calculation on opamp


Joined Dec 28, 2011
The output voltage is the gain * input.

Gain is figured by :
Rf / Ri + 1 = non-inverting op-amp gain
Rf / Ri = inverting op-amp gain
where Rf is the feedback resistance and Ri is the input resistance

The equations shown in the link given by luvv are useful if you know 2 of the following: input voltage, output voltage, gain.

Hope this helps a little.


Joined Dec 23, 2011
The output can be calculated by multiplying the input voltage by the gain of the op-amp stage.

Since the gain of the op-amp stage has been explained twice, I assume you're okay with that aspect.

The part that nobody explained was that the current source of 4 mA runs MOSTLY through the 10 ohm resistor. The voltage that appears at the +input (in front of the 10K input resistor) is 0.04v.

The GAIN (again) of the stage is that of a non-inverting op-amp stage and is 10K/941 + 1, or 11.62 ish. Multiplying the input of 0.04v by this gain yields the output the meter reads, approximately, 457 mV.

This analysis is only an approximation because the input bias currents, as well as other non-ideal op-amp properties, cause this value to be slightly off; CircuitMaker is o.k. at including these non-ideal parameters into its calculations.