.ac simulation of differential OPAMP

[patato]

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?

 

[sachinagg77]

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

 

[patato]

Thanks!

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

 

[shaikhsarfraz]

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

 

[sachinagg77]

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

 

[patato]

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

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?

 

[sachinagg77]

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

 

[patato]

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

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

 

[gte582w]

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.

 

[patato]

my favorite test circuit is using one AC source, then with two 0.5X VCVS
genernate two differential input signal

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.

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

 

[gte582w]

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.