measure op-amp open loop gain
Lego256,
Here's another way to measure/calculate open loop gain.
Connect the OpAmp in an inverting amplifier configuration.
Let
. Ri = input resistor
. Rf = feedback resistor
Set Rf/Ri to a value that is high, but lower than the expected open loop gain of the op amp by a factor of 10. For example if you expect the open loop gain to be 1^6, then set Rf/Ri = 1E5. You might have to experiment to get a factor that works best.
.
Accurately measure the closed loop gain. If noise is a problem, connect a capacitor across Rf to implement a simple 1st order low pass filter.
Let
. Ao = Open loop gain
. Ac = Closed loop gain.
. B = Feedback Factor = Ri/(Ri + Rf)
Ac = (Rf/Ri)[AoB/(1 + AoB)]
.
Solve the above equation for Ao.
.
To avoid problems with offset voltage, offset current, bias current, measure the output voltage at 2 different values of input voltage. and calculate the gain by using Ac = DeltaVout/DeltaVin. This way the errors cancel.
Regards,
Kral.