sizing cascode transistor

[sharkies]

I think I've read somewhere that usually the top is sized bigger than the bottom transistor when we have NMOS cascodes. Or I may be wrong and it could be the other way around, can't remember. anyways, Why is this so?

 

[zxcynosure]

for NMOS pair, bottom mirror should have larger L to obtain good matching. the top pair are used to enlarge the output impedance

 

[snafflekid]

The gm of the top transistor multiplies by the ro of the bottom transistor, roughly. So, a shorter channel length on the top increases gm and a longer channel length on the bottom increases ro.

But even just having two equal sized mosfets in cascode greatly improves the output resistance.

The widths are ususally matched between the top and bottom transistors for layout convenience.