What is a multiplexer? Operation, types and applications

A multiplexer (MUX) is a combinational logic circuit, which selects and delivers a single input as output from multiple input lines. It performs the similar operation of a switch. Hence, we can call it a digital switch.

In digital systems, several digital signals may be available at the same time, but only one of them may need to be transmitted or processed. There comes a need for a multiplexer circuit. It has multiple input lines and a single output line.

The multiplexer selects a particular input with the help of a control signal and routes it to the output. Hence it is also known as a data selector. Generally, a multiplexer has 2ⁿ inputs and n selection lines and one output.

For example, a 4 : 1 multiplexer has four data inputs, two selection lines, and one output. Similarly, an 8 : 1 multiplexer has eight data inputs, three selection lines, and one output.

The below diagram represents the general circuit for 2ⁿ : 1 mux circuit. It has 2ⁿ data inputs(D₀, D₁, D₂, … D_2ⁿ-1), n selection inputs(S₀, S₁,… S_n), 1 enable input(E) and one output(Y).

There are different types of multiplexers like 2 to 1 multiplexer, 4 to 1 multiplexer, 8 to 1 multiplexer and so on depending on the number of inputs.

For better understanding of the multiplexer circuits, let us look at some of the types.

2 : 1 multiplexer

The 2 : 1 multiplexer has 2 input lines(D₀, D₁), 1 selection input(S) and 1 output(Y). the block diagram and the circuit for 2 to 1 multiplexer is shown below.

When enable input is set to 0, the mux will not work. Enable should be set to 1, to get the output from the mux.

When the selection input S=0, it gets applied to the first AND gate. The output of this AND gate(1) is 0 irrespective of the other input. The inverted S input (S=1) is applied to the second AND gate(2), whose output will be equal to D₁.

When S = 1, it is directly applied to AND gate(1), the output of the gate will be equal to D₀. The inverted S input(0) is applied to the second AND gate(2). Thus the output of the AND gate(2) will be 0 irrespective of the other input.

Thus, when S=0, second AND gate is enabled and when S=1, first AND gate is enabled. The operation is explained with the function table below.

From the function table, it is clear that, the output Y = 1, when ES’D₁ = 1 and ESD₀ = 1.

The output expression for 2 to 1 multiplexer is found to be,

4 : 1 multiplexer

A 4 : 1 multiplexer has four data input lines (D₀, D₁, D₂ and D₃), two selection lines (S₁ and S₀), and single output line (Y).

The two selection lines can produce four different binary combinations. Each combination enables one of the four input paths and transfers the corresponding input signal to the output. The block diagram and circuit diagram is shown below.

For selection inputs, S₁S₀ = 00, first AND gate alone is enabled and the output produced is

For the next selection input S₁S₀ = 01, Second gate alone is enable, whose output is given by

When selection input S₁S₀ = 10, all the AND gates are disabled except the third gate from the top. The output will be

The fourth gate gets enabled for the selection input S₁S₀ = 11 and the output will be

Any type of boolean function can be implemented by using a multiplexer. For a better understanding of the implementation, some problems are solved in multiplexer.

Multiplexer ICs

There are several electronic multiplexer designed to perform the operation of different types of multiplexers. IC 74150 performs the operation of 16 : 1 mux, IC 74151 performs the operation of 8 : 1 mux, IC 74153 is used to perform 4 : 1 mux, and IC 74157 performs the operation of 2 : 1 multiplexer.

Applications of Multiplexer

A multiplexer is an important building block in digital electronics and combinational logic circuits. Some important applications include:

1. Data selection: It is used for selecting one data signal from several input signals.
2. Data routing: It is used for directing digital signals from different sources to a common destination.
3. Communication systems: It is used for sharing a common communication path between multiple data sources.
4. Processor and computer systems: It is used for selecting data between registers, memory locations, and processing units.
5. Digital control systems: It is used for selecting signals according to control inputs.
6. Combinational logic design: It is used in implementing Boolean functions using multiplexer circuits.
7. Signal switching: It is used for electronically switching between different digital signals.

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