---
title: "Short note: Logic gates"
subject: "Basic Electrical and Electronics Engineering"
chapter: 3
key: "semester_1/basic_electrical_and_electronics_engineering:short-note-logic-gates"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/short-note-logic-gates
site: BCT Engineering
---

# Short note: Logic gates

Basic Electrical and Electronics Engineering · Chapter 3: Introduction to Electronics Engineering

- Asked in: 2081 Baishakh (new course), 3 marks
- Video: [Understanding Logic Gates](https://www.youtube.com/watch?v=INEtYZqtjTo) — Spanning Tree
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/short-note-logic-gates
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_and_electronics_engineering/chapter-3

## Question

Write a short note on Logic Gates.

## Short answer

Logic gates are digital circuit elements that perform Boolean operations on binary inputs. Basic gates include AND, OR, NOT, NAND, NOR, XOR, XNOR; NAND and NOR are universal gates. They are used in adders, counters, memories, and control circuits.

## Answer

Logic gates are the basic building blocks of digital circuits. A gate has one or more binary inputs (0 or 1) and gives one binary output according to a logical rule. They are made from transistors and diodes in ICs (TTL, CMOS).

| Gate | Expression | Output is 1 when |
|---|---|---|
| AND | $Y=A\cdot B$ | all inputs are 1 |
| OR | $Y=A+B$ | at least one input is 1 |
| NOT | $Y=\bar A$ | the input is 0 (inverter) |
| NAND | $Y=\overline{A\cdot B}$ | at least one input is 0 |
| NOR | $Y=\overline{A+B}$ | all inputs are 0 |
| XOR | $Y=A\oplus B$ | inputs are different |
| XNOR | $Y=\overline{A\oplus B}$ | inputs are the same |

Truth table for two-input gates:

| A | B | AND | OR | NAND | NOR | XOR | XNOR |
|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 |
| 0 | 1 | 0 | 1 | 1 | 0 | 1 | 0 |
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 |
| 1 | 1 | 1 | 1 | 0 | 0 | 0 | 1 |

NAND and NOR are called **universal gates**, because any logic function can be built using only one of them. Gates are used in adders, counters, memories, alarms and control circuits.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a logic gate?
   A. A device that stores analog voltages
   B. A building block that gives one binary output from one or more binary inputs by a logical rule
   C. A gate that always gives the same output regardless of inputs
   D. A circuit that works only with decimal numbers
   Answer: B. Gates are basic digital building blocks with binary inputs and one binary output set by a logical rule. (Key point: Definition of a gate)

2. Which gates are called universal gates, and why?
   A. AND and OR, because they have the most inputs
   B. XOR and XNOR, because they are used in adders
   C. NAND and NOR, because any logic function can be built using only one of them
   D. NOT and AND, because every IC contains them
   Answer: C. NAND alone, or NOR alone, can implement any logic function, so they are universal. (Key point: Universal gates)

3. For a two-input NOR gate, when is the output 1?
   A. Only when all inputs are 0
   B. When at least one input is 1
   C. When the inputs are different
   D. When at least one input is 0
   Answer: A. NOR is the complement of OR, so its output is 1 only when all inputs are 0. (Key point: Gate output conditions)

## Related in other subjects

- [Universal gates; positive and negative logic](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Auniversal-gates-positive-logic) — Digital Logic, chapter 2
- [Positive/negative logic; XNOR using NAND](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-negative-logic-xnor-nand) — Digital Logic, chapter 2, asked 2×
- [Positive/negative logic; Ex-NOR using NOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-negative-logic-xnor-nor) — Digital Logic, chapter 2, asked 2×
- [K-map: x'yz + x'y'z + xyz with gates](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Akmap-xyz-realize-gates) — Basic Electronics Engineering, chapter 6, asked 2×