---
title: "Crossover distortion and its elimination"
subject: "Electronics Devices and Circuit"
chapter: 4
key: "semester_3/electronics_devices_and_circuit:crossover-distortion"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/crossover-distortion
site: BCT Engineering
---

# Crossover distortion and its elimination

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

- Asked in: 2082 Baisakh, 2+2 marks; 2081 Bhadra, 2+3 marks; 2078 Kartik, 3 marks; 2074 Ashwin, 2+4 marks; 2073 Shrawan, 3 marks; 2071 Shrawan, 4 marks (asked 6 times)
- Video: [Cross Over Distortion - Elimination of Cross Over Distortion in Class B Amplifier](https://www.youtube.com/watch?v=NwkooG8bwHw) — Padmasri Naban
- 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/crossover-distortion
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-4

## Question

What is crossover distortion? How it can be eliminated?

## Short answer

Crossover distortion is flat zero-output near zero-crossing in class B when input is below VBE. Eliminate it by class AB biasing using diode drops or a VBE multiplier, or by applying negative feedback.

## Answer

**Crossover distortion** is the distortion of the output waveform near the zero-crossing in a class B push-pull amplifier. In the region where the input signal is smaller than the base-emitter turn-on voltage (about 0.6 to 0.7 V for silicon), neither transistor conducts, so the output is zero (a dead zone) for that part of the cycle.

```text
 Input        Output (distorted)
   /\            /\
  /  \          /  \
 /    \        /    ___   <- flat dead zone
------\------/------/  \---   at zero crossing
       \    /
        \  /
         \/
```

When the input swings from positive to negative, the NPN turns off before the PNP turns on, so a flat step appears at the crossing. It is worst for small signals, and adds harmonics.

#### Methods to eliminate it
1. **Class AB biasing.** Apply a small forward bias to each transistor so that both are just conducting (VBE ≈ 0.6 V) at zero signal. The conduction angle becomes slightly more than 180°, so one device takes over smoothly as the other turns off.
2. **Diode biasing.** Place two forward-biased diodes (or a VBE multiplier transistor) between the bases of the complementary pair. The diode drops (about 1.2 to 1.4 V) bias both bases at the turn-on point and track temperature, which also prevents thermal runaway.

```text
        +VCC
          |
       [R1]
          |----- B of Q1 (NPN)
        --D1--
        --D2--
          |----- B of Q2 (PNP)
       [R2]
          |
        -VCC
```

3. **Negative feedback.** Include the output stage inside the feedback loop of an op-amp. The large loop gain reduces the dead zone.

The cost of class AB is a small idle current and a small drop in efficiency (still about 60 to 70 %).

## Check yourself

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

1. What causes crossover distortion?
   A. Neither transistor conducts while the input is below about 0.6-0.7 V
   B. Excessive supply voltage
   C. Saturation of both transistors
   D. Thermal runaway
   Answer: A. Below the VBE turn-on, both transistors are off, giving a dead zone at zero crossing. (Key point: Cause)

2. How does class AB biasing remove it?
   A. A small forward bias keeps both just conducting so one takes over smoothly
   B. It removes the PNP
   C. It doubles the supply
   D. It increases the signal
   Answer: A. Conduction becomes slightly over 180°, eliminating the dead zone. (Key point: Class AB cure)

3. What is the added benefit of diode biasing between the bases?
   A. It tracks temperature and helps prevent thermal runaway
   B. It raises the supply
   C. It removes the load
   D. It adds gain
   Answer: A. The diode drops (~1.2-1.4 V) set both bases at turn-on and track temperature changes. (Key point: Diode biasing)

## Related in Electronics Devices and Circuit

- [Crossover distortion and quasi-complementary class AB](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Acrossover-quasi-complementary-class-ab) — Electronics Devices and Circuit, chapter 4, asked 2×
- [Transformer coupled class B, max efficiency, crossover](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Atransformer-class-b-efficiency-crossover) — Electronics Devices and Circuit, chapter 4

## Related in other subjects

- [Errors eliminated by face observation](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aface-observation-errors-and-temporary-adjustment) — Surveying I, chapter 7