# Homework Help: Opamp circuit for Vout = 2v1 - 3v2

1. Nov 4, 2012

### Squizzel

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

Design an Opamp circuit for which the output voltage will be of the following form

Vout = 2v1 - 3v2

Check with Multisim

2. Relevant equations

3. The attempt at a solution

We have only learned the basics of Opamps thus far, so I am confused with the question. This is asking for 2 opamps with the first one a gain of 5 and then the second with a gain of -3?

So I would calculate the first opamp with a gain of 5. would that mean that V2 is 5x what ever v1 is?

2. Nov 4, 2012

### rude man

No, they want a single op amp circuit.

Hint: design the negative section first: Vo = -(Rf/Ri)Vi, then add a voltage divider to the + input to reduce the + gain to 2.

3. Nov 4, 2012

### Squizzel

Ok so for the v2 I got -R2/R1(v2) so that would be -3/1

so for v1 I get 1+3/1 * R4/R3+R4

So that means it needs to be 1/2 so all of the Rs must be 1?

4. Nov 4, 2012

### rude man

For the + input pin you'd get 3+1 = 4 as you state. So what voltage divider do you put at the + input pin to get an overall + gain of 2? You gave the answer: 1/2. So what are your choices of the voltage divider resistors?

Question - did you cover offset current compensation yet?

5. Nov 4, 2012

### Squizzel

We haven't gone over what offset current compensation is actually. I am also not sure what exactly a voltage divider is. But as to the question, I meant that the R3 and R4 would be 1k, not all of them.

Are you supposed to turn Vs2 off to calculate Vs1?

6. Nov 5, 2012

### aralbrec

The output of a linear circuit can be found with superposition.

Total output = output due to Vs2 with Vs1 zero + output due to Vs1 with Vs2 off.

That's why you can place all of this on one op amp.

7. Nov 5, 2012

### rude man

Yes. But 'turning off' means zero volts, not an open input!

Your choice of 1K for R3 and R4 is OK. When you get to offset current compensation you will find that R1||R2 should = R3||R4. You can do this and still have R1 = R2.