about rail to rail constant gm op-amp

[xz781122]

i am doing some research about rail to rail constant gm op-amp.But i don't know what is the "constant gm" meaning.Is "constant gm" input stage'gm constant or the whole amp's gm constant?i have read some books.They offer a equation such as sqrt(Ia)+sqrt(Ib)=const.But I don't know why?who can do me a favour.

 

[sunking]

it means when the common mode voltage change, the gm is constant.
For the mosfet,gm=KI**0.5, if keep Kn=Kp, then sqrt(Ia)+sqrt(Ib)=const

 

[ArchAngel]

This question arise from rail to rail input, commonly implemented using two diferential pairs. If ur bias point not choosen properly, there exists a situition that two pairs both operate, results a doubled gm.

search IEEE JSSC for rail to rail input opamp for suggestions.

 

[gevy]

See E-book

 

[xwcwc1234]

Many books existed for rail-to-rail opamp design such as "Low voltage CMOS Operational Amplifiers----Theory , Design and Implementation" ----by Satoshi Sakurai and M.Ismail

 

[DaBiz]

Be carefull with constant Gm Rail to Rail inputs cause input referred offset voltage and input referred errr in general will change dependant on common mode voltage