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sushan Junior Member Offline Posts: 21	Re: miller compensation Reply #30 - Dec 10^th, 2011, 6:28pm I have a basic question. Can the miller cap be seen as small signal current path between 2-stages, where it steals the small signal current form the dominant pole side and gives the samll signal current to the non-dominant stage, which looks like pole splitting? (So, it doesnt matter which is dominant)
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: miller compensation Reply #31 - Dec 11^th, 2011, 7:25am hello, could you please explain in-detail, sounds interesting and i am the great fan of answering miller compensation Questions. Thanks, Raj.
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sushan

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Re: miller compensation
Reply #32 - Dec 11^th, 2011, 10:40pm

Basically, I was thinking, when pole starts its effect, it means the small signal current from the main branch, is stolen by the R-C combination, which forms the pole(Lets say X-Amps). So, if we introduce a small signal current branch (the miller cap), at a much earlier frequency itself we can see that X-Amps being stolen from the main branch(amp-branch), which suggest that the pole is appearing at a much low frequency. The same holds good for the non-dominant pole. This is my intutive way and please correct me if I'm wrong.

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Praveen K

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Re: miller compensation
Reply #33 - Dec 11^th, 2011, 11:25pm

Hi analogbeginer,

when miller compensating, the non-dominant pole moves to a higher frequency because of the feedback at the second stage and NOT due to the current thru the miller cap.

if you consider, the indirect way of compensation thru a current buffer (ahuja compensation type) or thru a source follower, the feedward current ("where it steals the small signal current form the dominant pole side and gives the samll signal current to the non-dominant stage") is in fact blocked! to get rid of the zero.

I guess your intuition is wrong when you apply to pole splitting!

Your intuition is right when applied to creation of a pole in general.

and i think it does matters which pole is dominant, before using miller cap compensation!!!!

cheers,
Praveen

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: miller compensation Reply #34 - Dec 12^th, 2011, 12:52am hello analogbigner, i agree with praveen, what he is saying is correct, but still i didn't get your idea of steeling current (it would be great if you try to explain with some fig becaz for me it is very difficult to understand verbally). Thanks, Raj.
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HdrChopper

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Re: miller compensation
Reply #35 - Dec 12^th, 2011, 6:38am

sushan wrote on Dec 11^th, 2011, 10:40pm:

Basically, I was thinking, when pole starts its effect, it means the small signal current from the main branch, is stolen by the R-C combination, which forms the pole(Lets say X-Amps). So, if we introduce a small signal current branch (the miller cap), at a much earlier frequency itself we can see that X-Amps being stolen from the main branch(amp-branch), which suggest that the pole is appearing at a much low frequency. .

Correct...current stealing at high impedance node means gain dropping -> roll off started -> pole effect is there.

sushan wrote on Dec 11^th, 2011, 10:40pm:

The same holds good for the non-dominant pole. This is my intutive way and please correct me if I'm wrong

Not correct: Miller cap adds (does not steal) current to the non-dominant pole node (output node). Thus "delays" the effect of this second pole (ie. moves it to higher frequency).

Best
Tosei

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sushan

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Re: miller compensation
Reply #36 - Dec 12^th, 2011, 10:02pm

@Praveen,
I agree with you. Yes feedback increase the cap on the dominant side, and alter the resistance on the non-dominant side(Since there is a phase change, by the time it sees the non-dominant pole). But, in the classical miller experiment with vacuum tubes, he observed, "when a cap is connected between a gain stage, cap takes in a large amount of small signal current, which looks like as though the cap is boosted". So can't this current be termed as the steeled current? Correct me if I'm wrong.

And as you correctly pointed out, this cannot be directly applicable to cascode compensation(Ahuja).

@the hopper,
Actually I was saying the same. I meant the small signal current is added to the non-dominant side -->which explains pole splitting(if current is stolen, it should be given to some other baranch, which is non-dominant part). Pls correct me if I'm wrong.

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Praveen K

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Re: miller compensation
Reply #37 - Dec 12^th, 2011, 10:55pm

Hi analogbigner,

I agree with you on steeling current and boosting cap as a result of that. this intution will also hold good for creation of pole in general.

But I don't agree, when you say that the non-dominant pole is moved to a higher frequency because of addition of current that was stolen before from dominant pole.

Because of miller cap, a part of the output signal (capacitive division between miller cap and gate cap) will again appear at the gate of second stage, and because of this a significant amount of current will be added to the non-dominant node (thru gm2), which is the one actually moves it to a higher frequency. The stolen current added thru miller cap does nothing to do with nondominant pole moving away (which is proven by ahuja compensation). And in the unapproximated expression for output pole, miller cap (Cc) contribution is very negligent.

It seems like, this addition of the stolen current creates a RHP zero which is useless!!!! But I can't picture this in my mind, in terms of how an added current would make the amplitude response go up and phase response go down......... Shocked

??????!!!!!

cheers,
k.praveen

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HdrChopper

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Re: miller compensation
Reply #38 - Dec 13^th, 2011, 4:07am

Praveen K wrote on Dec 12^th, 2011, 10:55pm:

The stolen current added thru miller cap does nothing to do with nondominant pole moving away (which is proven by ahuja compensation). And in the unapproximated expression for output pole, miller cap (Cc) contribution is very negligent.

Not correct: the Miller cap creates a short at a certain frequency which lowers the impedance on the output -> this forces the non-dominant pole to move at higher frequencies. At such frequency, the "stolen" current is fully injected in the output: two ways of describing same thing.
Miller cap DOES affect the non-dominant pole moving away.

That is why Miller effect creates the pole splitting effect.

Tosei

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