Offset in Rail-to-Rail opamp

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What are the main reasons for offset in a R-R opamp ? the output stage is a class-AB ...
the opamp has a constant gain through out the common mode range... Can any one post some online links or suggest some books for this ?
Please help me in this regard.
 

mismatch is the main reasons for offset .
you can read the book by razavi.
 

As in most circuits, mis-match is caused by variation in circuit elements and I'm sure you can find a way to derive an equation to quantify it.

Although for R-R op-amps, you might have to be extra careful with keeping your gain constant through the range since you need both n-type and p-type differential sides to get the job done. Soo, not only would you need to pay attention to device mis-matches, pay very close attention to the variation of gain due to mis-match which is probably what you are noticing.

PM me if my explanation didn't make sense..haha. I had designed an R-R opamp for a R-R 4th order filter about a year ago so some details are still in short-term memory

Good luck!
 
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