This article will explain the concept of NOT gate operation in digital electronics along with its truth table, logic symbol, switching circuit diagram, etc. The NOT gate is one of the main building blocks of Digital Logic Circuits. A NOT Gate, also called an inverter, has only one input and one output. It is a logic gate whose output is always the complement of its input.

Here, we will cover NOT Gate using transistors with the working of the circuit and its applications, advantages, and disadvantages, accompanied by solved examples and answers to frequently asked questions.

What is NOT Gate?

In digital circuits, the NOT gate is a basic logic gate having only a single input and a single output. The output of the NOT gate is logic 0 when its input is logic 1 and the output is logic 1 when its input is logic 0. Thus, the NOT gate is used to perform the inversion operation in digital circuits. It complements the input and produces a corresponding output. Being a basic logic gate, the NOT gate is used in a variety of digital circuits to design and implement their basic logic circuits.

Symbol of NOT Gate

The logic circuit diagram of the NOT gate is shown in the following figure. It has only one input line denoted by the letter A and one output line denoted by the letter Y, where Y is the complement of the input signal applied to it. At the output line, the bubble denotes the complement operation and it is called the inversion bubble.

NOT Gate With Truth Table

Truth Table of NOT Gate

The truth table for the NOT gate is as follows. It shows the relationship between input applied and output generated and the working of the NOT gate for all the possible inputs. In a NOT gate, the output (NOT A) is the opposite of the input (A). So, when the input is 0, the output is 1, and when the input is 1, the output is 0.

Operation of NOT Gate

NOT Gate takes the Boolean value as input and returns the following results:

Returns 1, if the input is 0.

Returns 0, if the input is 1.

The Boolean expression of NOT Gate is as follows:

Y = Ā

or

Y = A’

Where the input variable to the NOT gate is represented by A and the output variable by Y. The symbol for NOT operation is ‘ – ‘ (bar) or ` ‘ ` (complement).

Use of Active-low Input Bubble

In digital logic the Active-low Input Bubble is represented by a small circle at the input of a gate symbol which indicates that the logic level is inverted. In the case of a NOT gate, which performs inversion, the active-low input bubble indicates that the gates operation is inverted. So, when the input signal has the active-low bubble, it means that the gate will produce an output that is the inverse of the input signal.

Bubble Notion for Input Inversion

Equivalent Gates

Now we will see how we can form not gate using universal gates(NAND and NOR gate).

NOT Gate Using NAND Gate

In digital electronics, we can implement the NOT gate logic by using the NAND gates only. For this, we tie all inputs of the NAND gate together and apply the input signal to the common input line as shown in the following figure.

NOT Gate using NAND Gate

The logical expression for this circuit is given by,

[Tex]Y = \overline{A} [/Tex]

NOT Gate Using NOR Gate

The NOR gate is another logic gate that can be used to implement the NOT gate logic. To realize the NOT gate circuit using NOR gate, we join all input lines of the NOR gate together and then apply the input signal to this common input line.

The operation of this circuit can be described using this equation,

[Tex]Y = \overline{A + A} = \overline{A} [/Tex]

NOT Gate by using NOR Gate

NOT Gate Implementation using Transistors

We can implement the NOT gate logic using a bipolar junction transistor. The circuit diagram of a NOT gate using BJT transistor is shown in the following figure.

Transistor Inverter

Working of the Circuit

A discrete NOT gate may be realized using a transistor as shown above. The input to the gate may be 0 V or +5 V. When A = 0V, transistor T is reverse biased and, therefore, remains OFF. As no current flows through R, no voltage drop occurs across R. Hence the output voltage Y= +5 V. When the input A= +5 V, T is ON and the output voltage Y= V_{cc (sat) ≈} 0V.

Truth Table for NOT Gate circuit: