# Op Amp Problem

1. Sep 15, 2009

### uzair_ha91

1. The problem statement, all variables and given/known data
Calculate the output of the op-amp as shown in the given circuit.
http://img9.imageshack.us/img9/7554/184v.png [Broken]

2. Relevant equations
Kirchhoff's Current Rule

3. The attempt at a solution

By applying KCL,
I1+I2=I3
V1/R1 + V2/R2 = Vo/R3
Substituing the values and rearranging the equation we get,
Vo= (5/10,000 + 2/4,000)*20,000
Vo= 20
But then I realized that if i use a minus sign between (5/10,000 + 2/4,000), I get Vo=0...
So does that minus sign come into the equation because we're trying to find out the output of an inverting amplifier, or is it simply because V2="-"2

Last edited by a moderator: May 4, 2017
2. Sep 16, 2009

### Mr.Green

This type of circuit is known as adder or summer

The solution is given by:

vo = - RF * [ V1/R1 + V2/R2 + ... + Rn/Rn]

SOLUTION:

Vo = - 20k * [ 5/10k + -2/4k] ; I think you forgot to use negative signs
Vo = 0

3. Sep 16, 2009

### uzair_ha91

Why did you put a minus sign before 20k?

4. Sep 16, 2009

### Mr.Green

Since the input is on the inverting input a negative sign should be used.
On yourpost you didn't specify the type of inputs of the op-amp, but it seems that the input is on the inverting input.

5. Sep 16, 2009

### uzair_ha91

Yeah you're right...thanks