Should I focus on gate-overdrive or VDsat for selecting operating regions for current mirrors with low supply?
If I have Vdd=1.2V and 55nm process, and for the first stage (folded cascode) of a two-stage ota where I'm not too concerned with outupt swing, I have been sizing the current mirror devices such that they have ~150mV gate overdrive (Vgs-Vth=150mV and gm/id in the range of 10-12). Vdsat turns out to be slightly less than 150mV. I make sure that Vds is greater than Vdsat by 50mV at the DC operating point. The point to notice is Vdsat is bounded by gate overdrive.
Now with Vdd=0.72V and 22nm process, for the same task I don't have the luxury to choose such large gate overdrives. In fact, even for 5*Lmin the Vdsat is actually greater than gate overdrive when the overdrive is in the 70-120mV range.
To minimize relative current mismatch, large Vgs-Vt helps in strong inversion. But If I can't afford Vds (margin included) more than say 150mV, then Vdsat is 100mV. Vgs-Vth actually turns out to be ~70mV (gm/id value ~20).
What's wrong with my thought process?