Is it feasible to use Vgs somewhat less than Vth ?

[jdp721]

Is it feasible to use Vgs somewhat < Vth ?

Hi,

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 ?
If yes, how much negative?

I am using UMC 180nm process...but any general info is also welcome.

Thanks..

 

[shanmei]

Re: Is it feasible to use Vgs somewhat < Vth ?

Yes, Vgs can be less than Vth.

Making them in the sub-threshold region has higher gm/id, more power efficiency and higher gm, which is good for noise minimizing and matching.

If you want very high bandwidth, you need to make it in the saturation region, making Vgs > Vth.