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Design >> Analog Design >> How would you compensate this ?
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Message started by Tareeq on Nov 19^th, 2008, 10:53pm

Title: How would you compensate this ?
Post by Tareeq on Nov 19^th, 2008, 10:53pm

Hi Guys,

please have at look at the schematic below:

http://ess000.free.fr/voltreg.png

How would compensate this (multistage) regulator ? the issue is that tradiontal miller or nested miller may give a weak transient response.

As you see part of it has been done, since the capacitor across Rf1 create a zeros that compensage for the pole created by the large Cgs between V- and gnd.

your thoughts and suggestions are most welcomed.

Regards,

Tareeq.

Title: Re: How would you compensate this ?
Post by aamar on Nov 20^th, 2008, 2:21am

in any case you can't use nested miller compensation for this circuit because all your stages are inverting.

Regards,

aamar

Title: Re: How would you compensate this ?
Post by raja.cedt on Apr 25^th, 2009, 7:36am

hi Tareeq,
i didn't understand why you have kept that 2nd stage ...i don't see any advantage with that..can you please clarify and what is the main purpose of this regulator?
Coming to your circuit u have dominate poles at O1,output..i don't know how pole at A,because it is in a small local feedback.So can cancel 2nd dominant pole with feedback cap, and after that if you are meeting your regulator dynamic response..then think about how to move 1st dominate pole?Because what type of compensation is depend on how your transient response? so please post that

Thanks,
rajasekhar.

Title: Re: How would you compensate this ?
Post by thechopper on Apr 25^th, 2009, 10:47am

It looks to me it this loop should not be too hard to compensate. The reason being the loop gain is not that large:
- you have only one large gain stage (the folded cascode one), the other gain stages do not contribute with too much gain to the loop.
- From the above, the most straightforward way to compensate this would be a cap in O1 which, - as rajasekhar said - is the dominant pole. Miller does not work since the gain from O1 to O2 is small.
- Things will get worse when you consider the compensation required with the regulator load connected: on one hand the output stage gain will further decrease if the load is low impedance (good for stability). On the other hand the capacitive load will create a secondary pole that might shift low enough in frequency in order to complicate the stability if the load cap is large.

Tosei