opamp : open loop gain vs output differential voltage

[student_of_analog]

Hi all,

Does anybody know hw to simulate open loop gain vs. output differential voltage for
a fully differential opamp ?
So I need to generate a Av(open loop gain) (Y-axis) Vs output differential voltage(Y axis) plot.

 

[LvW]

You "need" such a display? Why?
Please consider that the gain does not depend on the output voltage.
The other way round: You can fix the input voltage, vary the gain and plot Vout=f(Av).

 

[student_of_analog]

LvW, its required as part of project submissions. This plot will give an idea of the range of output voltage for which open loop
gain is constant I guess.

I found this old thread,
https://www.edaboard.com/threads/89279/.

I wanted some more clarity on the setup.

 

[FvM]

If I understand right, the question refers to amplifier non-linearity. You can perform a parametric small signal analysis with superimposed DC voltage as parameter.

You didn't exctcly specify gain in your question. If you refer to DC gain, you can also make a differential input DC sweep, calculate dVout/dVin and plot it over Vout.

 

[LvW]

OK, now it is clear to me: You do not need gain vs. output voltage but gain vs. max. output voltage range (dynamic properties of the amplifier.)
But this requires that you have a definition for the max. output voltage range: What is the maximum distortion (THD in %) you will allow at the limits of this range?
In case of integrated opamps this corresponds with the "full power bandwidth".
I am afraid, it`s not possible to create such a function in one simulation run: For several different gain values you need to increase the input signal and watch the output until it reaches the max. allowable distortion (that you have specified as output limit).

 

[FvM]

You do not need gain vs. output voltage but gain vs. max. output voltage range (dynamic properties of the amplifier.)

You can think of a small signal analysis with DC output voltage level as parameter. In case of a differential amplifier with common mode voltage zero, the DC level will refer to a differential output voltage. It can be only determined in multiple analysis and not directly displayed as a XY plot in the usual SPICE variants.

 

[student_of_analog]

thank you FvM and LvW for your response.

I have a fully differential amplifier. So should the test setup be as follows :

1)Unity gain feedback with OUTP connected to INM and OUTM connected to INP
2)DC source at INM swept from 0 to 2.5V (supply voltage is 2.5V)
DC source at INP swept from 2.5V to 0V.

Then do I take the slope of obtained o/p ( OUTP-OUTM) Vs i/p (INP-INM) ? to get the gain ?.
Then I take this dVout/dVin waveform points into excel along with o/p points into excel and plot them in excel to get the DC gain vs o/p voltage plot ?

Please correct me for any mistakes in setup.

 

[erikl]

Perhaps this thread may be helpful?