Visjnoe wrote on Mar 31st, 2010, 3:26am:Dear Rob,
Thanks for the feedback.
From handcalculcations on this basic structure, I can indeed see that you don't gain much in terms of output impedance. Therefore, a true cascoded current mirror (with both devices in saturation) looks like the better option (obtain gm*ro1*ro2 as output impedance).
Regards
In theory if the top device is deep enough into weak inversion both devices will be in saturation. However, it has to be pretty deep weak inversion. There is a transition zone between linear and saturation with an relatively low impedance even though the simulation says its in saturation. It seems like you need 200 mV or more of Vds on the bottom device before you get the full r
o impedance.
If you use a low-vt device the bottom device is definitely in saturation, but if that device's Vt is less than zero you can't diode connect the one side of the mirror which makes it pretty useless!
As a practical matter, the number of applications where this mirror works is pretty small. Mostly weak inversion applications.
rg