OR Gate Definition
An OR gate is a basic logic gate used in digital electronic
systems and circuits to implement the logical sum. In other words, a basic logic
gate that is used to implement logical disjunction or logical addition is
referred to as an OR gate.
An OR gate can accept two or more inputs and give only one
output. The output of the OR gate is HIGH or logic 1 if any of its inputs are
HIGH or logic 1. The output of the OR gate is LOW or logic 0, only when all of
its inputs are LOW or logic 0.
The OR gate operation is represented with the plus sign ‘+’
as it is used to perform logical addition.
Sometimes, the OR gate is also called Inclusive OR Gate to differentiate it from the XOR gate (Exclusive OR gate).
OR Gate Logic Symbol
The logic symbol of a two-input OR gate is shown in the
following figure.
The logic symbol of a three-input OR gate is shown in the following figure.
OR Gate Truth Table
The truth table of a two-input OR gate is shown below:
|
Inputs |
Output |
|
|
A |
B |
Y |
|
0 |
0 |
0 |
|
0 |
1 |
1 |
|
1 |
0 |
1 |
|
1 |
1 |
1 |
The truth table of a three-input OR gate is given below:
|
Inputs |
Output |
||
|
A |
B |
C |
Y |
|
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 Function
The Boolean function or logical function of a two-input OR
gate can be directly obtained from its truth table. The Boolean expression of a
two-input OR gate is given below.
Y = A + B
Where A and B are the input variables to the OR gate, and Y
is the output variable. This expression is read as Y is equal to A OR B.
Similarly, we can derive the Boolean expression of a
three-input OR gate from its truth table. The following is the Boolean expression
of a three-input OR gate.
Y = A + B + C
This expression is read as Y is equal to A OR B OR C.
We can extend this logical expression of the OR gate to any
number of input variables.
OR Gate Transistor Circuit
The transistor circuit of the OR gate is shown in the
following figure.
As we can see, a two-input OR gate can be implemented by using two BJT transistors. This circuit is also known as TTL OR Gate or Transistor-Transistor Logic OR Gate.
Operation:
This TTL OR gate will operate as follows:
- When the inputs A and B are LOW (0 V), both transistors T1 and T2 will be off. The output line will connect to the ground, making the output of the circuit LOW, i.e. logic 0.
- When A is LOW and B is HIGH, the transistor T1 will be off and transistor T2 will be on. The output line will connect to the power supply through the transistor T2. Hence, the output of the circuit will be equal to the supply voltage +VCC, making the output HIGH or logic 1.
- When A is HIGH and B is LOW, the transistor T1 will be on and transistor T2 will be off. The output line will connect to the power supply +VCC through the transistor T1. Thus, the output of the circuit will be equal to the supply voltage +VCC, making the output HIGH or logic 1.
- When both inputs A and B are HIGH, both transistors will be on. The supply voltage +VCC will be available at the output line, making the output of the circuit HIGH or logic 1.
This is how the above-shown transistor circuit acts as a logic
OR gate.
OR Gate Switching Circuit
The switching circuit of a two-input OR gate is shown in the
following figure.
In this circuit diagram, it can be seen that when both switches ‘A’ and ‘B’ are open, the circuit will be incomplete, and the lamp will remain off i.e. LOW output.
If both or any of the two switches is closed, the circuit
will complete and current can flow, turning the lamp on, i.e. HIGH output.
This is how the above-shown switching circuit is equivalent
to the logic OR gate.
OR Gate IC
The OR gate IC 7432 is one of the commonly used OR gate IC.
It is a Qaud 2-Input OR Gate IC,
having four 2-input OR gates.
The pin diagram of the OR gate IC 7432 is shown in the
following figure.
It has 14 pins and can operate with a power supply in the range of 2.0 V to 6.0 V. The IC 7432 OR gate is a TTL logic OR gate IC.
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