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Logic Gates


The inverter, often referred to as a NOT gate, is a logic device that has an output opposite of the input. It is sometimes called a negator. It may be used alone or in combination with other logic devices to fulfill equipment requirements.

When an inverter is used alone, it is represented by the symbol shown in the figure below. Symbols for inverters used in combination with other devices will be shown later in the chapter.

Symbol for an inverter
Symbol for an inverter.

If we apply signal A to the input of the inverter as shown in the figure below, then the output f will be the opposite of the input. At times t0 through t2, A is LOW. Consequently, the output is HIGH. At t2, A goes HIGH and as a result f goes LOW. f remains LOW as long as A is HIGH and vice versa. The output of an inverter will be the complement of the input. The Boolean expression for the output of this gate is f = A.

Inverter input and resultant output
Inverter input and resultant output.

The Truth Table for an inverter is shown below.

NOT gate Truth Table

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