---
title: "Role of hybrid transformer"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:hybrid-transformer-role"
source: https://www.bctengineer.com/q/semester_8/telecommunications/hybrid-transformer-role
site: BCT Engineering
---

# Role of hybrid transformer

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2069 Chaitra, 2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/hybrid-transformer-role
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

Explain the role of Hybrid transformer or circuit in case of telecommunications.

## Short answer

A hybrid transformer couples a two-wire subscriber line to four-wire transmit and receive paths. Its balancing network, matched to the line impedance, stops the received signal leaking into the transmit path, preventing echo and singing. It provides about 3 dB loss in each through path.

## Answer

A **hybrid transformer** (or hybrid circuit) is the interface between the **two-wire** subscriber line and the **four-wire** transmission/switching path.

Its roles:
- **2W→4W:** passes the subscriber's speech from the two-wire line to the four-wire transmit (GO) path.
- **4W→2W:** passes the received speech from the four-wire RETURN path to the two-wire line.
- **Isolation:** with a **balancing network** equal to the line impedance, it prevents the received signal from leaking into the transmit path (high transhybrid loss). This stops **echo** and **singing** on long four-wire circuits.
- Each through path has about 3 dB loss.

It is used in telephone sets (sidetone control), line cards of digital exchanges and the ends of trunk/carrier circuits.

## Check yourself

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

1. A hybrid transformer connects which two types of path?
   A. Two-wire subscriber line and four-wire transmission path
   B. Coax and fibre
   C. Analog line and radio link
   D. Two exchanges using the same wires
   Answer: A. The hybrid converts between the 2W subscriber loop and the 4W transmit/receive path. (Key point: Hybrid interface)

2. What is the purpose of the balancing network equal to the line impedance?
   A. To amplify speech by 3 dB
   B. To supply DC power
   C. To increase echo
   D. To stop the received signal leaking into the transmit path (high transhybrid loss)
   Answer: D. Proper balance gives high transhybrid loss, preventing echo and singing. (Key point: Balancing network)

3. Poor balance in a hybrid on a long four-wire circuit mainly causes what?
   A. Noise reduction
   B. Echo and singing
   C. Higher sidetone only
   D. Frequency shifting
   Answer: B. Leakage from receive to transmit path returns to the talker as echo and can cause singing. (Key point: Problems prevented)

## Related in Telecommunications

- [Media classes and hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amedia-classes-hybrid-transformer) — Telecommunications, chapter 2
- [Crosstalk causes and hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acrosstalk-hybrid-transformer) — Telecommunications, chapter 2
- [Guided media comparison and hybrid operation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-media-hybrid-operation) — Telecommunications, chapter 2
- [Hybrid, balancing network and singing](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ahybrid-balancing-singing) — Telecommunications, chapter 2
- [Wired vs wireless media; four-wire and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awired-wireless-four-wire-hybrid) — Telecommunications, chapter 2, asked 2×
- [Role of traffic engineering](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arole-of-traffic-engineering) — Telecommunications, chapter 6

## Related in other subjects

- [Equivalent circuit of 20 kVA 250/2500 V transformer from tests](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aoc-sc-20kva-250-2500) — Electrical Machines I, chapter 2, asked 3×
- [Equivalent circuit of 100 kVA 2200/220 V transformer](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aoc-sc-100kva-2200-220-hv) — Electrical Machines I, chapter 2, asked 3×
- [Equivalent circuit of 5 kVA 200/400 V transformer](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aoc-sc-5kva-200-400-primary) — Electrical Machines I, chapter 2, asked 2×
- [Equivalent circuit of 10 kVA 250/1000 V transformer](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aoc-sc-10kva-250-1000-primary) — Electrical Machines I, chapter 2, asked 2×