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some problem of DC operational amplifier design?

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ithink

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hi, i am designing a dc opam, the amplifier require input common-mode voltage range from 0 to 300mv,especially includes ground. After dc sweeping, we find output is good when input common-mode voltage range is between 50mv and 300mv. while when input common-mode voltage i s under 30mv, the output comes out larger deflection.for example, input common-mode voltage Vincm=10mv, the output should be 100mv after opam amplifying in theroy,but in fact output could be 103mv, which cause 3mv deflection. we expect the output is 0 when Vincm=0.

please help me decrease deflection by any method,thanks a lot!

best regards
ithink
 

use a rail to rail opa.
or you use a pmos as input stage.
 

I guess the deflection is due to the non-ideal effect, such as device mismatch, of mosfets. You can try to minimize it but it never disappears.
 

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