Overdrive voltage understanding

[viperpaki007]

Hi,

I read in Behzad Razavi CMOS book that overdrive voltage of MOS is dependent of W/L ratio. I am confused about this because Isn't overdrive voltage Vgs-Vth volts?. Can't i control overdrive voltage by changing vgs voltage by myself. For example, if Vth= 0.5V and i put Vgs =0.7 then overdrive voltage becomes 0.2V. I can change vgs voltage and make overdrive voltage according to my wishes. right? In this scenario, what will be the effect of W/L ratio on overdrive voltage?

regards

 

[skamthey]

Hi Viper,
Consider the case of current mirror (One MOS diode connected, providing VGS to other MOS), in reference branch the reference current is say 1uA. than what will be its overdrive voltage (Say Vth=600mV).
Here we need to know what is W,l of MOSFET.
Say for a particular W,L if this overdrive is not sufficient (say 50mV) then to improve it to 100mV, we need to change W,L of Mosfet,

 

[salehkhan]

Hi
I can't remember where it was mentioned. However, it could be seen from another point of view:
As in saturation region
VOD=sqrt(2*ID/Un*Cox*(W/L))
so, for a given current, VOD is dependent of W/L ratio.
We can discuss more if you say the page where you are referring.

 

[SIDDHARTHA HAZRA]

Can you mention the chapter / page number from the book.
For a stand alone MOS or say a simple CS Stage amplifier the over drive voltage is fixed and depends on the Vg voltage (neglecting any variation of Vth with W & L).
But in case of a differential amplifier the overdrive voltage of the diff MOS will change with change in W/L for fixed Vg voltage as because the current through the diff pair MOS is decided by a current source below it.

So we have to see what is the scenario .... kindly post the chapter / page number ....

 

[viperpaki007]

Hi Skamthey and Salekhan,

Thanks for the help. Skamthey, you mentioned a specific case of diode connected transistor. Here we do not have control to select Vgs by ourself. Therefore, in this particular example of diode connected transistor, maybe we can say that overdrive voltage is dependent on W/L ratio. However, i am asking for the case where we can arbitrarily choose Vgs voltage of MOS by ourself. As we do have control of Vgs so can't we choose our required value of overdrive voltage Vgs-Vth by ourself. Why i need to be dependent on W/L ratio?

Salekhan, i read about overdrive voltage on page 299. Example 9.5 opamps chapter.

 

[SIDDHARTHA HAZRA]

i read about overdrive voltage on page 299. Example 9.5 opamps chapter.

Check my previous post where i mentioned about differential amplifiers.
In case of op-amps (differential amplifiers) the current is not controlled by the MOSFETs overdrive voltage as current is fixed from a current source MOS sitting below the input pair MOS.
So depending upon the current drawn the overdrive voltage is adjusted for a given size of MOSFET i.e. if current is fixed and you increase W/L the overdrive voltage will reduce (check saturation current equation of MOS).

In you example M1 and M2 size will control its overdrive voltage .... Try to visualize the same for M3-M6 if Vb1 & Vb2 are fixed ...

Hope it will help ...

 

[diarmuid]

Classically it is stated that the optimum Vov is ~ 200mV (see Willey Sansens book).

For modern processes the Berkely boys have shown Vov of ~ 200mV to yield the max ft*gm/Id product.

So setting Vov as 200mV is often a good starting point.

Note that for a current mirror however, a lower Vov is not optimum as this increases your gm and hence noise mirroring.

So if your starting point is a current mirror, I would recommend higher than 200mV (say 500/600mV).