Folded cascode OpAmp
"structural offset"? I know only systematic and random offset, what do u mean?
MOST can operate in cut-off, triode, saturation, breakdown regions. During saturation it can operate at weak, moderate and strong inversion level. The transconductance effectiveness has maximum at weak inversion level. The keyword for better understanding this is "Gm/Id methodology".
Read this:
look for 1212king.pdf on
Assuming threshold voltage mismatch as the main source one can show that
sigma(Vos)^2=sigma(VthDiffPair)^2+sigma(VthMirrorMOSTs)^2*(GmMirrorMOST/GmDiffPair)^2
That is why better to increase transconductance of diff. pair MOSTSs regarding mirroring MOSTs.
Mismatch of cascode transistors has negligible effect on offset (if mirror MOSTs have enough Rds).
If u have questions it's easy to send me private message. I'm no able to discuss at work.