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Subject: Re: Wide Swing Folded Cascode OTA
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Date: 06/20/01 at 3:27 PM
Posted by: Ron H
In Reply to: Wide Swing Folded Cascode OTA posted by George Shafik on 06/19/01 at 1:17 PM:
If you look at every transistor, you will see that the PMOS transistors all have their bulks tied to their sources, while the NMOS transistors all have their bulks at node 0. This implies to me that the part is made without using separate well isolation (triple-well) for each NMOS transistor. In other words, the NMOS transistors all have a common bulk region (the substrate). NMOS sources which are not tied to common (ground or Vee) cannot be connected to bulk, because that is ground (or Vee).
The PMOS transistors can have their bulks tied to their sources, because each PMOS transistor must be built in an N-well anyway. The only advantage to using a common N-well (all PMOS bulks tied together) would be a savings in die area. This is because each N-well must have some space separating it from the others if they are not common.
The advantage of using separate N-wells lies in avoiding the body effect, wherein the bulk acts like a weak JFET gate. Reverse bias from bulk to gate tends to pinch off the channel, reducing transconductance. This is also true of the NMOS transistors when all bulks are tied to substrate, as is the case here. Not using triple-well saves at least one process step, as well as some die area. This all means less cost. Since NMOS transistors have higher transconductance than equal-sized PMOS transistors, this is not too much of a disadvantage.
I hope this answers your question.
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