I've always used for my cascode a short L (0.5 µm), then increasing the gm.
I've heard that some peolple are using a long L for the cascode. I don't understand why, and I don't understand how it should works.
Have you some explanations for that ?
Thanks in advance
you normally use cascode for increasing rout. One transistor "shields" the other one, so its drain voltage experiences less variations. In that way, the output resistance is "amplified" by approx. gm x rout.
As for the L, the longer the transistor, the greater the output resistance. Cascode generaly has large outupr resistance, but sometimes you need to get as much as you can by increasing the length of your transistors.
But if you increase the lenght of the cascode, you will then decrease the gm and decrease by the same way the output resistance.
Or you mean, keeping the same W/L ratio and increasing L ?
Leo, also the excess gain added by cascode in this case is reduced by 2^0.5, you gain some headroom for the transistor near power rail, to prevent it from entering triode region, then its rds will be enhanced.
So, it is very difficult to judge (by simulation) whether shorter or longer L is better.
For me, I prefer to short one, still, I have reduced output parasitic cap!