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CMOS gates consist of an output node connected
to a pull-up circuit that contains only PFETs

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and a pull-down circuit that contains only
NFETs.

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In our example, our gate computes F(A,B,C,D).

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It is observed that F(1,0,1,1) = 1.

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With that information, what can you say about
the value of F(1,0,0,0)?

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All CMOS gates are inverting which means that
if all your inputs are 0, then only the PFETs

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in the pull-up are on, and so the output = 1.

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If all your inputs are 1, then only the NFETs
in the pull-down circuitry are on, and the

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00:00:45,250 --> 00:00:47,550
output = 0.

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If we are told that F(1,0,1,1) = 1 and we
want to find out what F(1,0,0,0) is, we notice

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that the difference between the first set
of inputs and the second is that the third

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and fourth inputs have been changed from 1
to 0 while the others remain unchanged.

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Changing an input from 1 to 0 will turn more
PFETs on, and more NFETs off, which means

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that there are more possibilities of the pull-up
circuitry to pull the output up to high, and

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fewer chances of the pull-down circuitry pulling
the output down to 0.

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Since, we were already producing a high output
with even fewer PFETs turned on, we are guaranteed

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that by turning more PFETs on and turning
some NFETs off, the only thing we can do is

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maintain our high output.

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So F(1,0,0,0) = 1.