Mar 123 min read

Understanding the Logic Gates Properties: A Guide for Beginners

Logic gates are fundamental building blocks of digital circuits that perform logical operations on binary inputs (0 and 1) to produce a binary output. These gates are the building blocks of digital systems and computers, where information is represented in binary form. There are several types of logic gates, each with its unique function. The basic logic gates include AND, OR, NOT, XOR (exclusive OR), NAND (NOT-AND), and NOR (NOT-OR).

Here's a brief explanation of each logic gate and its properties:

1) AND Gate:

Operation: Output is high (1) only if all inputs are high (1).
Symbol

Truth Table:

A	B	OUTPUT
1	0	0
1	0	0
0	1	0
1	1	1

2) OR Gate:

Operation: Output is high (1) if at least one input is high (1).
Symbol :

Truth Table:

A	B	OUTPUT
0	0	0
1	0	1
0	1	1
1	1	1

3) NOT Gate:

Operation: Output is the complement of the input (1 becomes 0, and vice versa).
Symbol :

Truth Table:

A	OUTPUT
0	1
1	0

4) XOR Gate:

Operation: Output is high (1) if the number of high inputs is odd.
Symbol :

Truth Table:

A	B	OUTPUT
0	0	0
0	1	1
1	0	1
1	1	0

Properties:

1. If any one input is connected to Gnd, XOR gate will act as Buffer.

2. If any one input is connected to Vcc, XOR gate will acts as Inverter.

3. Even parity generator and Odd parity detectors.

5) NAND Gate:

Operation: Output is low (0) only if all inputs are high (1).
Symbol :

Truth Table:

A	B	OUTPUT
0	0	1
1	0	1
1	0	1
1	1	0

6) NOR Gate:

Operation: Output is low (0) if at least one input is high (1).
Symbol :

Truth Table:

A	B	OUTPUT
0	0	0
0	1	1
1	0	1
1	1	0

7) Ex-NOR gate

Operation: The Ex-NOR gate outputs a HIGH (1) only when both its inputs, A and B, are equal (either both LOW (0) or both HIGH (1). If one or both inputs are different, the output becomes LOW (0).
Symbol:

Truth table

A	B	OUTPUT
0	0	1
1	1	1
0	1	0
1	0	0

These gates can be combined in various ways to create more complex digital circuits, allowing for the implementation of mathematical and logical operations essential for computer functionality. The properties of these gates form the basis for Boolean algebra and digital circuit design.

Also Read: Introduction to CMOS

Que. What are universal gates?

Ans - In digital electronics, a universal gate is a logic gate that can implement any possible Boolean function or logic circuit without additional gate types. This makes universal gates incredibly versatile components.

The most common universal gates are the NAND gate and the NOR gate. Let's understand why:

Que. Why NAND and NOR Gates are Universal?

Ans- for the following reason NAND and NOR are universal gates: -

Expressing Other Gates: You can combine NAND or NOR gates in specific ways to replicate the functionality of the basic logic gates:
NOT: Connect both inputs of a NAND or NOR gate together.
AND: Use a NAND or NOR gate followed by a NOT gate (created as explained above).
OR: Use a NAND or NOR gate with its outputs inverted (again, created as above).
Complete Set of Operators: NAND and NOR gates provide a functionally complete set of logical operators. This means that any Boolean function, no matter how complex, can be expressed using a combination of only NAND or only NOR gates

Que. which gate is faster NAND and NOR gate?

Ans - In CMOS technology we use silicon as semiconductor material. Problem with silicon is the mobility of electrons is almost 3 times as that of holes. When we make a 2 input NOR gate, we have 2 PMOS in series and 2 NMOS in parallel. The basic requirement of CMOS is to have its rise time and fall time to be equal, in order to have risen time and fall time of NOR gates equal you end up widening the gate size of PMOS to around four times that of NMOS. Whereas in the NAND gate the PMOS are in parallel and NMOS in series, we need the PMOS gate to be just twice as big as NMOS.

NAND gates consume lesser area, give higher speed, less leakage current, and hence are preferred.