jdp wrote on Jun 2nd, 2016, 4:19am:In an opamp, we can set the input-pair MOSFETs to be in Sub-threshold region of operation.
To what extent can we put them in sub-threshold?...like, can we use (Vgs - Vth) negative?
As I searched in forum posts, people say they use (Vgs - Vth) ~ 100-200mV which makes the MOSFET near subthreshold..but is it feasible to use a Vgs somewhat < Vth ?
(I would be using the opamp in a lower freq. application (say max. 5 MHz))
Sorry for the late response... you can make it as wide as you want and make Vgs-Vt negative, but at some point the leakage will really affect performance especially at hot temperature. Also, the gate to source capacitance gets larger.
I settled on Vdsat=150mV a few years ago as a reasonable starting point for a differential pair. I'm not sure what Vgs-Vt is for that operating point as it is slightly different than Vdsat, but I remember driving it negative before. Reducing Vdsat further by making the device wider has diminishing returns in gm, but the Cgs cap increases linearly.