XNOR Gate

The XNOR (exclusive NOR) function is simply an XOR followed by a NOT function, and therefore it is the complement of XOR with reversed output Q. This implementation is using a NOR gate (circuit 1), which we can replace with an AND gate with inverted inputs (circuit 2). However, students should be familiar with both implementations, because either can come up in the exam to fool you.

This formula uses a NOR gate.

We can use De Morgan's theorem to remove the NOR gate and replace it with an AND gate and two NOT gates.

This is an alternative form of a XNOR gate.

A typical question often starts with a formula that you can apply De Morgan’s theorem to convert the circuit into an alternative form of XNOR implementation.

XOR Truth Table

With the NOR gate circuit 1, the intermediate terms and final Q outputs are as shown in the truth table above.

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