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Design >> Analog Design >> High speed OTA
https://designers-guide.org/forum/YaBB.pl?num=1159359998

Message started by chase.ng on Sep 27^th, 2006, 5:26am

Title: High speed OTA
Post by chase.ng on Sep 27^th, 2006, 5:26am

Hi all,

It appears to me that most high speed CMOS OTA like the one utilized in pipeline ADC (sampling rate > 100MHz) are usually implemented as single stage OTA such as folder cascode coupled with other tricks like gain boost, rather than 2-stage. My understanding is that compensation cap in the 2 stage structure will slow down the speed regardless of what kind of compensation is being used and therefore more power needs to be burned to increase the speed. I try to design some high speed 2 stage OTA before, but I found that at some point, burning more power will not really increase the speed without compromising the stability, I think that has something to do with the parasitic capacitance.

Is anybody here experienced in designing high speed OTA? Especially 2 stage OTA. I would really appreciate if someone can share his/her experience on the design, such as what is limitation? How fast (in terms of gain bandwidth product or unity gain frequency) can an OTA given a particular process?

Thanks and Regards,
chase

Title: Re: High speed OTA
Post by Sarig on Sep 29^th, 2006, 2:04pm

Hi,
First you need to look at output Cap. and Gain which you need, maybe 1 stage is fine for you ?
Mainly your output Cap will limit you for a given supply current. we are missing here numbers.
Sarig

Title: Re: High speed OTA
Post by chase.ng on Oct 1^st, 2006, 6:03pm

Hi,

My concern is that single stage normally will not able to provide large voltage swing. I would think it is very crucial to increase signal to noise ratio especially when supply is becoming lower and lower.

Thanks,
chase

Title: Re: High speed OTA
Post by Faisal on Oct 2^nd, 2006, 4:02am

Well, you need to first put some specifications down. What supply voltage you are on, what speed you are looking at etc ? Output swing, Gain etc will decide btw 1-stage or 2-stage. In 2-stage you can use a class-AB output stage for improved slew rate.

Hope this helps.

KR,
Faisal

Title: Re: High speed OTA
Post by ywguo on Oct 2^nd, 2006, 7:11am

Hi Faisal,

Would you give me an example, a paper, a patent, or any literature else, to show an amplifier with class-AB output stage that is used in a pipelined ADC? It is interesting.

Best regards,
Yawei

Title: Re: High speed OTA
Post by Faisal on Oct 2^nd, 2006, 11:49am

Hi,

A 1.8-V digital-audio sigma-delta modulator in 0.8-μm CMOS from Rabii, S.; Wooley, B.A.;

A low-voltage low-power fast-settling operational amplifier for use in high-speed high-resolution pipelined A/D converters: Lotfi, R.; Shoaei, O.;

KR,
Faisal

Title: Re: High speed OTA
Post by loose-electron on Oct 6^th, 2006, 2:59pm

Guys:

Two comments -

1. Google it - a lot of folks post their papers online.

2. Get a copy of the IEEE JSSC and search that database. If you are a memeber you can get the whole thing on DVD for about $80 USD.

jerry