ACWWong wrote on Nov 9th, 2006, 12:54pm:Hi aval,
try posting a diagram.. it might help you get a better replies... but your startup circuit sounds iffy.
btw i am not quite clear how this/why this 6*L thing works... can;t you just use cascode load to increase output impedance ?
cheers
aw
Hi,
I agree that a cascode may also be a good idea when the headroom is available. However, to answer your question, placing several
devices in series effectively increases the L, just as placing many in parallel would increase the W. You can think of it more easily
if you consider that even a single MOS transistor may be visualized as a distributed device, with several smaller devices combining in
series (split the channel into equal parts), and tied to the same Gate and Bulk. Although the channel pinches off towards
the drain end, there is still a channel present below the pinchoff point.
Considering the MOS device once more as a series of smaller MOS devices, it is easy to see that only some of the devices at the end
have their channels pinched-off, while the rest do not. Hence, the simulation shows all but one devices as operating in the linear region.
Regards
Vivek