For me Vdsat is the required drain-source voltage at which Id enters the saturation region and the transistor starts to operate in strong inversion. When Id is saturated it depends only from Vgs and not from Vds, just like an ideal current source. Basically strong inversion is achieved by increasing the overdrive voltage (Vgs - Vt), which also ensures that random Vt variations would have less impact on Id. Therefore, better matching is achieved as well.

Regards!

Hi PalmRunner &  Tlaloc,

Thanks for your replies. PalmRunner's point is really something new!! But I'm not sure how much Vt variation ocurs after manufacturing. I mean if Vt  variation is large then the circuit that is generating the Vgs will also undergo some changes, which will make Vgs to vary also. So, I'm not sure whether this is the reason. That is my doubt, I may be wrong. May be you were referring to some other vt variation, not due to manufacturing.

What is the dependency of lambda on vdsat? As far as I know vdsat = (vgs-vt)/1.2 in strong inversion. I agree increasing L reduces mismatch, but I do not agree that it also increases vdsat!!

I have heard of some other explanation., I

In strong inversion the mismatch problem is reduced!!

I wonder why??

If the above statement is accepeted then yes, we need to have large vdsat values, as it implies strong inversion.

But why strong inversion....

It seems to be accepatable becoz strong inversion means all the square law equations of MOS (that we typically use for hand calculation) hold more accurately, the more we go to weaker inversion, the models become less reliable.  So the simulation results are no longer  reliable  :-X

Although in many other circuits where volage swing becomes important, we cannot afford to have such large vdsat values. For example, in low power supply cases. But we still maintain a reasonable vdsat value, >= 140mV!!  i.e. we still try to maintain strong inversion whenever possible.

Is reliabilty of the simualtion models the only issue???

Rajdeep

Hi Berti,

Thanks a lot. This should answer a lot of queries, nevermind give rise to some new ones  :)

Thnx!