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Design >> Analog Design >> question about gainboost OTA
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Message started by lonemy on Apr 23^rd, 2009, 7:58am

Title: question about gainboost OTA
Post by lonemy on Apr 23^rd, 2009, 7:58am

dear all:
i am working on a gainboost OTA. the gain of main OTA(folded cascode) is 40dB, phase margin is 80deg and GBW is 200MHz. the gain of booster OTA(N and P are folded cascode as well) are 30dB, phase margin are 60deg and GBW are 300MHz.
after boost, the gain is about 70dB, but the GBW change to 270MHz. we know the GBW is gm/CL, but the booster OTA never change gm and CL, why this happens?

Title: Re: question about gainboost OTA
Post by subgold on Apr 23^rd, 2009, 9:13am

gm/CL is a very rough 1st order estimation. Now, neglecting the impact from all the nondominat parasitic poles, you already have at least two poles in the loop, the pole from the boost amp will result in a pole-zero doublet, that's why the GBW of your main amp increases. you must be careful about the selection of your poles, otherwise you may get some ugly settling. and i doubt it is a good scheme to have GBW of the boost amp larger than that of the main amp.

Title: Re: question about gainboost OTA
Post by raja.cedt on Apr 23^rd, 2009, 9:59am

hi,
can you please post the schematic,and please check the parasitic poles,i guess there are some poles near to the dominant pole or near to the UGB.

Thanks,
Rajasekhar.

Title: Re: question about gainboost OTA
Post by lonemy on Apr 24^th, 2009, 7:56am

hello everyone:
i re-designed my circuit today, this time i changed the phase margin of my boost OTA to 85deg. after boost, the change of GBW is very small (from 205M to 220M).
i think the change of GBW is come from the feedback of the boost OTA. because the phase margin is not enough, there will be imaginary poles exist in my circuit after boost. the imaginary poles will enlarge GBW of the whole OTA. am i right?
the attachment is the schematic of my circuit.

Title: Re: question about gainboost OTA
Post by lonemy on Apr 24^th, 2009, 7:58am

i am sorry i forgot the boosters.

Title: Re: question about gainboost OTA
Post by lonemy on Apr 24^th, 2009, 8:12am

subgold wrote on Apr 23^rd, 2009, 9:13am:

there is a famous paper "A fast-settling CMOS op amp for SC circuits with 90-dB DC gain", in this paper the authors point out the fu1(fu1 is the GBW of the boost OTA) should be larger than β*fu2(fu2 is the GBW of main OTA, β is the feedback factor) and less than the p2(p2 is the second pole of main OTA). so i choose the fu2=200M and fu2=300M, did you mean 300M is not enough? so how much should be enough?

Title: Re: question about gainboost OTA
Post by raja.cedt on Apr 24^th, 2009, 9:12am

hi,
i understood the problem, you have to keep boosters ugb grater than the ugb of the main OTA,so you are keeping one booster 200mhz and another booster at 300mhz.so please increase its 200mhz booster also to 300mhz, then i am sure your problem will solved. While designing this kind of circuits you keep chaging the booster ugb and find pole and zeros .
if you are interested you just watch the below 11th lectureslectrehttp://webcast.berkeley.edu/course_details_new.php?seriesid=2009-B-25830&semesterid=2009-B

Thanks,
Rajasekhar.

Title: Re: question about gainboost OTA
Post by lonemy on Apr 25^th, 2009, 1:44am

dear Rajasekhar:
i set the UGB of the main OTA at 200M, and the UGB of both two boost OTAs at 300M.
i think the key of this question is the phase margin of the boost OTAs. and you?