= \frac-18.3(-6.66Vi)=121.878Vi$

$\frac{Vo}{Vi}=121.878$

 

[gneill]

The result looks good.

You might want to sprinkle a few comments around in your derivations so that it's obvious what your steps are accomplishing. It was a bit puzzling at first to see the same symbols, R1 and R2, take on different values with no apparent reason.

 

[Eucliddo]

I've read this exact problem this morning and got 122.22. It may not be as exact as your answer but it's just the same. Just make your solution clearer to avoid confusion.

 

[BvU]

Coincidence that this is the same exercise ?