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# OR Gate Using Diode Resistor Logic

A **logic gate** is a digital electronic circuit that is used to implement different kinds of logical
and comparison operations. We have several types of logic gates such as OR Gate, AND
Gate, NOT Gate, NAND Gate, NOR Gate, etc. In this article, we will discuss the realization
of **OR Logic Gate** using **diode resistor logic**, i.e. DL OR Gate. But before going into the
realization of DL OR gate, let us discuss the basic of OR Gate.

## What is OR Gate?

**OR Gate** is a logic circuit which may have two or more binary inputs and gives an output
according to the combination of the inputs.

The OR Gate produces a logic 1 (HIGH) output, even if one of its input is in the logic 1
(HIGH) state. The OR Gate gives a logic 0 (LOW) output, only when each one of its inputs is
in the logic 0 (LOW) state. Therefore, the OR gate is also referred to as **any or all gate**. It
may also be called as an **inclusive OR gate** because it includes the state where both the
inputs can be present. The symbols of a two input and a three input OR gate are shown in
Figure-1.

## OR Operation

The OR operation is represented by ‘+’. The Boolean expression for the output of OR gate is given below,

$$\mathrm{Y = A+B+C+ \cdot \cdot \cdot }$$

Where, Y is the output of the OR Gate, A, B, C… are the input variables to the OR gate.

The operation of a logic gate is explained in terms of a **truth table** that represents the logical
relationship between inputs and output. The following table is the truth table of a three input
OR Gate.

Inputs | Output | ||
---|---|---|---|

A |
B |
C |
Y = A + B + 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 |

## OR Gate Using Diode Resistor Logic

We may realize the discrete OR gate by using diodes and resistors (called **diode resistor
logic**). The circuit diagram of the OR Gate using diode resistor logic is shown in the
following Figure-2.

Here, the inputs A and B to the gates may be either 0 V or +5 V. The operation of the OR Gate using diode-resistor logic is explained as follows −

When A = 0 V and B = 0 V, both the diodes D

_{1}and D_{2}are off. Hence, there is no current flow through the resistor R, so there is no voltage drop across the resistor R. Consequently, the output voltage Y = 0 V.When A = +5 V and B = 0 V, then the diode D

_{1}is on and the diode D_{2}is off. Thus, the output voltage Y = 5 V.When A = 0 V and B = +5 V, then the diode D

_{1}is off and the diode D_{2}is on. Therefore, the output voltage Y = 5 V.When A = +5 V and B = +5 V, then both diodes D

_{1}and D_{2}are on. Therefore, the output voltage Y = 5 V.

**Note** − In practice, there is a small voltage drop (about 0.7 V) takes place in diode itself.
Hence, the output voltage Y = 5 – 0.7 = 4.3 V. This output voltage is regarded as logic 1.

## Conclusion

From the above discussion, we can conclude that the OR Gate is a basic logic gate that is used to implement the operation of binary sum. The OR gate gives a logic 1 as output even if any of its inputs is at logic 1 state. It gives logic 0 output, only when all its inputs are at logic 0 state.