---
title: "Transformer coupled push-pull efficiency, 20 V supply"
subject: "Electronics Devices and Circuit"
chapter: 4
key: "semester_3/electronics_devices_and_circuit:class-b-transformer-numeric-20v"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/class-b-transformer-numeric-20v
site: BCT Engineering
---

# Transformer coupled push-pull efficiency, 20 V supply

Electronics Devices and Circuit · Chapter 4: Output Stages and Power Amplifiers

- Asked in: 2069 Chaitra, 3+3 marks
- Video: [Class B Push Pull Amplifier - Solid-state Devices and Analog Circuits Day 7, Part 2](https://www.youtube.com/watch?v=HGOLNANVUTQ) — Vocademy - Electronics Technology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/class-b-transformer-numeric-20v
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-4

## Question

Calculate the efficiency of transformer coupled push pull Power Amplifier for a supply voltage of 20V and output of (i) VP = 20V (ii) VP = 16V.

## Short answer

Efficiency of transformer-coupled class B push-pull amplifier is η = (π/4)(Vp/VCC). For VCC=20 V: (i) with VP=20 V, η=78.5%; (ii) with VP=16 V, η=62.8%.

## Answer

**Given:** Transformer-coupled class B push-pull, VCC = 20 V. Efficiency is

η = (π/4)(VP/VCC) × 100 %

where VP is the peak output (collector) voltage swing across one half of the primary. Derivation: Po = VP²/(2R'L), Pi = VCC · 2VP/(π R'L), so Po/Pi = (π/4)(VP/VCC), independent of the load resistance.

#### (i) VP = 20 V
η = 0.7854 × 20/20 = 0.7854 → **78.5 %**

This is the maximum possible value, since VP = VCC.

#### (ii) VP = 16 V
η = 0.7854 × 16/20 = 0.7854 × 0.8 = 0.6283 → **62.8 %**

| VP | VP/VCC | η |
|---|---|---|
| 20 V | 1.0 | 78.5 % |
| 16 V | 0.8 | 62.8 % |

**Answer: (i) 78.5 %, (ii) 62.8 %.**

## Check yourself

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

1. Which formula gives the efficiency of the transformer-coupled push-pull amplifier?
   A. η = (π/4)(VP/VCC) x 100%
   B. η = 50% x (VP/VCC)^2
   C. η = (π/2)(VP/VCC) x 100%
   D. η = 25% x (VP/VCC)
   Answer: A. Po/Pi = (π/4)(VP/VCC), independent of R'L. (Key point: Efficiency formula)

2. What is the efficiency for VP = 20 V, VCC = 20 V?
   A. 62.8%
   B. 50%
   C. 78.5%
   D. 100%
   Answer: C. η = 0.7854 x 1 = 78.5%, the maximum since VP = VCC. (Key point: Full swing)

3. What is the efficiency for VP = 16 V with VCC = 20 V?
   A. 78.5%
   B. 62.8%
   C. 50.3%
   D. 39.3%
   Answer: B. η = 0.7854 x 16/20 = 62.8%. (Key point: Reduced swing)

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