NAND Gate

A NAND gate is a logical Gate that
creates a Low Output Signal only when all Inputs are High otherwise it creates High output signal. A NAND Gate is nothing but a combination of AND Gate and NOT Gate. Today we are going to discuss the Truth Table, Internal Circuit Design, and Symbol of the NAND Gate.

Basically, a NAND gate has three terminals- two input terminals, and one output terminal.

Equivalent Circuit of NAND Gate:

We know that the AND gate performs the logical AND operation and the NOT gate inverts the signal. So the NAND gate first performs the AND operation of the input signals and then inverts the signal.

The Concept and Operation of NAND Gate:

According to the truth table of the NAND the operation of a Two-Input NAND Gate given below,

As the NAND Gate has two input terminals so we can apply two input signals together. Suppose two Inputs are A and B.
1. When inputs A and B both are low then the NAND gate will produce a High output signal.
2. When input A is low and input B is high then the NAND Gate will produce a High output signal.
3. When input A is high and input B is low then the NAND Gate will produce a High output signal.
4. When inputs A and B both are high then the NAND gate will produce a Low output signal.

Truth Table of NAND Gate:

The inputs of the NAND Gate are represented by 'A', 'B', and 'C' and Output is represented by 'Q'.

Truth Table of Two-Input NAND Gate:

 A B Q 0 0 1 0 1 1 1 0 1 1 1 0

The Boolean expression of the two-input NAND gate is,

Truth Table of Three-Input NAND Gate:

 A B C Q 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 0

The Boolean expression of the three-input NAND gate is,

Internal Circuit Design of NAND Gate:

The internal circuit design of the NAND gate using a Transistor is shown below,

Circuit Design of NAND Gate using Transistor:

As you see in the above figure here two transistors are connected. When we applied high signals to both input terminals and Vcc is applied then the current will flow from the supply to the ground through the transistors T1 and T2. In that time no current will flow through the output terminal. So the output will be low.

When anyone input or both inputs are low then the current cannot flow through the transistors it will flow through the output terminal to the ground.

So only when both inputs are high then the output of the logic circuit will be low otherwise the output will be high.

Why NAND Gate is Called Universal Gate?

NAND Gate is called the Universal Gate because they are mostly used and they are also used to create other logical gates like OR Gate, NOT Gate and AND Gate etc.

AND Gate using NAND Gates:

You can Build a AND by using two NAND Gates. Connect both input terminals of the second NAND together and then connect the output terminal of the first NAND gate to the input terminal of the second NAND gate.

OR Gate using NAND Gate:

To build an OR gate you need three NAND gates.

NOT Gate using  NAND Gate:

By connecting two input terminals of a NAND together you can build a NOT gate.

Examples of NAND Gate ICs:

• Two-Input NAND Gate: IC 7400
• Three-Input NAND Gate: IC 7410
• Four-Input NAND Gate: IC 7420
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NAND Gate Truth Table, Internal Circuit Design, Symbol Reviewed by Author on February 04, 2019 Rating: 5