The circuit in problem 6 does use 6 transistors, but that does not implement
a 3-input OR gate. Rather, it implements ~a~b+~c. But what that circuit
doesn't show you is that if you want to use any of those input signals (~a,
~b, ~c), you need to use more transistors to invert the original signals (a,
b, c). Remember, for problem 8 it explicitly states, "For this problem,
please do not assume that you can complement inputs for zero cost."

Hopefully this helps.

Neal N. Oza

On 4/11/08, Byron Cheng <Byron@u.northwestern.edu> wrote:
>
> I'm looking over this question again and I'm still having problems
> understanding why three NMOS and three PMOS transistors don't work. For
> instance, isn't the circuit in problem 6 made up only of NMOS and PMOS
> transistors? I thought that even though they aren't good at transmitting
> both types of signals, because they are used in conjunction it works out?
>
> I know it's late, but I'd appreciate any clarification. Thanks
>
> _______________________________________
>
> Byron Cheng
> byron@u.northwestern.edu
> byroncheng@gmail.com
>
> -----Original Message-----
>
> >> It seems like problem 8 is very simple to do with three of each. Am I
> missing something?
> >>
> >> Jeremy
>
> Remember that NMOS and PMOS aren't good at transmitting both high and low
> voltages. Each is only good for one of the two. Additionally, remember
> that you cannot assume access to complemented inputs for this problem.
>
> -- David
>
>

-- 
Neal N. Oza
Graduate Student Researcher
Northwestern University
nealoza@u.northwestern.edu