Difference Between Class A, AB and Class D Amplifiers - Explained

February 24, 2022
Classes of Amplifiers
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amplifier classes

The way an amplifier combines power and signal defines its Class. Moving forth, do you wonder what terms like Class A, AB, G/H or Class D mean in audio amplifiers? 

Also, what are the differences between Class A, AB, and Class D amplifiers?

In this blog, we will go over the various classes of amplifiers, elaborate on their associated strengths and weaknesses and how the technology differs, to help you make a more informed purchasing decision.

Each class brings something unique

First things first, these aren't grading systems with which to evaluate amplifier quality, but are accurate descriptions of an amplifier's topology. Each class brings something unique to the table, but the objective is always the same: amplifying the input signal without introducing distortion.

Amplifier Class Synergizes Power and Signal

Amp class differs from amp to amp with efficiency and sound fidelity contingent upon the design that gets used. In all designs, banks of output transistors, each a little amp by itself, add their collective power together to deliver the amplifier's final output.

It is significant to remember that the different amplifier classes produce different amounts of heat. Too much heat will damage the amplifier's output signal and internal components.

Also Read: How to Choose Amplifier for Speakers - An Easy Guide

Detail about Amplifier Classes

  • Class A Amplifiers
Class A Amplifier

This is a high fidelity amplifier class. Class A amplifier's output transistors run at full power whether there's an input signal or not. When there's no signal, the transistors' power transforms into heat. When there is a signal, the power goes out the speaker terminals.

Also, each Class A output transistor amplifies both the negative voltage and the positive voltage parts of the signal's AC waveform, increasing the workload and raising more heat. Class A amps function at a 25% efficiency level and the remaining 75% of their power is turned into heat.

There are no turn-on, turn-off, warming, or cooling cycles that would affect the signal flow. In this class, you get distortion-free performance. Besides there is no switching going on, meaning there is no induced high-frequency interference. Pure Class A amplifiers are rare and expensive.

  • Class B Amplifiers
Class B Amplifier

These ease the workload of each output stage by replacing the single transistor there with two transistors set up in a commonly referred to as "push-pull" arrangement. Out of the two, one of the transistors amplifies the negative voltage parts of the signal's AC waveform, while the other one manages the positive voltage. 

The two synergize into a unified output. The class B amplifiers are more efficient with low fidelity, but produce much more distortion.

  • Class AB Amplifiers
Class AB Amplifier

These amplifiers offer higher fidelity and efficiency. The push-pull pair of output transistors in a Class AB amp are each on more than half the time, and they don't turn on and off abruptly. There's an optimum bias current for each amplifier that cuts down the crossover distortion of Class B design.

Class AB amplifiers have much higher efficiency than Class A amps for upto 60% and less distortion than class B amplifiers. Class AB amplifiers are a practical and apt choice for home theatre and stereo amplifiers.

  • Class D Amplifiers
Class D Amplifier

Class D amplifiers are not digital devices. Most of the amplifier circuits will be strictly analog. They pack a punch with almost 90 % efficiency. Onboard circuitry creates very high-frequency (often over 100K Hz) pulses of DC current. IT is equipped with pulse width modulation or PWM. 

These DC pulses are run through the amplifying output transistors resulting in a high-power output.  This is the most efficient way of running these transistors — as much as 90% efficient in some cases. However, most audiophiles won't use Class D amplifiers in their systems stating the need for filtering out generated distortion.

  • Class G/H Amplifiers
class g/h amplifier

They offer enhanced  efficiency over the Class AB amplifiers. They function on the power supply section. A high-voltage power supply is required for high-output power. Class G utilises more than one stable power rail. While Class H is a notch above the class G as it consistently varies the supply voltage at any time.

However, they both require considerably more complex power supplies that leads to a higher cost of implementing these features.

Also Read: Understanding the difference between AV Receivers and Amplifiers

Key Take Aways

  • Class A design is the least efficient but has the highest sound fidelity.
  • Class B design is a little more efficient, but has a lot of distortion.
  • Class AB design packs a punch with power efficiency and superb sound.
  • Class D design offers the highest efficiency but isn't quite as high-fidelity. 

  • Hopefully, this blog has been helpful and by getting informed on those differences, you can narrow your focus and make educated decisions on which one to opt for. And if you have any questions or want some help, contact our experts at Ooberpad.

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