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Design >> Analog Design >> booster amp in gain boosting ota design
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Message started by jszair on Jul 1^st, 2010, 11:07am

Title: booster amp in gain boosting ota design
Post by jszair on Jul 1^st, 2010, 11:07am

Hi all

the attached is a booster amp used a gain boosted ota, from one berkeley student's project report.

I don't understand why he used cascoded tail current source with one transistor controlled by biasing network and another by cmfb. Any particular advantage of this topology compared to normal cmfb tail current source (two parallel transistor)?

Thanks!

Title: Re: booster amp in gain boosting ota design
Post by Mayank on Jul 1^st, 2010, 9:00pm

Quote:

I don't understand why he used cascoded tail current source with one transistor controlled by biasing network and another by cmfb.

Just to make the TCS cascoded to improve Rout. Here, the current is controlled by the lower CMFB guided NMOS only, NOT the biasing ckt.

Quote:

Any particular advantage of this topology compared to normal cmfb tail current source (two parallel transistor)?

Just the advantage of Higher rout & closer to ideal current source. Other than that, it has a disadvantage of 100% cmfb control instead of only a part as is the case with parallel bias + cmfb control NMOSes.

Title: Re: booster amp in gain boosting ota design
Post by love_analog on Jul 29^th, 2010, 7:06am

Perhaps he using this method to close the CMFB loop. Never seen anyone do it like this before. If you plan to use it, make sure you write the equations well and simulate it (esp startup conditions since you will could have a zero operating point, ie the output is zero and the ckt still converges).