I am trying to determine which of the CMOS circuits in the question below have the logic function Y, but I am having trouble understanding how to do so. I have tried looking at the circuits and analyzing the inputs and outputs, but I am still unsure of how to determine the logic function. Can anyone help explain this to me or provide any resources that could assist me in understanding this? Any help would be greatly appreciated.

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    \$\begingroup\$ Homework questions require a visible attempt at a solution. \$\endgroup\$
    – Andy aka
    Jan 2, 2023 at 17:42

1 Answer 1


I cannot give you the answer, but instead, I'll give you the basic idea to solve any problem of the sort.

A logic 1 (normally, VDD) applied at the input of an NMOS will result in its drain going to 0 (i.e. it will invert the signal). Likewise, applying a logic 0 to a PMOS will make its drain go to 1 or VDD.

Knowing this, now you can imagine what would happen if you put two NMOS in series as those of #1 for A and B inputs. If either signal is 0, then its NMOS will be turned off. It doesn't matter if the other signal is a 0 or a 1, because the turned off NMOS will prevent the other NMOS to sink current.

On the other hand, the PMOS, when having 0 as an input, will drive their drain to VDD. It doesn't matter if only one input has a 0 and the other 1, since both PMOS are connected in parallel, one is enough to drive their drain node up.

Therefore, that stage is effectively implementing a NAND function.

Hope you can take it from there.


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