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.ac simulation of differential OPAMP

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patato

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when doing .ac analysis for a single-ended OPAMP, we put a "ac 1" source at VINP, "ac 0" at VINN. then we can run simulation for gain and phase.

But what do we do when it comes to a differential OP?
put "ac 1" on both VINN and VINP , and probe (VOUTP - VOUTN) to see the OP's gain and phase?

Am i correct?
 

Dear Patato

For differential op-amp, you will have to use AC=0.5 for positive input and AC=-0.5 for the negative input. As you mentioned, you will need to probe VOUTP-VOUTN for phase and gain.

I hope this would be of help.

Regards
Sachin
 

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    patato

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Thanks!

one more question,
so the phase margin should be
[ phase of (VOUTP-VOUTN)+180 ] when gain(VOUTP-VOUTN) = 1 right?
 

Hi,
For running the ac analysis, connect VSIN with '+' terminal connected to VINP and '-' terminal connected to VINN.

Take ac magnitude as 2mV and an Offset of 1V.

After that as you have suggested you can you will need to probe VOUTP-VOUTN for phase and gain.

Thanks
Shaikh Sarfraz

Added after 9 minutes:

Hi Patato,
As far as your second question is concerned, phase margin is the phase at 1dB axis crossing of the gain plot.

Thanks
Shaikh Sarfraz
 

Dear Patato

You are correct about the Phase margin and Gain calculation.

Please note than you will have to use a CMFB circuit to set the output common mode level of the differential op-amp. In case you are using a Switched Capacitor based CMFB, please replace it with an ideal circuit before you perform the AC analysis.

With Regards
Sachin
 

sachinagg77 said:
Dear Patato

You are correct about the Phase margin and Gain calculation.

Please note than you will have to use a CMFB circuit to set the output common mode level of the differential op-amp. In case you are using a Switched Capacitor based CMFB, please replace it with an ideal circuit before you perform the AC analysis.

With Regards
Sachin
Click to expand...

wow,you pre-answered my question before i ask it.
ya i am using a SC CMFB structure,
does it mean i should replace the CMFB op with an ideal one(infinite gain OP)?
or just bias the whole circuit in saturation and run AC analysis?
 

You will need to replace the Switched Capacitor section of the CMFB loop with an equivalent ideal circuit. The op-amp in the CMFB circuit is not a switched circuit and can be retained as such.

One implementation of ideal CMFB circuit is mentioned at
 

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    patato

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sachinagg77 said:
You will need to replace the Switched Capacitor section of the CMFB loop with an equivalent ideal circuit. The op-amp in the CMFB circuit is not a switched circuit and can be retained as such.

One implementation of ideal CMFB circuit is mentioned at
Click to expand...

Great! my ideal CMFB works now. Thanks for your help!
 

my favorite test circuit is using one AC source, then with two 0.5X VCVS
genernate two differential input signal, for DC biasing/CM, you can stack one
DC source there. The easy way to check the phase margin and gain margin
is to use stb simulation avaiable in spectre.
 

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    patato

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gte582w said:
my favorite test circuit is using one AC source, then with two 0.5X VCVS
genernate two differential input signal
Click to expand...
umm, i dont get it. could you be more speciftc?

, for DC biasing/CM, you can stack one
DC source there. The easy way to check the phase margin and gain margin
is to use stb simulation avaiable in spectre.
Click to expand...

in fact i dont know spectre got such funtion :|
maybe i'll check it later. Thanks.
 

since the two vcvs coming from the same AC source with +-0.5 gain,
they will be +0.5vin and -0.5vin, exactly what you want for the
differential input signals.
 

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