# Designing an Op Amp Circuit

1. Sep 15, 2014

### Vishera

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

2. Relevant equations

3. The attempt at a solution

Here is my attempt: http://i.imgur.com/oKjwI8O.png

The problem is at the end, I get 0=0. What did I do wrong?

2. Sep 15, 2014

### Staff: Mentor

Can you post a drawing of your opamp circuit, with the resistor names labeled? Thanks.

3. Sep 16, 2014

### Vishera

Sure, here is the drawing of the general circuit that I am aiming to design:

For that general circuit, the following equation applies:

Here is an example in the textbook:

4. Sep 17, 2014

### rude man

You sure it isn't Vo = 3V2 - 2V1?

5. Sep 17, 2014

### donpacino

I agree with rude man.

In all cases an input to the negative side of an single op-amp circuit will be inverted, while an input to the positive side will not be inverted.

6. Sep 17, 2014

### Vishera

Is there any specific reason why you can't the following equation to:

to:

vo=3v1-2v2?

I understand that I get 0=0 but why do I get 0=0? Algebraically speaking, why would it matter if it were vo = 3v2 - 2v1 or vo=3v1-2v2?

7. Sep 17, 2014

### Staff: Mentor

It might be helpful to write:

$3 v_1 - 2 v_2 = v_1 + 2(v_1 - v_2)$

Then a really simple circuit solution might come to mind.

8. Sep 17, 2014

### donpacino

combing your two equations the way you initially had them -R2/R1 would have to equal three.

that would imply that you have a negative resistance, which is impossible

9. Sep 17, 2014

### donpacino

I would solve this problem through an iterative method.

1. determine the relationship between R2 and R1 based on the V1 coefficient (hint R2/R1 = ?)
2. choose resistor values such that the relationship is true
4. determine R3 and R4 values based on the V2 coefficient and R1 and R2.
5. If needed go back to step 1

10. Sep 20, 2014

### Vishera

I understand that negative resistance is impossible in the real world, but shouldn't I at least get an answer with negative resistances in the math? I didn't even get an answer in the algebra which is confusing me.

11. Sep 20, 2014

### donpacino

you got unlucky with the numbers. for that particular (wrong) arrangement of equations there is no solution. why don't you try it again with the correct number orientation. If you don't believe you got unlucky, change coefficients in the equation to some random combination and see what happens.

I do want to point out that the best way to solve these problems is an iterative process.
I recommend following the process I gave you above.