vivkr wrote on Jun 24th, 2008, 11:02pm:Hi
I think you must have figured out from the previous old discussions that you basically have only 1 transistor when you connect 2 in series and have the same gate and bulk. So, there is really no need to worry about body effect as it is not relevant.
When we speak of body effect, we are speaking of the source end of the channel and the V(G,S) needed to create inversion there, but in your "pseudo-cascode" connection, that source end is the lower transistor, which anyway has its source and bulk tied together. The "channel" of the pseudo-cascode extends from the source of the lower device to the drain of the higher one.
Hi Vivek,
What I meant is that I still do not see clearly the differences between: A) Tying the both bulks together
B) Tying each bulk to the corresponding device´s source
According to source referred models (BSIM) in case A) the body effect should play a role in the device under saturation mode, since there would be a difference between its bulk and its own source voltages. Therefore Iout should not be exactly 1/2*Iref (I agree with your description of considering the actual source the one for the lower device and therefore body effect does not play a role, but I understand source referred models are not following such assumption)
For case B) source referred model predicts an output current of exactly 1/2*Iref since body effect on the upper device is eliminated (this is not true if we base the description of the problem based on considering the two devices as only one).
So, are we talking here about incompatible models (source referred vs body referred?) I would expect both models give the same results. It is highly likely I might be interpreting something wrong though.
Thanks and regards
Tosei