---
title: "Class B maximum efficiency and condition"
subject: "Electronics Devices and Circuit"
chapter: 4
key: "semester_3/electronics_devices_and_circuit:class-b-max-efficiency-condition"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/class-b-max-efficiency-condition
site: BCT Engineering
---

# Class B maximum efficiency and condition

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

- Asked in: 2068 Baisakh, 4 marks; 2072 Kartik, 4+4 marks; 2070 Chaitra, 4 marks (asked 3 times)
- Video: [Lec-160 Efficiency of Class B Power Amplifier-Derivation | A E | R K Classes | Hindi |](https://www.youtube.com/watch?v=ZvE2_kd_lZ8) — R K Classes
- 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-max-efficiency-condition
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-4

## Question

What is the maximum efficiency of class B amplifier? State the condition when it occurs.

## Short answer

The maximum efficiency of a class B amplifier is 78.5% (π/4). This occurs when the peak output voltage Vm equals the supply voltage VCC, i.e., full swing without saturation.

## Answer

**Maximum efficiency of a class B amplifier = π/4 × 100 = 78.5 %.**

#### Condition
It occurs when the peak output voltage equals the supply voltage, Vm = VCC (the full possible swing, ignoring the transistor saturation voltage VCE(sat)). In other words, the peak load current is Im = VCC/RL, so the output swings across the entire supply range.

#### Reason
For a sinusoidal load current of peak Im:

Po = Im² RL/2,  Pi = VCC · 2Im/π

η = Po/Pi = (π/4)(Im RL/VCC) = (π/4)(Vm/VCC)

The efficiency rises in direct proportion to Vm, so it is highest at Vm = VCC:

ηmax = π/4 × 100 % = 78.5 %

At smaller outputs the efficiency is lower; for example, at Vm = VCC/2, η = 39.3 %. In practice VCE(sat) and the dead-zone bias reduce the maximum to about 60 to 70 %.

#### Note
Maximum efficiency (full swing) is not the same point as maximum transistor dissipation. The dissipation in the two transistors is largest at Vm = 2VCC/π ≈ 0.64 VCC, where P2Q(max) = 2VCC²/(π² RL).

## Check yourself

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

1. What is the maximum efficiency of an ideal class B amplifier?
   A. 25%
   B. 50%
   C. 78.5%
   D. 95%
   Answer: C. ηmax = π/4 x 100 = 78.5%. (Key point: Maximum efficiency value)

2. When does maximum efficiency occur?
   A. At zero signal
   B. When the peak output voltage Vm equals VCC
   C. When Vm = VCC/2
   D. When Vm = 2VCC/π
   Answer: B. Efficiency is proportional to Vm/VCC, so it is highest when Vm = VCC, the full supply swing. (Key point: Condition)

3. What is the efficiency at Vm = VCC/2?
   A. 39.3%
   B. 78.5%
   C. 50%
   D. 25%
   Answer: A. η = (π/4)(0.5) = 39.3%, because efficiency varies linearly with Vm. (Key point: Efficiency vs swing)

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