Question about Miller compensation

• Jun 18, 2005

• #1

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xiexi

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There is a classical op with MIller compensate, but because the load capacitor is very large (15p), so I think it is not suitable to compenate the second stage, I connect the Cc to output node. So there is a problem that a right half-plane zero comes out, why, I only can find a left half-plane zero generated by the compensate R, and how to cancel the right half-plane zero.

 

• Jun 18, 2005

• #2

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xiexi

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About compensation

The AC waveform.

 

• Jun 19, 2005

• #3

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ezt

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Re: About compensation

Hi.
First of all, I think at least in this circuit, you have no problem with zeros or other poles, because all of them have been located after the unity gain frequency. Besides, the magnitude of resistance in series with C1 (compensation capacitor) should be carefully chosen in order to move the RHP zero to the left half plane. maybe you should enlarge your resistance (but I don't think so). On the other hand, I think usually in Miller compensation, the magnitude of compensating cap should be larger than load cap. But in your circuit it is not that case. I don't know the aspect ratio of your devices. maybe the current of second stage is big enough to relax the compensation. Maybe, just maybe this peaking in frequency response is because of two complex poles.

Regards,
EZT

 

• Jun 21, 2005

• #4

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avinash

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Re: About compensation

hi xiexi,
why this resistance R1=1k has been introduced.

 

• Jun 23, 2005

• #5

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simov

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Re: About compensation

ezt said:

Hi.
I think usually in Miller compensation, the magnitude of compensating cap should be larger than load cap.

Click to expand...

why????

 

• Jun 23, 2005

• #6

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dog1357

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About compensation

normally they put resistors between the top transistors and Vdd or bottom transistor and ground, I guess it is because they want to take the effect of finite resistance of connection, so add one resistor can dominate the side effect of the connection.

 

• Jun 23, 2005

• #7

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ezt

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Re: About compensation

simov said:

Hi.
I think usually in Miller compensation, the magnitude of compensating cap should be larger than load cap.

why????

Click to expand...

Hi.
Because we want to make sure the dominant pole (the pole at the ouput of first stage) is far enough from other non-dominant poles specially the pole at the ouput node. But in Cascode-miller compensation the first non-dominant pole and RHP zero will move to upper freqs and hence the compensation capacitor can be smaller.

Regards,
EZT

 

• Jun 30, 2005

• #8

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vpecxuwei

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Re: About compensation

Where can I find the analysis of this circuit?
Thank you,

 

• Jul 19, 2005

• #9

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telleric

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About compensation

Does the opamp behave as a single pole one, if the RHP zero is moved to LHP to cancel the first non_dominant pole and the third and forth pole are very far away from the dominat pole? Thank you very much.

 

• Jul 20, 2005

• #10

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layes2

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Re: About compensation

do not cancel Lhp pole just move it to infinite

 

• Jul 22, 2005

• #11

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bunda_bindaas

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Re: About compensation

layes2 said:

do not cancel Lhp pole just move it to infinite

Click to expand...

Hi, what do you mean by just move it to infinity?? Usually pole-zero cancellation is done, to achieve better GBW response. What are you implying here?

 

• Jul 22, 2005

• #12

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nxing

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About compensation

pole-zero cancellation is very hard to control considering the process and manufacture variation. So, usually the zero just be moved to far enough.

 

• Jul 22, 2005

• #13

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020170

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Re: About compensation

in my opinion, the best method is pole - zero canceling among the frequency compensation methods although it is hard to control.