Question

Define CMOS NAND gate and its logic operation.

Answer 1

A CMOS (Complementary Metal-Oxide-Semiconductor) NAND gate is a fundamental digital logic gate that performs the logical AND operation followed by a logical NOT operation on its input signals. It is constructed using CMOS technology, which is a widely used method for designing and fabricating integrated circuits. CMOS NAND gates are commonly found in digital circuits and are used to perform various logical operations.

The basic components of a CMOS NAND gate are two complementary pairs of MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors): one pair of p-channel MOSFETs (pMOS) and one pair of n-channel MOSFETs (nMOS). The p-channel MOSFETs are responsible for the logical NOT operation, while the n-channel MOSFETs perform the logical AND operation.

Here's how a CMOS NAND gate works and its logic operation:

Input Stage (AND operation):

The inputs to the NAND gate are labeled as A and B.
The n-channel MOSFETs (nMOS) are connected in parallel between the output node and ground (GND) for each input. When the input is high (logic 1), the nMOS is turned on and provides a low-resistance path to ground.
If both inputs A and B are high (logic 1), both nMOS transistors conduct, creating a low-resistance path to ground. This effectively pulls down the output node to ground and creates a logical low (0) output.

Output Stage (NOT operation):

The output of the input stage is connected to the gates of the p-channel MOSFETs (pMOS).
When the input stage produces a logical low output (0), the pMOS transistors are turned on since their gates receive a logical high voltage (VDD or supply voltage). This creates a low-resistance path between the output node and the supply voltage (VDD).
When the input stage produces a logical high output (1), the pMOS transistors are turned off, creating a high-resistance path between the output node and VDD.

Combining these stages, the overall behavior of the CMOS NAND gate is as follows:

If both input A and input B are high (logic 1), the output will be low (logic 0).
For any other combination of inputs, the output will be high (logic 1).

In summary, a CMOS NAND gate performs the logical AND operation on its inputs and then negates the result, producing a logical NOT operation. This behavior makes it a versatile building block in digital circuits for implementing various logical functions and operations.