Calculating Output of Op-Amp in Given Circuit

[uzair_ha91]

Homework Statement

Calculate the output of the op-amp as shown in the given circuit.
http://img9.imageshack.us/img9/7554/184v.png
(Answer=0)

Homework Equations

Kirchhoff's Current Rule

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

 

[Mr.Green]

This type of circuit is known as adder or summer

The solution is given by:

v_o = - R_F * [ V₁/R₁ + V₂/R₂ + ... + R_n/R_n]

SOLUTION:

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

 

[uzair_ha91]

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

Why did you put a minus sign before 20k?

 

[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.

 

[uzair_ha91]

Yeah you're right...thanks