A multiplexer is a combinational digital 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 a digital circuit, it is sometimes necessary to select single input from many input lines. There comes a need for a multiplexer circuit. It has multiple inputs and a single output.
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. It is also termed as mux in simple words.
The above diagram represents the general circuit for 2n : 1 mux circuit. It has 2n data inputs(D0, D1, D2, … D2n-1), n selection inputs(S0, S1,… Sn), 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
For 4 to 1 multiplexer, 4 data inputs, 2 selection lines and 1 output is needed. 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.
There are several ICs 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.