**Design of CMOS Full Adder Circuit**

A CMOS Full Adder circuit is the logic has three inputs: A, B and Carry-in and two outputs: Sum and Carry-out. CMOS Full Adder 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 Full Adder (FA) Circuit**

Let us see the basic Block Diagram of a Full Adder circuit.

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

**Step 1:** Write the Boolean expression of the given function.

**Step2:** Take the complement of the output. As CMOS always perform automatic inversion.

**Step 3:** For more than one number of MOS transistor, follow the design rules said above (a) & (b).

**Step 4:** Design firstly a CMOS HA circuit. As the output Carry (c1) is taken as fresh input of second CMOS HA circuit, therefore for designing final output Carry-out, c1 and c2 (carry of second CMOS HA) need to be added (OR operation).

**Step 5:** Draw the Truth Table of the logic gates.

**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 FA is available in our Youtube Channel.

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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.