Thanks~. FvM
Are you saying 'source degeneration'? Then I understand.
But I don't understand that someone used pmos r2r amp(2'nd pic) as BGR.
It doesn't need to lessen gain.
I don't know, why the designer wanted to set a specific gain. But he did, in both cases. Regarding input stage, they are only slightly different (N versus P, split currents sources versus split resistors).
That is an old trick used to improve slew rate at higher input
differentials. But personally I've found it hard to distinguish
from the simple single tail.