# High-gain Op Amp open loop gain test circuits

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#### Lego256

##### Newbie level 3
op amp gain test

Hi everyone,

I have to test a high gain (100dB+) op amp as accurately as possible so I tried the circuit on page 5 of this course :
http://www.macs.ece.mcgill.ca/~roberts/ROBERTS/COURSES/EE334/Gain.pdf
using a nulling amplifier. It is quite widely used over the internet.

But couldn't make it work.
The high gain of the op amp tested makes the DUT oscillate because of his own noise I think. And as I need a close loop gain as precise as possible, I need well matched resistors and I can't therefore have a huge ratio between them, around 10000 is a maximum. So the level of E0 I look for as output is a few millivolts and a 2V oscillation is... very disturbing...

Anyone has ever tried that kind of test? How would you do it?

Thanks !
Lego

#### watersky

##### Member level 5
op amp open loop gain

I don't know your aim of this op amp. If you use this op amp as a part of close loop. Perhaps the mismatch of different resisters is a primary reason.

#### Lego256

##### Newbie level 3
how to measure op-amp open loop gain

I failed to notice that the test I do is of course a DC gain test.

The nulling amplifier is supposed to set the output of the DUT amplifier to $-V_{SRC1}$ (+ the nulling amp offset) so I know the DUT output voltage value. Then the DUT input voltage is $V_{0null}/A_{cl}$. So I know about the input then I know the open loop gain, $\frac{V_{SRC1} \cdot A_{cl}}{V_{0null}}$

Unfortunately, $V_{0null}$ oscillates...

#### Kral

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.

mugdha37@gmail.com

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### mugdha37@gmail.com

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#### Lego256

##### Newbie level 3
circuit for measuring op-amp open loop gain

Sounds great.
Thank you very much Kral, I'll try that.

Best regards,
Lego

#### mineurthreat

##### Newbie level 1
Using a 9V battery as a power supply for rails and another for your signal will help with the noise.

#### kwkam

##### Full Member level 5
The null opamp test circuit can do the job to find open loop gain of the opamp. And the pdf link above was fail. You can goto read some book to find the way to use this great circuit. Give you some hint, Av=Vo/Vi

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