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Failed an exam.PLS help me about a offset voltage in a two stage opamp

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paul871017

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Hallo,
professor asked me about the offset voltage in a two stage opamp.All the M are same , M1 and M2 have the same gate voltage .

but why does VA not equal to VF????


please help me .
Unbenannt.JPG
 

Because M3/M4 only has half the current of M6.
The Vgs of M6 can therefore never be the same as that of M3.
This means that M3/M4 have different vds, and hence from channel length modulation, will never have identical current.
This is the cause of the offset.

BTW, this is not an easy question IMO to a student. Failing an exam because of this, your institution's standards must be quite high.
 
Because M3/M4 only has half the current of M6.
The Vgs of M6 can therefore never be the same as that of M3.
This means that M3/M4 have different vds, and hence from channel length modulation, will never have identical current.
This is the cause of the offset.

BTW, this is not an easy question IMO to a student. Failing an exam because of this, your institution's standards must be quite high.

Thank u very much.
He just asked me this question,we stuck in it .and i failed.
 

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