9/16/04

A student writes:

     Dr. Patt,

     I have a question about transistors.

     Why can't we realize an AND gate by just switching the positions of the
     p-type transistors and the n-type transistors on a NOR gate?

     Thank you,

     **************  (a.k.a - the baffled one)
     << name withheld -- he calls himself the baffled one.  I would rather
     say "the insightful one >> 

Looks like tonight is my night for questions outside what I expect you to worry about. So, feel free to delete this one and move on. Or, save it for a rainy day. Or, spend time on it tonight. Up to you!

In any case, the answer is well beyond the scope of 306.

I should tell you to save it for some later course when they start discussing the electrical details of these CMOS circuits.

For now, let me say that what we want the transistors to do is provide an open circuit or a closed circuit, depending on the voltage applied to the gate. It turns out that we get pretty much exactly that when P-types are connecting to the power supply and N-types are connected to ground. Do it the other way and you get a small voltage instead of no voltage (closed circuit). This makes the separation between 0 and 1 smaller which is not terrific. Therefore, we do it the way we said in class.

That is, we connect them this way, rather than the way you suggest for "electrical" reasons, not for logic reasons. I hope that is enough to hold you for now. If not, talk to me after class, or if you can, save it for next year!

Yale Patt