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Is the method right to judge the CMFB's stability?

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didibabawu

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cmdmprobe

The diagram of a full differential opamp with CMFB is in the left
I want to know whether it is stable, I broken the point a and make
the cmfb point as the input and point a as the output as can be seen in the right.
Cla is the load of the point a.
Can I judge the stability of the CMFB circuit by seeing the bandwidth and phase margin of the point out?
Thank you very much.
 

wjxcom

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cmfb stability

Hi, I use the same method to test the CMFB, I think this method is right! Mybe somebody can give us some advices!
 

bigpop

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stb analysis spectre

if you use spectre. you need not break the loop. just add a iprobe in the line a and do stability analysis. you can find the phase margin from the result
 

shaq

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common mode feedback stability

didibabawu said:
The diagram of a full differential opamp with CMFB is in the left
I want to know whether it is stable, I broken the point a and make
the cmfb point as the input and point a as the output as can be seen in the right.
Cla is the load of the point a.
Can I judge the stability of the CMFB circuit by seeing the bandwidth and phase margin of the point out?
Thank you very much.
Dear didibabawu,

If I wanna test the CMFB's stability, what kind of input signals do I need to inject?
 

nxing

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iprobe spectre

run the "stb" analysis in Spectre will be easier and more accurate than this traditional way.
 

didibabawu

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middlebrook stability criteria

shaq said:
Dear didibabawu,

If I wanna test the CMFB's stability, what kind of input signals do I need to inject?
Hi shaq,
Thank you for your reply.But I don't know what you mean,
I do the ac analysis for the circuit to see the bandwidth and phase margin.

Added after 20 minutes:

nxing said:
run the "stb" analysis in Spectre will be easier and more accurate than this traditional way.
Hi nxing,
Thank you for your help. Can the 'stb' analysis be used in the SC-CMFB circuit?
 

shaq

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common mode feedback circuit stability

didibabawu said:
shaq said:
Dear didibabawu,

If I wanna test the CMFB's stability, what kind of input signals do I need to inject?
Hi shaq,
Thank you for your reply.But I don't know what you mean,
I do the ac analysis for the circuit to see the bandwidth and phase margin.

Added after 20 minutes:

nxing said:
run the "stb" analysis in Spectre will be easier and more accurate than this traditional way.
Hi nxing,
Thank you for your help. Can the 'stb' analysis be used in the SC-CMFB circuit?
Hi didibabawu,

I just mean that as shown below.

inn=dc '0.5*vdd'
inp=dc '0.5*vdd'
in= ac 1v

then perform the ac analysis.

Am I right?
 

xshou

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stability analysis iprobe

Thanks to Middlebrook, his great double-injection method. Thanks to Tian and Kundert, you have iprobe and cmdmprobe in spectre. Stb analysis is just one click away.

You know all the basics of feedback control and the idea behind Nyquest stablity criteria. But, watch out!
The key question is: have you broken all the loops at the same breaking point? You are not suppose to break one loop while allow other loops. Stb analysis at such breaking-point is very very dangerous.

As in the above example, I assume your Gm block would have certain feedback configuration on a higher level -- e.g. resistor/capactor feedback between inn/inp and outn/outp. If so, that diff mode feedback also functions as common mode feedback (specially at high frequency). Therefore, breaking point a doesn't really break the new loop mentioned above.

Finally, I would suggest you break both common mode and differential mode loop at outp/outn by using cmdmprobe.

Of course, I will check breaking point a too but outp/outn is the main concern.

Good luck
 

    didibabawu

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didibabawu

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spectre stb analysis

xshou said:
Thanks to Middlebrook, his great double-injection method. Thanks to Tian and Kundert, you have iprobe and cmdmprobe in spectre. Stb analysis is just one click away.

You know all the basics of feedback control and the idea behind Nyquest stablity criteria. But, watch out!
The key question is: have you broken all the loops at the same breaking point? You are not suppose to break one loop while allow other loops. Stb analysis at such breaking-point is very very dangerous.

As in the above example, I assume your Gm block would have certain feedback configuration on a higher level -- e.g. resistor/capactor feedback between inn/inp and outn/outp. If so, that diff mode feedback also functions as common mode feedback (specially at high frequency). Therefore, breaking point a doesn't really break the new loop mentioned above.

Finally, I would suggest you break both common mode and differential mode loop at outp/outn by using cmdmprobe.

Of course, I will check breaking point a too but outp/outn is the main concern.

Good luck
Thank you very much for your help.
But I cannot use Spectre now! I can only use Hspice.
So how can i do the simulation for the circuit.
the circuit is used in diff mode feedback as you say above.
then when I do the simulation, need I use only the amplifier and the cmfb and remove the diff mode feedback?
thank you very much
 

jnuhope

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.stb analysis in spectre

xshou said:
Thanks to Middlebrook, his great double-injection method. Thanks to Tian and Kundert, you have iprobe and cmdmprobe in spectre. Stb analysis is just one click away.

You know all the basics of feedback control and the idea behind Nyquest stablity criteria. But, watch out!
The key question is: have you broken all the loops at the same breaking point? You are not suppose to break one loop while allow other loops. Stb analysis at such breaking-point is very very dangerous.

As in the above example, I assume your Gm block would have certain feedback configuration on a higher level -- e.g. resistor/capactor feedback between inn/inp and outn/outp. If so, that diff mode feedback also functions as common mode feedback (specially at high frequency). Therefore, breaking point a doesn't really break the new loop mentioned above.

Finally, I would suggest you break both common mode and differential mode loop at outp/outn by using cmdmprobe.

Of course, I will check breaking point a too but outp/outn is the main concern.

Good luck
If I am using hspice, how should I break both loops at once? And how do I tell the result is cmfb's loop stability or diff loop's stability? Or am I completely mis-understood?:?:

Thanks!
 

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