---
title: "Short note: Amplitude modulation"
subject: "Basic Electronics Engineering"
chapter: 5
key: "semester_2/basic_electronics_engineering:short-note-am"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-am
site: BCT Engineering
---

# Short note: Amplitude modulation

Basic Electronics Engineering · Chapter 5: Communication System

- Asked in: 2067 Mangsir, 6 marks
- Video: [Understanding Amplitude Modulation](https://www.youtube.com/watch?v=I46eP8uZh_Y) — Rohde & Schwarz
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-am
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5

## Question

Write a short note on Amplitude Modulation (AM).

## Short answer

Amplitude modulation (AM) varies carrier amplitude proportional to message signal, with modulation index m=Vm/Vc≤1 to avoid distortion. The modulated wave contains carrier and sidebands (fc±fm), bandwidth 2fm. It is simple and cheap but power-inefficient and noise-prone, used in AM broadcasting.

## Answer

**Amplitude modulation (AM)** is a modulation method where the amplitude of a high-frequency carrier is varied according to the instantaneous amplitude of the low-frequency message signal. The carrier frequency and phase remain constant.

#### Equations

Carrier vc = Vc sin ωc t, message vm = Vm sin ωm t.

vAM = Vc (1 + m sin ωm t) sin ωc t

Modulation index: m = Vm/Vc. For no distortion m ≤ 1 (100 % at m = 1). Overmodulation (m > 1) distorts the envelope.

Expanding:

vAM = Vc sin ωc t + (mVc/2) cos(ωc − ωm)t − (mVc/2) cos(ωc + ωm)t

so the AM wave contains the carrier, the lower sideband (fc − fm) and the upper sideband (fc + fm).

#### Waveform and spectrum

```text
        /\          /\
     /\/  \/\    /\/  \/\
     \/\  /\/    \/\  /\/
        \/          \/

 amplitude
   |        |  carrier
   |    |   |   |
   |____|___|___|____ f
     fc-fm fc fc+fm
```

#### Features

- Bandwidth = 2 fm (for 5 kHz audio, 10 kHz).
- Power: Pt = Pc (1 + m²/2). Only m²/4 of Pc is in each sideband, which carry the information, so it is inefficient (at m = 1, only 33 % of the power is in the sidebands).
- Advantages: simple, cheap transmitters and receivers (an envelope detector); long range.
- Disadvantages: low efficiency, noise-prone, limited fidelity.
- Applications: medium-wave and short-wave broadcasting, aircraft communication.

## Check yourself

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

1. What is the total AM power?
   A. Pt = Pc(1 + m)
   B. Pt = Pc m²
   C. Pt = Pc/2
   D. Pt = Pc(1 + m²/2)
   Answer: D. Each sideband carries m²/4 of Pc. (Key point: Power)

2. At m = 1, what fraction of the power is in the sidebands?
   A. 100 %
   B. About 33 %
   C. 50 %
   D. About 10 %
   Answer: B. Sidebands carry m²/2 over (1+m²/2) = 0.5/1.5 = 1/3. (Key point: Efficiency)

3. Which frequencies does a single-tone AM wave contain?
   A. fm only
   B. fc and 2fm
   C. fc and fc + 2fm
   D. fc, fc − fm and fc + fm
   Answer: D. Carrier plus lower and upper sidebands. (Key point: Spectrum)

4. What happens at m > 1?
   A. Power efficiency becomes 100 %
   B. The envelope is distorted
   C. Bandwidth halves
   D. Noise disappears
   Answer: B. Distortion occurs, so m is kept ≤ 1. (Key point: Overmodulation)

## Related in Basic Electronics Engineering

- [Modulation; AM and FM modulated waves](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amodulation-am-fm-wave) — Basic Electronics Engineering, chapter 5
- [Communication system and AM block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-am) — Basic Electronics Engineering, chapter 5
- [Define AM and FM with waveforms](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aam-fm-waveforms) — Basic Electronics Engineering, chapter 5

## Related in other subjects

- [AM wave diagram, modulation index; why near 100%](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-diagram-index-100-percent) — Communication Systems, chapter 2
- [Quadrature amplitude modulation (QAM)](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aqam-explanation) — Data Communication, chapter 5, asked 4×
- [Need for modulation; single tone AM expressions](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-need-am-single-tone) — Communication Systems, chapter 2, asked 2×
- [Two-tone AM: net modulation index, power, spectrum](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-two-tone-50-20-12) — Communication Systems, chapter 2, asked 2×