NOT gate

Also called: inverter, logic inverter, inverter gate

A one-input logic gate, also called an inverter, whose output is always the opposite of its input: 0 becomes 1 and 1 becomes 0.

The NOT gate, or inverter, is the simplest gate there is. It has one input A and one output Y, and Y is always the complement of A:

A = 0 gives Y = 1.
A = 1 gives Y = 0.

In expressions it is written , read "not A" or "A prime". Many books draw a bar over the letter instead; the meaning is the same.

The symbol is a triangle pointing in the direction the signal travels, with a small circle at its tip. That circle, the bubble, is what does the inverting. A triangle with no bubble is a plain buffer, which passes the signal through unchanged.

Why you need it: AND and OR alone can never turn a 1 into a 0, so any function that needs a complemented input, like , needs an inverter somewhere. In practice you rarely need a separate NOT gate. A NAND or NOR with its inputs tied together works as one, and bubbles on other gates' inputs or outputs do the same job.

Two inverters in a row cancel out, a fact called double inversion.

Worked example

Example

A chain of three inverters

Three NOT gates are connected in series. The input is A = 1. Find the output of each gate.

	Y
0	1
1	0

1.
First inverter: 1 in, so 0 out.
2.
Second inverter: 0 in, so 1 out.
3.
Third inverter: 1 in, so 0 out.
4.
An odd number of inverters gives ; an even number gives back A. Three is odd, so Y = = 0.

Common mistakes

Thinking the triangle does the inverting. The bubble inverts; a triangle without one is a buffer.
Confusing with . A bar over a whole group inverts the result of the group, not each letter.
Forgetting that a NOT gate has exactly one input. Gates that combine two or more inputs and invert are NAND, NOR or XNOR.

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NOT gate is taught in Logic Gates.

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