Thur, 17th Sept 2015, 23:31 Re: AND and OR Gate Question

This question often comes from a student who thinks there is a better way
to use the transistors to design the AND and OR gates.  There is a problem
with it, but the reason is subtle, and with everything else I want you to
understand, I felt it not necessary to burden you with this.  So, if you
have enough to do with everything else in 306, feel free to delete this
message and move on.  However, I seem incapable of not answering a student's
question, so for those who are interested, feel free to read on.

A student writes:

> Dr. Patt,
> I was working on homework set 2 and thought of a question while drawing the
> AND gate. Why can we not switch the n and p type transistors in a NOR gate
> to form an AND gate? It seemed to work for each example I tried. I attached
> a picture to show you what I am trying to explain.
> Thanks in advance,
> <<name withheld to protect the student who is unaware of a subtle difference>>

Good question.  At first blush, it would certainly seem so!

The thing you are missing, and it is not your fault since I did not tell you
that it matters whether a P-type transistor is connected to the power supply
or ground, and similarly for an N-type transistor.

That is, as we pointed out in class, if the P-type transistor is connected to
the power supply, and the input to the gate is 0, the transistor acts like a
piece of wire with no voltage drop across it.  Similarly for an N-type
transistor connected to ground with the input to the gate = 1.

However, the electrical properties of the transistor are such that if you
connected a P-type transistor to ground, there would be a voltage drop across
the transistor, called the transmission voltage.  Similarly, for the N-type
connected to the power supply.

So, consider your AND gate constructed from transistors as you suggested.
Suppose the power supply is 2 volts, and the transmission voltage is 0.7 volts.
The design we did in class produces an output of 2 volts when we want an
output 1 and 0 volts when we want an output 0.  If we do it your way, we would
get an output of 1.3 volts when we want an output 1 and 0.7 volts when we want
an output 0.  We prefer the difference of 2 volts rather than a difference of
0.6 volts.

Clear?  If not, we can talk about it in class on Monday.

Good luck finishing the problem set on time.

Yale Patt