P14 Prototyping Techniques: Attempting to Measure Op Amp Open-Loop DC Gain

RE: P14 Prototyping Techniques: Attempting to Measure Op Amp Open-Loop DC Gain

colporteur — Thu, 18 Sep 2025 22:30:50 GMT

It is a prototyping technique I would consider trying. It doesn't look pretty but if it gets you the proof of concept results then it has form in my books.

RE: P14 Prototyping Techniques: Attempting to Measure Op Amp Open-Loop DC Gain

Loulou — Mon, 15 Sep 2025 13:56:52 GMT

I have done an OpAmp tester base on this méthod of measurement.

Jean-Louis

RE: P14 Prototyping Techniques: Attempting to Measure Op Amp Open-Loop DC Gain

jc2048 — Tue, 15 Mar 2022 17:54:32 GMT

I took the advice of Shabaz and increased the amount of attenuation from the integrator output to the input of the
DUT. Rather than increase the 100k further, I decreased the 100R resistors to around 9.6R by putting 10R resistors in
parallel with them (it was easier to do that than pull the SMD 100R resistors off the board). That gets a larger
swing at the integrator output for the 1V change and gives me something I can at least measure, if not very
accurately. I had to increase the resistance in series with the integrator to get it stable for all four of the parts
I was testing. Take these figures with the proverbial pinch of salt, but that gets me measured gains of 944k
(119.5dB), 1039k (120.3dB), 692k (116.8dB), and 577k (115.2dB).

This is what the original datasheet showed for typical figures (I'm measuring with 15V supplies), so they're in the
right ballpark, though somewhat high.

RE: P14 Prototyping Techniques: Attempting to Measure Op Amp Open-Loop DC Gain

dougw — Tue, 15 Mar 2022 16:52:45 GMT

This brings back some memories of a circuit I designed with an amplification of 1 million. I also used OP27s because of their low noise. I had to use low value resistors and multiple stages to minimize resistor noise. Can you just reduce all resistor values by a factor of 10? Maybe try an OP27 as the DUT just to see if you can get better results from a low noise amp. You could also try running it at as cold a temperature as possible because the noise is also lower at lower temperatures.

RE: P14 Prototyping Techniques: Attempting to Measure Op Amp Open-Loop DC Gain

shabaz — Mon, 14 Mar 2022 15:05:39 GMT

Hi Jon,

Nice prototyping! And interesting circuit as usual : ) interesting that they've got a circuit with such a high capacitance.

Also, was the switch intended to be controlled manually or by say a pulse generator?

I've only briefly looked, maybe I've not thought it fully though, if the attenuation is reduced (I.e. not 10,000 but say 1000) does that help? Then the output may be high enough that the random walk is less observable.

RE: P14 Prototyping Techniques: Attempting to Measure Op Amp Open-Loop DC Gain

Jan Cumps — Mon, 14 Mar 2022 14:49:56 GMT

Do I try and refine this, or do I give up and try a different technique?

I tried one of the simpler techniques. I don't think I have a result I can use either - except for finding the 90° and 180° phase shift frequencies.
What helped me with the noise was using the averaging function of my oscilloscope in its maximum setting. Still not very good, but at least a signal that isn't out-drowned by noise.
But I didn't get any farther than building the test circuit and not having good results.