Random offset for a differential pair operating in sub threshold region

The random offset for a differential pair reduces if we increase the area of the differential pair. When we consider a differential pair which is operating in sub threshold region, then the area of that differential pair will be huge compared to a differential pair operating in saturation region.

So, can we conclude that all differential pairs operating in sub threshold will have low random offset by default?

Thanks,
Rama

First of all, operation mode is not considered, only area counts. However, distance mismatch could dominate local-area offset voltage mismatch, so careful high symmetry (interdigitation, common centroid) layout is necessary.

The offset voltage is related with two mismatch sources (in first order) - 1) differential pair threshold voltage mismatch and 2) current mismatch in the load of diff-pair.
As long as the mosfets are without halo implants, the first component is bias independent and equal to A_VTH/sqrt(AREA).
The second one is transferred to the input by input device transconductance as Delta_Id/gm_in. Transconductance is maximized for weak inverted mosfets and close to Id/n·Vt (≈30mS·Id), so for weak inversion devices we can minimized the influence of current mismatch in diff-pair. There is also an additional factor which is related with the current gain factor mismatch (oxide thickness and mobility) and inversion level, and for strong inverted mosfets is a product of overdrive voltage and µ·Cox mismatch.

So, we can concluded, biasing diff pair close to weak inversion is a good idea for offset minimization (also for noises) however has to be a trade-off with other parameters like bandwidth.