---
title: "Short note: Varactor diode"
subject: "Basic Electronics Engineering"
chapter: 2
key: "semester_2/basic_electronics_engineering:short-note-varactor"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-varactor
site: BCT Engineering
---

# Short note: Varactor diode

Basic Electronics Engineering · Chapter 2: Diodes

- Asked in: 2072 Magh, 4 marks
- Video: [Varactor Diode Explained (Working, Specifications and Applications of Varactor Diode)](https://www.youtube.com/watch?v=OnVNOGL1nOc) — ALL ABOUT ELECTRONICS
- 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-varactor
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-2

## Question

Write a short note on varactor diode.

## Short answer

A varactor diode is a PN junction operated in reverse bias as a voltage-controlled capacitor. Increasing reverse voltage widens the depletion region, decreasing capacitance per C = C0/(1+VR/V0)^n. Capacitance ranges from few pF to ~100 pF. Applications include electronic tuning, VCOs, and FM modulators.

## Answer

A **varactor diode** (varicap, voltage-variable capacitor) is a PN junction diode designed to be used as a voltage-controlled capacitor. It is operated in reverse bias.

#### Principle
In reverse bias, the depletion region acts as a dielectric between the p and n regions, which act as the capacitor plates. When the reverse voltage VR increases, the depletion region widens, so the capacitance decreases. When VR decreases, the capacitance increases.

C = C0 / (1 + VR/V0)ⁿ

where C0 is the capacitance at zero bias, V0 is the barrier potential and n = 1/2 for an abrupt junction (1/3 for a graded junction).

```text
 C
 |\
 | \
 |  `.
 |    `--.___
 +---------------- VR (reverse)

 Symbol:  --|>|--| (diode with a capacitor symbol)
```

#### Characteristics
- Capacitance range from a few pF to about 100 pF, tuning ratio about 2:1 to 10:1.
- Needs a reverse dc bias; the signal must be small enough not to turn the diode on.
- High Q (low losses).

#### Applications
- Electronic tuning of radio and TV receivers (replacing tuning capacitors).
- Voltage controlled oscillators (VCO) and phase-locked loops.
- Frequency modulation (FM) generators and frequency multipliers.
- Automatic frequency control (AFC).

## Check yourself

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

1. How is a varactor operated?
   A. Forward bias
   B. Zero bias only
   C. Breakdown
   D. Reverse bias
   Answer: D. It uses the reverse-biased depletion capacitance. (Key point: Biasing)

2. What happens to capacitance as reverse voltage increases?
   A. Increases
   B. Decreases because the depletion region widens
   C. Stays constant
   D. Becomes zero
   Answer: B. A wider depletion region means plates farther apart. (Key point: Capacitance vs VR)

3. Which is a typical use?
   A. Rectification
   B. Voltage regulation
   C. Electronic tuning of receivers and VCOs
   D. Light emission
   Answer: C. Voltage-controlled capacitance suits tuning and VCOs. (Key point: Application)

## Related in other subjects

- [Direct FM generation using varactor diode](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adirect-fm-varactor) — Communication Systems, chapter 3