# Analyze op amp circuit

I want to know how to calculate the output of the below op amp. I know this is supposed be an inverting amplifier with offset but I have never seen the connection of the offset to the negative side of op amp.

simulate this circuit – Schematic created using CircuitLab

I have come up with this formula:

$$V_{\text{out}} = V_{\text{in}} \times \frac{R_2}{R_1} + \frac{R_2}{R_4+R_7}V_\text{p}$$

Is it correct? $$\V_\text{p}\$$ is the voltage out of the potentiometer.

• Hint: print this circuit out on paper, and use different colored highlighters to trace out the different nodes. – The Photon Sep 4 '19 at 16:33

R7 does not do anything of value, it is actually harmful (increases output noise and output offset voltage- and related drift). Ideally there is no voltage across R7 so it has no effect, but op-amps are not ideal and if you think about the resistor approaching zero it's clear there will be problems. You can calculate the effects if you model the op-amp with Vos and Vn voltage in series withe one of the inputs (offset and noise).

Vout ~= $$\ -V_{IN} \cdot \frac{R_2}{R_1} -V_P \cdot \frac{R_2}{R_4}\$$, ignoring op-amp gain and offset voltage.

You can find that by applying KCL to the non-inverting input node, assuming zero input current to the op-amp and then find Vout such that V- = V+ = 0V.

• Isn't the R7 and R1 a voltage divider for the input VIN? Could you please explain why does not do anything to the circuit? Apart from the harmful evects you mentioned – MrBit Sep 4 '19 at 20:02