opamp input transistor corner variation

[iamxo]

I use 0.18um cmos process to design my opamp, and the telescopic structure with Nmos input transistor is used. The body of input transistor is connected to gnd, however i find that when i do corner simulation, the opamp works abnormally at some corner condition, and the problem comes from the large vth variation of the input Nmos.

My common-mode voltage is fixed to 1.5v (that is gate voltage), and vth of input Nmos could be 1.14v at the worst corner, which makes the Nmos in the off region.

There is a deep-Nwell mos in the process, should I use this type Nmos (because the body could be tied to the source, which leads to no body effect)? People here usually use mos tied to gnd or tied to source when it is used as input transistor??

Thank you all ~

 

[dick_freebird]

I would want either BTS (your deep well), a much larger
and lower current density tail source and diff pair (getting
lower working Vgs there, and a usable lower Vds in the
tail device), or a PMOS follower input.

 

[iamxo]

What is BTS? could you explain in detail?

 

[dick_freebird]

Body Tied (to) Source, as you were contemplating.
Just another acronym for the collection.