# How do you calculate the output voltage of an op-amp with DC voltage

Suppose the following inverting op-amp.

How does one calculate the voltage output when we connect a DC voltage source of for example 750mV to it.

With an ideal amplifier we can just multiply the gain with the input voltage. But that is not the case with inverting op-amps. But i don't know how to then calculate the output voltage with a inverting op-amp when we give a dc voltage input. I know that in this case the output voltage should be 2.05V. But i do not know how this is calculated.

simulate this circuit – Schematic created using CircuitLab

Since there is a negative feedback there is no current going into the input terminals of the op-amp. Thus the currents of R1 and R2 are the same. U = IR, so I = U/R.

For R1, I = 750mV/100 and for R2 I = $$\V_{out}\$$/300. These are equal to each other

So we get $$\ 750mV/100 = V_{out}/300\$$

which leads to $$\ V_{out} = 2250mV\$$, which is still not the correct answer

• There are two principles that are used in analyzing such a circuit: 1) When there is a negative feedback then the voltage on the two input terminals is equal. 2) There is no current flow into/from the input terminals. Now, using these principles write down the voltages and currents on this schematic. – Eugene Sh. Oct 30 '19 at 19:39
• I have edited the post, but with your information i get to 2250mV instead of 2050mV – elcie Oct 30 '19 at 19:50
• You need to take account which direction current flows, or which way you look voltage levels. 2.25V is from negative input to output pin - so -2.25V to ground level. – jalaffo Oct 30 '19 at 19:54
• Apart from the sign, you calculated it correctly -2.25V is the correct answer. Note that $I_{R1} = -I_{R2}$. I think your reference "I know that in this case the output voltage should be 2.05V" is incorrect. That answer is lacking a sign too, BTW – Huisman Oct 30 '19 at 19:58
• You don't really show it in the schematic, but is the op-amp + terminal connected to ground? – user4574 Oct 29 '20 at 0:11