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Design >> Analog Design >> AB operation in a class-A amp???
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Message started by vivkr on Jan 18^th, 2008, 8:09am

Title: AB operation in a class-A amp???
Post by vivkr on Jan 18^th, 2008, 8:09am

Hi,

I built a simple class-A amp with 2 stages, and each stage has its own CMFB. I am suprised to see that the output settles to the
maximum accuracy even when I go on reducing the time available for settling. It looks very good of course, but can someone
suggest why this might be happening?

Thanks
Vivek

Title: Re: AB operation in a class-A amp???
Post by thechopper on Jan 19^th, 2008, 7:09pm

hi Vivek,

Is it the differential or output CM signal the one setting that fast? May be a drawing of your circuit might help better understand the behavior.

Regards
Tosei

Title: Re: AB operation in a class-A amp???
Post by vivkr on Jan 20^th, 2008, 11:16pm

Hi Tosei,

It seems to only be the differential output. Unfortunately, I cannot post a drawing on my circuit because
I would get into trouble if I did that. However, I can describe the architecture.

2-stages, first stage with NMOS input, NMOS cascode. Compensation cap returns to cascode node.
Switched-cap CMFB for both stages separately so that compensation is simpler. This scheme has been
around in many different forms, and although I have added a CMFB in each stage, I believe that people have
also done this.

I will see if I can find out something from my side.

Regards
Vivek