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For a lab, I need to calculate the gain due to the input voltage offset of an op amp. This is the circuit below: enter image description here

This looks like an inverting op amp to me; so I was thinking the gain would be -1M/100K. But does the R3 resistor have an effect? I also wasn't sure if the fact that both terminals (V+ and V-) are grounded has an effect; I believe this is just to establish an input voltage of 0V.

Thanks for your help.

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  • \$\begingroup\$ Yes, gain is -1M/100K. The resistor R3 is used to null the additional offset due to bias current. Notice R3 is R1||R2. Assuming the same bias current flows ( in same directions) in both terminals. The voltage generated by the bias current at both inputs will be same and hence will be same for both inputs. So no output due to this voltage. The output will be voltage offset multiplied by the gain. \$\endgroup\$ – Ash Jun 10 '17 at 0:32
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Offset voltage is a voltage that can be considered to be an independent voltage source that is in series with either the inverting or the non-inverting input of the op-amp itself. It is not applied to the (left) end of R1. There is a difference.

Put a voltage source in series with either input calculate the output voltage assuming an otherwise ideal op-amp. You will find the magnitude of the gain is the same either way (and it is not R2/R1 or -R2/R1).

R3 has no effect on the gain (at least in the context of this simple question). Its only purpose is to equalize the resistances looking out of the op-amp inputs so that input bias currents do not cause an addition error term (ideally).

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Depending on the tolerance of resistors and Input bias current, R3 should generate the same input bias voltage in mV or uV as R1//R2 such that the "differential" gain of 100x does not amplify this input bias current (Iin*R) found in specs as Iin. It is one of many sources of offset error. Be sure the OA has a split supply or can operate on a single supply with inputs at negative rail (Gnd) THese are called Rail-to-Rail inputs with certain designs of OA's and usually not in BJT types.

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