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Design of CMOS combinational circuits (Multiplexer)

Ritam Dutta
13 Feb 2017
Read Time : 7 Minutes
Design of CMOS combinational circuits (Multiplexer)

Design of CMOS combinational circuits (Multiplexer)

The term multiplexer means many to one. Multiplexing is a process of transmitting a large number of information over a single line. A CMOS 2:1 Multiplexer circuit is the logic has two Data inputs: A and B, one Selection input (S) and one output (Y). CMOS 2:1 Multiplexer uses more than one nMOS and one pMOS transistor(s). The nMOS(s) is used in Pull Down Network (PDN) and the pMOS(s) is used in Pull Up Network (PUN).

Basic Operation: When input logic is low (0), the nMOS is OFF and the pMOS is ON. Hence, the output is connected to VDD through pMOS. When the input logic is high (1), the nMOS is ON and the pMOS is OFF. Hence the output is connected to ground through nMOS.

Now for more number of transistor design using CMOS VLSI Design, there are two rules to be followed:

a)    If AND operation (multiplication) is to be designed, then nMOS transistor(s) need to be connected in SERIES at PDN and pMOS transistor(s) in PARALLEL at PUN.
b)    If OR operation (addition) is to be designed, then nMOS transistor(s) need to be connected in PARALLEL at PDN and pMOS transistor(s) in SERIES at PUN.

Design of CMOS 2:1 Multiplexer Circuit

To design such basic logic gates the steps need to be followed:

Step 1:    Write the Boolean expression of the given function.
Step 2:    Make Truth Table of 2:1 Multiplexer.
Step 3:    Take the complement of the output. As CMOS always perform automatic inversion.
Step 4:    For more than one number of MOS transistor, follow the design rules said above (a) & (b).
Step 5:    Now design firstly a CMOS Pull Up Network and then connect with Pull Down Network using CMOS.
Step 6:    Perform each combinations by providing logic HIGH (1) or logic LOW (0) values at the corresponding input(s) and check the desired output.

The detail of working of CMOS 2:1 Multiplexer is available in our Youtube Channel.

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Authored By Ritam Dutta

Ritam Dutta is an official partner of "Day On My Plate". He is an entrepreneur in mind and passionate blogger by heart. Moreover, he is also an academician, author, public speaker, investor, and internet personality.

 

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