OR Gate Internal Circuit Design, Truth Table, Symbol

OR Gate 

OR gate is a logic gate, basically, it has two inputs and one output. The output of the OR gate will be HIGH when any one input or both input is HIGH. There are also more than two input OR Gates are available.

The logic gates are very important things in digital electronics. In fact, the concepts of digital electronics start with logic gates. Today we are going to discuss what is OR Gate, how we can make an OR gate, the internal circuit, truth table, and symbol.

OR Gate Symbol:

The concept of OR Gate:

We know that in Digital Electronics, the signal has two states HIGH or LOW. The states of the digital signals are also represented by the binary numbers '1' and '0'. Suppose in a digital circuit power supply and a load are connected. We can allow or stop the flow of current from the power supply to the load by a switch. When we allow flowing of current from the supply to load then it is called the signal is high and when we stop then it is called the signal is low. 

This is the concept of the states of the digital signal.

Let's understand the concept of an OR gate with a basic OR Gate which has two inputs and one output. When we apply a HIGH signal to only one input or both inputs of the OR Gate the Logic OR Gate creates a HIGH signal to its output. But if we apply a LOW signal to both inputs of the OR gate then the OR gate creates a LOW signal to its output. 

As we represent the High signal as '1' and the Low signal as '0' so you can understand the operation of the OR Gate from the Truth Table of the OR gate.

Truth Table of OR Gate:

The inputs of the OR Gate are represented by 'A' and 'B' and 'C'. The output of the OR Gate is represented by 'Q'

Truth Table of Two Input OR Gate:

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

The Boolean expression for Two Input OR Gate is, Q = (A+B)

Truth Table of Three Input OR Gate:

A	B	C	Q
0	0	0	0
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	1

The Boolean expression for Three Input OR Gate is, Q = (A+B+C)

How Logic OR Gates are Made?

We can make a logic gate using PN Junction Diodes or Transistors. Actually, the logic gates are made using transistors which come in the form of ICs.

Now let's know how to make OR Gate using Diodes and Transistors. The circuits which are given below are not the same as the OR Gate ICs. To convince you the basic circuits are given below.

Design of Logic OR Gate using Diodes:

Here You can see in the above figure, two diodes are used. The outputs of the two diodes are connected together and the output of the whole OR Gate circuit is also connected to the outputs of the Diodes.  Now if we apply a high-level signal to any Diode or both diodes the output of the OR gate circuit will be high.

When we apply a high-level signal to only one diode then the signal passes through that diode and goes to the output, this signal can't pass through the other diode as it will be reverse bias.

Design of Logic OR Gate using Transistors:

In the above figure, you can see two Bipolar Junction Transistors are used. The Emitter of each transistor is connected together and the output of the whole OR Gate circuit is also connected to the emitter of the Transistors. The Base terminal of the transistors is taken as input. When we apply a high-level signal to the base of the transistor the current starts flow from the collector to the emitter.

Design of Multi-Input OR Gate using Two-Inputs OR Gates:

Here six Inputs OR Gate shown below,

OR Gate ICs:

The Two-input OR Gate IC is: IC 7432

The Three-input OR Gate IC is: IC 4075

The Four-input OR Gate IC is: IC 4072

Read Also:

• NAND Gate Truth Table, Internal Circuit Design, Symbol
• AND Gate Truth Table, Internal Circuit Design, Symbol
• Exclusive OR [XOR] Gate Truth Table, Internal Circuit Design, symbol
• NOR Gate Truth Table, Internal Circuit Design
• NOT Gate Truth Table, Internal Circuit Design, Symbol

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