The Multiplexer

The Multiplexer which sometimes are simply called "Mux" or "Muxes", are devices that act like avery fast acting rotary switch. They connect multiple input lines 2, 4, 8, 16 etc one at a time to a common output line and are used as one method of reducing the number of logic gates required in a circuit. Multiplexers are individual Analogue Switches as opposed to the "mechanical" types such as normal conventional switches and relays. They are usually made from MOSFETs devices encased in a single package and are controlled using standard logic gates. An example of a Multiplexer is shown below.

Addressing      Input

Selected

b      a

0      0      A

0      1      B

1      0      C

1      1      D

The Boolean expression for this 4 to 1 Multiplexer is given as:

Q = abA + abB + abC + abD

In this example at any instant in time only one of the four analogue switches is closed, connecting only one of the input lines A to D to the single output at Q. As to which switch is closed depends upon the addressing input code on lines "a" and "b", so for this example to select input B to the output at Q, the binary input address would need to be "a" = logic "1" and "b" = logic "0". Adding more control address lines will allow the multiplexer to control more inputs. Multiplexers can also be used to switch either analogue, digital or video signals, with the switching current in analogue circuits limited to below 10mA to 20mA per channel in order to reduce heat dissipation.

Multiplexers are not limited to just switching a number of different input lines or channels to one common single output. Here, the voltage gain of the inverting amplifier is dependent upon the ratio between the input resistor, Rin and its feedback resistor, Rf as determined in the Op-amp tutorials. A single 4-channel (Quad) SPST switch configured as a 4-to-1 channel multiplexer is connected in series with the resistors to select any feedback resistor combination from a single value of Rf to all the resistors connected together in parallel. The combination of these resistors will determine the overall gain of the amplifier, (Av). Then the gain of the amplifier and can be adjusted digitally by simply selecting the appropriate resistor combination.

Digital multiplexers are sometimes also referred to as "Data Selectors" as they select the data to be sent to the output line and are commonly used in communications or high speed network switching circuits such as LAN´s and Ethernet applications. Some multiplexer IC´s have a single inverting buffer (NOT Gate) connected to the output to give a positive logic output (logic "1", HIGH) on one terminal and a complimentary negative logic output (logic "0", LOW) on another different terminal.

It is possible to make simple multiplexer circuits from standard AND and OR gates but commonly multiplexers/data selectors are available as standard i.c. packages such as the common TTL 74LS151 8-input to 1 line multiplexer or the TTL 74LS153 Dual 4-input to 1 line multiplexer.