Logic OR Gate

By Basar

Posted on May 29, 2021

Logic OR Gate Tutorial

An OR gate is a digital logic gate circuit that perform logic OR operation. The logic OR gate operation is when any of the inputs are high or “1” the output will be high or “1”, otherwise output is low “0” when all inputs are low “0”.

The logical symbol of two inputs OR gate is shown below:

Logic OR Gate

The Boolean expression of digital logic OR gate is the logical addition denoted by “+”. Boolean expression of OR gate is Q=A+B.

The operation of a 2-input logic OR gate as:

“If any of the inputs A or B is true, then output Q is true”

circuit diagram of OR Gate

2-input Transistor OR Gate

The circuit diagram of 2 inputs logic OR gate inclusive of Resistor–transistor logic – RTL connected together as shown in below the figure. In this circuit diagram the both of the inputs is directly connected with the transistor T1 and T2 with the base of transistor. Either of the transistors must be saturated “ON” for the output at point “Q”. If both of the transistors are in cut off region “OFF” at same time than we will not get any output from “Q”.

Logic OR Gate

The uses of logic gate to produce desire logic operation for the special purpose.

2-inputs OR Gate Circuit Diagram with diode

The OR gate can also be implemented by diode. The circuit contain two diodes whose inputs is A and B and one resistance the output terminal is X component as follows.

Logic OR Gate

In the above circuit diagram if any of the diode is forward bias the output will appear at X is high and when both of the diodes are not conduct its mans it is “OFF” the output will be low “0”.

Digital Logic “OR” Gate Types

2-inputs Logic OR Gate

In 2-inputs logic OR logic has two inputs terminal “A” & “B”. The output terminal is “X” if any of the input is high the output will also high and both the inputs is low the output will also low. The truth table of OR logic are shown below.

Symbol	Truth Table
2-input OR logic	B	A	Q
	0	0	0
	0	1	1
	1	0	1
	1	1	1
Boolean Expression Q = A+B	Read as A OR B gives Q

Boolean expression of two inputs OR logic is Q=A+B

3-inputs Logic OR Gate

In 3-inputs logic OR logic has three inputs terminal “A”, “B” &”C”. The output terminal is “X” if any of the three inputs is high the output will also high and if all three inputs are low the output will also low. The truth table of 3-inputs OR logic are shown below.

Symbol	Truth Table
3-input OR logic	C	B	A	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
Boolean Expression Q = A+B+C	Read as A OR B OR C gives Q

Boolean expression of two inputs OR is Q=A+B

Just like an AND logic, the OR logic can any numbers of input terminal. However the OR logic is available in market in 2, 3 and 4 input types. If the additional inputs will requires the logic to be cascaded together.