[SOLVED] Measure close loop gain op fully op amp

Hi
Can anyone help me how to measure close loop gain of the op amp in the picture in hspice simulation. Thanks

Along with your frequency plots of the individual signals, add a plot of (inp-inn)/(outp-outn). Your simulator should be able to plot this function.

Analog Ground said:

Along with your frequency plots of the individual signals, add a plot of (inp-inn)/(outp-outn). Your simulator should be able to plot this function.

Click to expand...

I would suggest tp plot the invers of the given expression, since the gain is defined as output/input.

LvW said:

I would suggest tp plot the invers of the given expression, since the gain is defined as output/input.

Click to expand...

Yes, I made a silly error. (outp-outn)/(inp-inn)

Thanks Analog Ground and LvW for your suggestion. I make a new schematic to run with cadence spectre (attached picture). I plot differential gain vesus frequency (close loop gain) , the result from spectre is not what i expect. From formular for close loop gain Acl = Aol/ (1 + B*Aol) B is feed back factor
From schematic B = C2 / (C1 + C2 + Cin) where Cin is total op amp input capacitor (Cin = 0.7pF in my simulation)

So the feedback factor B = 1 / (1 + 3 + 0.7) = 0.2
My open loop gain is 54dB, and from Acl = Aol/ (1 + B*Aol) ---> Acl = 1/B
---> Acl = 1/0.2 = 5
The spectre simulation shows Acl = 3 only
I thinking the open loop gain is a litle low but this is not the case because 54dB is OK.
Anyone has any idea to fix this problem please let me know. Thanks a lot





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I make a mistake in caculate Aol:

Aol ~Cin/Cfb = 3/1 = 3. The simulation is correct