Overdrive voltage of MOS transistor

[viperpaki007]

Hi I am confused about how to control overdrive voltage of MOS. I read that overdrive voltage can be made smaller by increasing W/L ratio of MOS. Why is like that? I think that as overdrive voltage definition is Vgs-Vth, therefore, overdrive voltage should only be controlled by increasing or decreasing Vgs. Moreover, sometimes i have seen in simulations that shown overdrive voltage is different than actual values of Vgs-Vth voltages. From this i get a confusion that does overdrive voltage and Vgs-Vth are same thing?

 

[crutschow]

Increasing the W/L ratio reduces the ON resistance so a lower overdrive voltage (Vgs-Vth) can be used to obtain a given ON resistance. There is no particular fixed value for the overdrive voltage but beyond a certain value, the ON resistance no longer is significantly reduced.

 

[viperpaki007]

Hi crutschow, Thanks for the comments. My question is that is it possible to have a have a different overdrive voltage (chosen by W/L ratio) than Vgs-Vth voltage?

Vgs-Vth voltage can be controlled by applying a different Vgs voltage while overdrive voltage can be controlled by W/L ratio.