Mon, 21st Sept 2015, 00:15 Re: Ambiguity in Logic Gates?

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Often, a student's question allows me to reinforce what we discussed
in class, and also gives me the opportunity to teach you something else.
Such is the case here.

A student writes:

> Hi Dr. Patt, my name is *************** and I'm in your EE306 class. I just
> had a quick question or two about logic circuit ambiguity.
> If a logic circuit is supposed to return 1, is it okay if the n-type
> transistors at the bottom of the circuit are also connecting the output to
> the ground?

No, it is not okay.  Recall with the NAND and NOR, either there is always
a connection from output to the power supply or there is a connection to
ground, but not both.

What would happen if the output was connected to both power supply and
ground.  Answer: As I told you in class, you would have a wire going from
power supply to ground, and it would burn out the power supply.  ...or burn
down the house if the power supply was the wall outlet.  ...unless, of course,
the house's electrical system was protected with fuses or circuit breakers.
...which is why we have fuses and circuit breakers!

> And another question:
> If a logic circuit is supposed to return 0, is it okay if the n-type
> transistors at the bottom of the circuit are NOT connecting the ground to
> the output?

No, it is not okay.  If the n-type transistors are not connecting the ground
to the output, how does the output get to be 0?

OK?

As promised, since I have your attention: Suppose I have a transistor
circuit constructed like the NOT-gate with one important exception:
We have two inputs A and B.  A is connected to the gate of the P-type
and B is connected to the gate of N-type.

You know that if A=B=0, the output is 1, and if A=B=1, the output is 0.
(That is, if A=B, these two inputs are really one input, and we have a
NOT-gate.)

You know from my answer to the student's first question above that if
A=0 and B=1, you burn out the power supply.

What happens if A=1 and B=0?  In that case, the output is not connected
to the power supply and it is not connected to ground.  That is the output
is not connected to anything and we do not have a complete circuit.  We say
the output is "floating" (since it is not connected) and it is useless.

> Thanks,
> <<name withheld to protect the student who is figuring out
all the ways one can mess up a transistor circuit>>

Good luck with the rest of the course.  See you in class later today.

Yale Patt

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