# What is a NOT Gate? Logic Symbol and Truth Table

The NOT gate is also known as the inverter gate. It inverts the input logic. If the input of the NOT gate is 1, then the output is 0, and vice versa.

In digital electronics, the NOT gate is a basic logic gate consisting of a single input and a single output. The NOT or inverter gate gives a HIGH (Logic 1) output when its input is LOW (Logic 0), and it gives a LOW (Logic 0) output when its input is HIGH (logic 1). Therefore, it operates as an inverter, i.e. converts HIGH into LOW or LOW into HIGH.

## NOT Gate Definition

In digital electronics, the NOT gate has a single input and a single output. It gives a HIGH (Logic 1) output when its input is LOW (Logic 0), and it gives a LOW (Logic 0) output when its input is HIGH (logic 1). Therefore, it operates as an inverter, i.e. converts HIGH into LOW or LOW into HIGH.

The basic logic symbol of a NOT gate is depicted in the following figure.

In the logic symbol, the bubble at the right corner of the triangle is called the inversion bubble. It denotes a complement/inversion of the input signal.

## Truth Table of NOT Gate

The truth table is a table of input and output that shows a relationship between them and is used to analyze the operation of the logic. The truth table of the inverter logic is given below.

From the truth table, it is clear that the NOT gate performs the inversion operation by converting 0 into 1 or 1 into 0.

## Boolean Expression

Boolean expression is a logical function that provides a mathematical relationship between the input and output of a logic. The Boolean function of a NOT gate is given below.

Where A’ or (A bar) denotes the complement of the input signal.

## Electrical Equivalent Circuit

The electrical equivalent circuit of the NOT gate is shown in the following figure.

The circuit consists of a battery, a switch, and a lamp. This realizes the NOT operation as follows:

• When the switch is open, i.e. A = 0, the current through the circuit follows the path from the battery to the lamp and comes back to the battery. This makes the lamp glow, i.e. output is high, i.e. Y = 1.
• When the switch is closed, i.e. A = 1, the current follows the path through the switch, as it provides a short-circuited path with low resistance. This time no current will flow through the lamp, making it turned off, i.e. output is low or Y = 0.

This way the above shown circuit realizes the inverter logic operation.

## NOT Gate in Transistor Logic

The NOT gate can be implemented by using a bipolar junction transistor (BJT). The circuit diagram of an inverter using a bipolar transistor is shown in the following figure.

This circuit is also known as a transistor inverter.

When a high voltage (say +5 V) is applied at the input A, the transistor becomes ON, and the current flows from +VCC to the ground through the transistor, making the output low, or 0.

When the input voltage at A is low (say 0 V), then the transistor becomes OFF, and the current does not find any path to the ground, and the entire supply voltage (+VCC) will be available at the output Y, making it high, i.e. 1.

## NOT Gate ICs

The following table gives a list of commonly available inverter ICs in TTL and CMOS logic:

## 7404 NOT Gate IC

The IC 7404 is a commonly available NOT IC in the market. It has six NOT gates in TTL logic. The schematic diagram of the 7404 IC is shown in the following figure.

## Applications

The following are some major applications of NOT gate:

• It is widely used in CMOS inverters to produce waveforms.
• Used in microprocessors and microcontrollers as one of their fundamental components.
• It is used in printer-integrated circuits.
• Used in temperature control circuits.