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Design >> Analog Design >> pole splitting
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Message started by raja.cedt on May 17^th, 2009, 9:08am

Title: pole splitting
Post by raja.cedt on May 17^th, 2009, 9:08am

hi
i have a basic question on pole splitting.
I have read about miller compensation that it makes dominant pole more dominant and non dominant more non dominant.
My intuition about this is first stage load cap becomes more because of miller effect of 2nd stage and 2nd stage o/p impedance will decrease because of feedback, so first stage o/p pole frequency reduce and second stage o/p pole frequency increase.So if originally first stage o/p is dominant pole is more dominate and then splitting happens but if 2nd stage pole dominates then poles come towards...can any body explain this flaw.....

Thanks,
Rajasekhar.

Title: Re: pole splitting
Post by jqjqjq on May 18^th, 2009, 12:35am

Hi,raja, I have the same intuition with you from Allen's book. For your question, I think that op amps is designed in the way that the first stage gives the dominant pole and the second stage give the non-dominant one, just as the first stage is always the transconductance stage and the second be buffer with high output current.
If what you assumed happen, we should use other ways to compensate the opamp and perhaps Miller compensation will fail.
Hope helpful.
Best regards.
jqjqjq

Title: Re: pole splitting
Post by raja.cedt on May 18^th, 2009, 1:09am

hi
jqjqjq, thanks for your reply but what i heard is whatever the dominate pole still miller compensation will works,you can refer the following paper

Considerations for Fast Settling Operational Amplifiers

Thanks,
Rajasekhar.

Title: Re: pole splitting
Post by ndnger on May 18^th, 2009, 2:45am

Raja,

You should derive the exact transfer fuction using Gm and Ro models for first and second stage to get the complete picture which can confirm the intuitive results. Here's a couple of things which will happen in this case:

1. Given: second stage output pole is dominant.
2. Application of miller is valid only upto the second stage pole frequency as after this frequency the second stage gain starts falling with 20 dB/decade. You might still be able to use miller gain but then you should use A2(s), and not the DC value of A2.

3. Intuitively, I can say that the second stage pole will decrease by a smaller amount as long as A2 at first stage pole frequency is greater than 0 dB. Second stage pole frequency will decrease because Cc is not acting as a short between gate and drain of second stage MOSFET if it is smaller than the ouput capacitance of second stage. Hence, both second stage pole and first stage pole will decrease while first stage pole will decrease rapidly.

4. When compensation cap is made larger than the output capacitance of second stage it will act as a short between gate and drain of second stage mosfet and the second stage pole will increase as Cc is increased.

5. At around this frequency or when compensation cap is made very large (compare to output cap of second stage) first stage pole becomes dominant and the usual pole splitting behaviour should be observed thereafter.

All these observations are intuitive and only a complete analysis could establish their reliability.