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jjv New Member Offline Posts: 5	miller compensation Aug 24^th, 2011, 7:52am In an uncompensated two stage opamp if load pole is dominant than first stage pole will miller compensation help. Won't it make the nondominant first stage more dominant.
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raja.cedt

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Re: miller compensation
Reply #1 - Aug 24^th, 2011, 8:15am

hello be clear while asking you question. I think what you are asking if load pole is dominate than first stage o/p pole how miller compensation will help.

the answer for this question is what-ever the location before compensation, after that dominate becomes more dominate as well as non-dominate becomes more non-dominate.

Get back to me if you have any question.
Thanks.

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jjv

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Re: miller compensation
Reply #2 - Aug 24^th, 2011, 8:35am

Hi raja,
You understood my question correctly. But how can the dominant become
more dominant and non dominant become more nondominant. The miller
multiplied cap is going to come at the first stage output making it dominant from non dominant condition.

Also output remain close to same. So both poles should come closer after compensation.

Am I right.

Thanks,
jjv.

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jjv New Member Offline Posts: 5	Re: miller compensation Reply #3 - Aug 24^th, 2011, 8:38am I meant Also output pole remain close to same location before compensation. So both poles should come closer after compensation. Am I right. Thanks,
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buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: miller compensation Reply #4 - Aug 24^th, 2011, 8:55am i jv, do you know what it means when one pole is "dominant"? Can you describe under which condition a pole is dominant? Only then you can find and understand the answer to your question.
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jjv New Member Offline Posts: 5	Re: miller compensation Reply #5 - Aug 24^th, 2011, 9:05am buddy, A node is dominant when the RC product at that node is greater than other nodes
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raja.cedt

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Re: miller compensation
Reply #6 - Aug 24^th, 2011, 9:13am

hello,
first off all they don't approch each other. I can tell explain through graph. Think compensation cap is feedback around the opamp. So the resultant transferfunction is having well seperated poles (dominate becomes more dominate and non dominate as well). I can explain through some otherway (because this is a special case, i spend so much of time to found this. here you dont see miller multiplied cap however you will get miller multiplied resister) bit complicated to explain. But i am sure this fig gives very good explanation.

Hope it helps.

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miller.png

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Rakesh Community Member Offline Posts: 60	Re: miller compensation Reply #7 - Aug 24^th, 2011, 11:35am Hi, After miller compensation irrespective of the initial pole locations Always the first stage pole will be dominant and the second stage pole will be non dominant. Rakesh
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buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: miller compensation Reply #8 - Aug 24^th, 2011, 12:10pm jjv wrote on Aug 24^th, 2011, 9:05am: buddy, A node is dominant when the RC product at that node is greater than other nodes I suppose you mean "pole" (rather than "node")? Or do you define dominant nodes? Your explanation holds for real poles only. A general definition is based on the pole location in the complex s-plane.
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Praveen K

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Re: miller compensation
Reply #9 - Sep 26^th, 2011, 3:06am

hi jjv,

as far as i know, the miller compensation will be helpful only when the pole of first stage is dominant. In your case the pole at the second stage is dominant, which occurs when the load cap is huge.

In this case my suggestion is just leave output pole to be dominant and put a buffer in between the two stages, so that the resultant non dominant poles at the input and output of buffer are far away from dominant.

praveen

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: miller compensation Reply #10 - Sep 27^th, 2011, 6:06am hello praveen, miller compensation will be usefull irrespecive of the dominate poles location. I would say "miller capacitor makes dominate pole more dominate and non-dominate more non-dominate" I didn't understand your 2nd point, could plz explain more.... Thanks, Raj.
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Praveen K

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Re: miller compensation
Reply #11 - Sep 27^th, 2011, 11:45pm

hi raj,

I'm attaching the STB analysis of a LDO driving a load of 10nF. The first stage is an high swing OTA, second stage is a PMOS of large size to drive the 10nF. In this case the dominate pole is the output pole and first non dominant is the gate of PMOS.

in the fig, (red--case with no compensation cap , green--with miller compensation of 100pf) you can see that adding a 100pF miller cap moved the nondominant pole(@ gate of PMOS) significantly to lesser frequency and doesn't help in anyway in PM. the dominant output pole remains almost same.

So i'm afraid i can't agree with what you say, cause miller cap can't help here. I agree with jjv, with his original argument.

Instead of this say we add a buffer stage in between the OTA and PMOS. here we will have three poles, dominant one again is the output pole, the two other non dominate poles( one at the output of OTA, another at the gate of PMOS) will be at a higher frequency because your buffer will have lesser input cap and low output impedence.

correct me if i'm wrong.

regards,
Praveen

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ACsim_load10nF.PNG

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: miller compensation Reply #12 - Sep 28^th, 2011, 6:00am hello praveen, let me think about this, but mean while you can read this paper. Re-examination of Pole Splitting of a Generic Single Stage Amplifier Thanks, raj.
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Praveen K Junior Member Offline Posts: 25 Bangalore	Re: miller compensation Reply #13 - Sep 28^th, 2011, 6:54am hi Raj, I don't have access to IEEE transactions on circuit and systems. will be thankful, if possible you can post me that paper. interested in reading it, praveen
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: miller compensation Reply #14 - Sep 28^th, 2011, 7:33am i can't. It's ieee, soplease give your mail id so i can send you. Thanks.
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