Op-Amp Simulation Measurements

crutschow

Joined Mar 14, 2008
31,138
To measure the open loop gain at 1MHz just open the loop and apply some voltage at 1MHz and measure the output voltage AC.
For proper measurement of an opamp's open-loop gain, you need to provide DC feedback to stabilize the operating point, as I show in my simulation, otherwise it will be at one of the rails and likely give an incorrect gain value.
 

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stinkimy

Joined Apr 9, 2021
8
Here is another example for measuring Offset Voltage from an Analog Devices Application Note MT-037. It also demonstrates the use of the .measure SPICE command.
Thank you, will have to figure out more SPICE commands. So far I've only done the basic analyses.
 

Thread Starter

stinkimy

Joined Apr 9, 2021
8
You might also want to consider how the slew rate affects the output at various input AC frequencies. It is not really enough to consider just the open loop gain because the slew rate could distort the waveform changing it from a clean sine wave to a strange triangular shape wave.
You also have to watch out for clipping and if you do see that you have to lower the input voltage for the test.

You can also get some idea what to expect by looking at the data sheet for the op amp you are using.
If it is a distorted sine wave, is it right that the rise/run is not the slew rate because the output isn't able to change at the rate of the input changes?
 

MrAl

Joined Jun 17, 2014
9,633
For proper measurement of an opamp's open-loop gain, you need to provide DC feedback to stabilize the operating point, as I show in my simulation, otherwise it will be at one of the rails and likely give an incorrect gain value.
Yes it would take a very very small signal and offset adjustment. I should have mentioned that too.
 

MrAl

Joined Jun 17, 2014
9,633
If it is a distorted sine wave, is it right that the rise/run is not the slew rate because the output isn't able to change at the rate of the input changes?
Yes. What happens is the way the op amp works is it has to have time to ramp up to the required output voltage and that time depends on the slew rate. For example the LM358 op amp is often quoted as having a 0.5v/us slew rate, and since the max dv/dt of the output is where the sine wave crosses zero, the slope of the output has to be equal to or less than the slew rate of the op amp. If it is not, the output will have a 'ramping' part as well as a partial sine part.
It also varies with the output peak voltage because the dv/dt near zero degrees increases when the gain is increased (output has to ramp up higher)
 
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