# OR Gate: Among The Basic Logic Gates

*An OR gate is a fundamental digital logic gate used in electronic circuits, producing a high output (1) when any of its inputs are high. It operates based on Boolean algebra, symbolizing the logical addition operation. The gate’s output is low (0) only when all its inputs are low.*

OR gate refers to the digital logic gate having two or more inputs with one output to perform logical or inclusive disjunction.

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### Table of Contents

## What is an OR gate?

This logic gate is represented with a plus (+) sign as it is used for logical addition. It is one of the three basic logic gates using which you can construct any Boolean circuit. Any function in binary mathematics can be implemented with them. To distinguish it from the XOR gate, it is also called as the inclusive OR gate. OR gates are available in the families of TTL and CMOS ICs logic. Standard 4000 series CMOS IC is 4071 which includes 2-input OR gates. TTL device is 7432.

The above diagram represents the switching circuit of the OR gate operation. In this switching circuit, A and B are the two switches with one lamp connected to the voltage source. According to the switching circuit, lamp L will be lit up in two cases.

- Either when both switches A and B are closed.
- One of two switches i.e. A or B is closed.

In case both the switches are open, then the lamp will not get lit up.

An OR gate operates on the basis of boolean algebra. An OR gate follows the logic operation of input and output signals allowing the signal to pass and stop through it. The combination of inputs is 2n. Here n denotes the number of inputs.

- The output of OR gate is true (1) even if at least one of the inputs is true.
- Unless every input is 0, the output of the OR gate will always be 1.

## Representation

In circuit diagrams, OR logic gate is represented by a curved shape that has two inputs and one output side. It is symbolized as one of the two following logic designs:

**American Logic Gate Symbol (MIL/ANSI Symbol)**

**European Logic Gate Symbol (IEC Symbol)**

Here, “≥1” represents that the output is activated by at least one active input.

Analytical Representation:

*f***(***a***,***b***) = ***a*** + ***b*** – ***a*** * ***b*

*f*(0,0) = 0 + 0 – 0 * 0 = 0

*f*(0,1) = 0 + 1 – 0 * 1 = 1

*f*(1,0) = 1 + 0 – 1 * 0 = 1

*f*(1,1) = 1 + 1 – 1 * 1 = 1

## Types of OR Gate

Technically, you can have an OR logic gate with any number of inputs. The output will be true if any of the N inputs are true. Here, we are discussing the two common types of OR gates.

## 2-Input OR Gate

Also known as Basic OR logic gate, this type of OR gate takes in two input values to produce a single output value. Here there are two input values, hence the possible combination of inputs will be 4.

### OR Gate Truth Table For 2-Input Gate

The following table represents the OR gate truth table. Remember that here, 0 represents false and 1 represents true.

A |
B |
A OR B |

0 | 0 | 0 |

0 | 1 | 1 |

1 | 0 | 1 |

1 | 1 | 1 |

## 3-Input OR Gate

This OR logic gate has three inputs and gives an output of true if any one, two, or all three of its inputs are true.

### OR Gate Truth Table For 3-Input Gate

The following table is the 3-input OR truth table:

A |
B |
C |
A OR B OR C |

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 |

## Addition Using OR Logic Gate

Let us understand how addition can be performed using 2-input OR logic gate considering the below-given table for reference:

A |
B |
A OR B |

0 | 0 | 0 |

0 | 1 | 1 |

1 | 0 | 1 |

1 | 1 | 1 |

**Case 1:** If A = 0 and B = 0

Using the truth table, A OR B = 0.

It means that neither A nor B is true, so the output is 0.

**Case 2:** If A = 0 and B = 1

Using the truth table, A OR B=1.

This means at least one of A or B is true, so the output is 1.

**Case 3:** If A = 1 and B = 0

Using the truth table, A OR B =1.

Again, since one of A or B is true, so the output is 1.

**Case 4:** If A = 1 and B = 1

Using the truth table, A OR B = 1.

Both A and B are true, so the output is 1.

## FAQs

**How many inputs can an OR gate have?**

While the most common OR gates have two inputs, they can be designed with multiple inputs. There's no theoretical limit to the number of inputs.

**What is the Boolean expression for an OR gate?**

For a 2-input OR gate with inputs A and B, the Boolean expression is A + B or A ∨ B.

**How is an OR gate different from an AND gate?**

An OR gate outputs '1' if any input is '1', while an AND gate outputs '1' only if all inputs are '1'.

**Can OR gates be used to create other logic gates?**

Yes, OR gates can be combined with other gates to create more complex logic functions.

**What are some real-world applications of OR gates?**

OR gates are used in various electronic systems, including alarm systems (where any triggered sensor activates the alarm) and computer memory addressing.

**Can OR gates be implemented with transistors?**

Yes, OR gates can be built using transistors in various configurations, such as with bipolar junction transistors (BJTs) or field-effect transistors (FETs).

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