# Expression for op-amp circuit

Joined Feb 3, 2016
42
The expression of Vout=-Vin1+Vin2+Vin3

I apply the virtual short condition
V+=V-
i1=(V+ - Vout)/R
Vout=R*i1 + V+
Vout=(V+ - Vout)+V+
=2V+ - Vout

It seems the voltage divider cannot be applied for V+
How can I find the V+ in this circuit?

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#### Jony130

Joined Feb 17, 2009
5,487
Try to use superposition principal .

#### MrAl

Joined Jun 17, 2014
11,281
Hi,

In short, if you want to derive an expression to show the relationship between three inputs and one output, then you have to at least include those three inputs in the expression. That means Vin1, Vin2, and Vin3 have to be somewhere in the expression too.

#### WBahn

Joined Mar 31, 2012
29,888
The expression of Vout=-Vin1+Vin2+Vin3

I apply the virtual short condition
V+=V-
i1=(V+ - Vout)/R
Vout=R*i1 + V+
Vout=(V+ - Vout)+V+
=2V+ - Vout

It seems the voltage divider cannot be applied for V+
How can I find the V+ in this circuit?
Go back and learn where the voltage divider equation comes from. Then use that same approach to find V+.