# Homework Help: Op Amp : Design the circuit such that

1. Oct 10, 2012

### ElijahRockers

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

Assume the op amp is ideal.
Design the circuit such that Vo = Va + 2Vb + 3Vc

3. The attempt at a solution

I started with

$\frac{V_a}{R_a}+\frac{V_b}{15k}+\frac{V_c}{R_c} = 0$ since the op amp is ideal, the sum of the currents going into the non-inverting input must be 0.

From here I haven't got any ideas how to proceed. But what is the voltage at the inputs? I'm assuming it's zero because Vn/20k = 0 : Vn = 0

EDIT: Alright I found Ra to be 30k, Rc to be 10k. Now I have to find the currents ia, ib, and ic, when Va=.5, Vb=.7, and Vc=1.2. Shouldn't that just be Va/Ra =ia? I know one of them must be negative since the total current flowing in must be 0, but how do I know which one?

I found Vo =5.5 at those values for Ra,Rb,Va,Vb,Vc.

Ok.... I figured it out. I guess sometimes it just helps to type it out on a forum haha. For anyone wondering, Vo=6Vn (by KCL at the negative terminal). Since Vo = 5.5, then Vn = .9166...

Last edited: Oct 10, 2012
2. Oct 10, 2012

### rude man

This equation is incorrect. Write down the correct KVL equations.

3. Oct 11, 2012

### Staff: Mentor

You really can't get "Vo = 5.5" when the op-amp has supply voltages of ±5 volts.

4. Oct 11, 2012

### rude man

How can you get numerical answers of any kind when your inputs are symbols?
Are you pulling somebody's leg? :uhh:

5. Oct 11, 2012

### hoangkyem

I founded an ebook that very helpful for you: Op amp for every one

www.ti.com/lit/an/slod006b/slod006b.pdf [Broken]

Last edited by a moderator: May 6, 2017
6. Oct 11, 2012

See post #1