# Differential Op Amp Output voltage

I was just wondering if someone could tell me if I am doing this right, because I don't really have any sample problems but just want to be prepared for anything. If we wanted to find the output voltage of this system could we find:

The gain by Rf/Rs Then multiply Va*Gain? simulate this circuit – Schematic created using CircuitLab

*It would not be square rather just a 5v source.

So gain would be: 36k/12k or 3; so Va*3 would be the output?

Gain from the positive input is 1 + (R1/R4) = 4.

The voltage at the positive input is R2/(R2+R3) times the input V1.

So the voltage gain from V1 to the output is (1+R1/R4)*R2/(R2+R3) ~= 2.222.

• So I ran those numbers and got 2.77 at voltage Va, and then gain would be 4. So output would be ~11.1V, does that sound about right? Apr 27, 2017 at 18:24
• 2.222 * 5V is 11.11V, yes. Apr 27, 2017 at 18:27

No.

What you've drawn there is a non-inverting amplifier.

The amplifier keeps the voltage across R4 equal to Va. The voltage across R1 is 3 times Va, so the total output voltage is 4 times Va.