What is NOR Gate in Digital Electronics

In this article, we shall discuss the definition, logic symbol, truth table, Boolean expression, transistor, and switching circuit of the NOR gate. NOR gate is basically a combination of two basic logic gates namely OR Gate and NOT Gate. It is used to implement various Boolean or logical functions in digital electronic systems. Let’s begin with the basic definition of NOR gate.

NOR Gate Definition

The NOR gate is a type of logic gate that is used to implement the inverted operation of the OR gate. It is a combination of NOT gate and OR gate, hence it is also called NOTed OR gate.

The NOR gate is a type of universal logic gate, as it can be used to generate any other type of logical function.

The NOR gate can have two or more input lines, but only one output line. Where, the output of the NOR gate is HIGH (logic 1), only when all its inputs are LOW (logic 0). The output of the NOR gate is LOW (logic 0) if one or more inputs to it are HIGH (logic 1).

NOR Gate Symbol

The logic symbol of a two-input NOR gate is shown in the following figure.

The logic symbol of a three-input NOR gate is shown in the following figure.

NOR Gate Truth Table

The truth table of a two-input NOR gate is shown here:

Inputs		Output
A	B	Y
0	0	1
0	1	0
1	0	0
1	1	0

The truth table of a three-input NOR gate is given below:

Inputs			Output
A	B	C	Y
0	0	0	1
0	0	1	0
0	1	0	0
0	1	1	0
1	0	0	0
1	0	1	0
1	1	0	0
1	1	1	0

From these truth tables, it is clear that the NOR gate produces a HIGH (logic 1) output when all its inputs are LOW (logic 0).

NOR Gate Boolean Expression

The Boolean expression or logical function of a two-input NOR gate can be directly written from its truth table. The following is the Boolean expression of a two-input NOR gate.

`\Y=(A+B)'=\overline{A+B}`

Here, A and B are the inputs, and Y is the output. This expression is read as Y is equal to A NOR B.

Similarly, the Boolean expression or logical function of a three-input NOR gate can be written from its truth table. The following is the Boolean expression of a three-input NOR gate.

`\Y=(A+B+C)'=\overline{A+B+C}`

NOR Gate Transistor Circuit

The transistor NOR gate circuit is depicted in the following figure.

This is a two-input NOR gate transistor circuit. It has two bipolar junction transistors (BJTs) in its circuit. This circuit is also known as TTL NOR Gate or Transistor-Transistor Logic NOR gate.

Operation:

This transistor NOR gate circuit will function as a logic NOR gate as follows:

• When the inputs A and B are LOW (logic 0), i.e. a low voltage (say 0 V) is applied to A and B. Both transistors T₁ and T₂ will be off and do not conduct. Therefore, the total supply voltage will appear at the output terminal, making the output of the circuit HIGH (logic 1).
• When the inputs A is LOW and B is HIGH. The transistor T₁ will be off and transistor T₂ will be on. Hence, the supply voltage will go to the ground through the transistor T₂, in other words, the output line will directly connect to the ground potential through the transistor T₂. Hence, for this input combination, the output of the circuit will be LOW (logic 0).
• When the input A is HIGH and B are LOW. The transistor T₁ will be on and transistor T₂ will be off. Hence, the supply voltage will go to the ground through the transistor T₁, in other words, the output line will directly connect to the ground potential through the transistor T₁. Hence, for this input combination, the output of the circuit will be LOW (logic 0).
• When both inputs A and B are HIGH, both transistors will be on and the output line will directly connect to the ground. Hence, in this case, also, the output of the circuit will be LOW (logic 0).

This way, the above-shown transistor circuit acts as a NOR gate.

NOR Gate Switching Circuit

The switching circuit of a two-input NOR gate is shown in the following figure.

In this circuit, when both switches A and B are open, the current will flow from the battery through the lamp, turning the lamp on. If one or both switches are closed, there will be a short-circuited path for current flow through the closed switch and no current will flow through the lamp, turning it off.

Hence, in this switching circuit when all inputs are LOW (open switches), the output is HIGH. When any of the inputs is HIGH (closed switch), the output is LOW.

This way, the above-shown switching circuit is equivalent to a two-input NOR gate.

NOR Gate IC

The most commonly use NOR Gate IC is the IC 7402. The NOR Gate IC 7402 is a quad 2-input logic NOR gate. That means it has four two-input NOR gates. The pin diagram of the NOR Gate IC 7402 is shown in the following figure.

Hence, this is all about NOR gate, its circuit diagram, logic symbol, and truth table.