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Class A, B, AB, and C Amplifiers

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    Rosa Tiara

What is Amplifier?

The generic term amplifier comes from the word amplify. The definition of amplify is to expand, larger, greater, increase, or strengthen something. Coming up from that definition, amplifiers are just the term used to describe a circuit that produces an increased version of its input signal. An amplifier's job is to take a weak audio signal from a music player and boost it to generate a signal that's powerful enough to drive a speaker.

There are usually two parts of an amplifier:

  • Pre-amplifier: controls volume and input selection.
  • Power amplifier: drives speakers.

In an integrated amplifier, those two parts are combined in one box called chassis.

Why classify them into four classes?

In fact, there are more than four amplifier classes, such as D, E, F, G, S, and T. These classes belong to a set of amplifiers which are usually called switching amplifiers. In this article, we're only going to discuss fully-on amplifier and see them based on their biasing scheme. Biasing scheme helps us to see how much is the amplifier ON when there's no signal being played. This classification helps us to distinguish the electrical characteristics of the different types of amplifiers according to their configuration, operation method, and efficiency.

Kindly Reminder

This article only focuses on the power amplifier. A power amplifier is essentially a 'push/pull' device. It creates the backward/forward signal that is needed to drive the speakers by creating a pair of output transistors, one in the positive domain and one in the negative.

Class A

Class A Amplifier is an amplifier that never turns off and always uses the maximum capacity of its power whether it has an input signal or not. Let's say we have a 200-watt amplifier & connect it to a speaker without any input signal. In this condition, Class A Amplifier will still deliver current into the speaker. Because it has no input, the current will be converted into heat instead. Class A Amplifiers are always giving their ALL even though it is unnecessary. Inefficient, right?

Besides its inefficiency, class A amplifiers are considered to be the best class of amplifier design due to their awesome linearity, high gain, and low signal distortion levels. The high linearity and gain were achieved by the "fully-on" feature they have by keeping the output stage biased "ON" all the time.

The main principle of Class A operation is that all of an amplifier's devices must be conducting through the full 360-degree cycle of a waveform. Class A can be classified again into single-ended and push/pull amplifiers. Push/pull is the feature that mainly reduces the harmonic distortion relative to single-ended designs. This feature is introduced by the non-linearity of the transfer characteristics of a single transistor amplifier and is designed by utilizing output devices in pairs.

In the real world, Class A Amplifier's efficiency rates are between 15-35%, with the potential to drop into single digits by using highly dynamic source material.

Here's a table to sum up Class A Amplifiers:

No crossover & switching distortionConsumed a lot of power
Simple circuitHigh heat dissipation requirements
Has ideal amplification effectHigh-cost for the whole machine

Class B

Class B Amplifier was invented as a solution due to Class A's efficiency and heating problems. The main difference with Class A is that Class B amplifiers have a conduction angle of 180-degree, which means that only half of the input signal is processed to realize the amplification. In the class B amplifier, there's no DC base bias current as its quiescent current is zero so the DC power is so small. Therefore, class B efficiency is much higher than class A amplifier.

The problem with a class B amplifier is that each time the current is switched from positive to negative, it is reduced to zero before crossing over. This is called a crossover point. At this point, both of the transistors are in the off position, before one of them switches back on again to receive the current from the other side. Then, an audio distortion called crossover distortion is created because there's no current traveling through either of the transistors.


Even though the Class B amplifier is more efficient than Class A, the crossover distortion that Class B exhibits is so large that harms the sound quality of the amplifier.

Very low standing bias currentCreates crossover distortion
Can be used for much more powerful outputs than Class ASound quality is not good compared to Class A
More efficient than Class AMore complicated circuit than Class A

Now here's the thing. Class B amplifier has more efficiency than Class A but the sound quality it produces is not as good as Class A does. To solve this problem, the Class AB amplifier comes in handy!

Class AB

As the name suggests, the Class AB amplifier is a combination of Class A and Class B type amplifiers that we've discussed earlier. This type of amplifier is one of the most commonly used types of audio power amplifier design and operates even for small power outputs. Unlike class B, the electrical current in the class AB amplifier is always traveling through at least one of the transistors, keeping crossover distortion to as minimum as possible because the current is never reduced to zero. There will always be an electrical current that travels. In other words, class AB amplifier has conduction angle between 180 degree and 360 degree depending upon the chosen bias point. Class AB amplifier conducts signal more than 50% of the time but less than 100%.

The advantage of the small bias voltage applied to the circuit (provided by series diodes or resistors) is that the crossover distortion that Class B created has been overcome, but is still efficient. In terms of conversion efficiencies, class-AB amplifiers reach about 50% to 60%.

No crossover distortionComplex circuit construction
Higher efficiency than class ALess efficiency than class B
Provides high-frequency responseHigh-cost
Less harmonic distortionProduces spikes in the output signal

Class C

Class C amplifier is a type of amplifier where the transistor conduct for less than one half cycle of the input signal. Less than one-half cycle means that the conduction angle is less than 180% but its typical value is about 80 to 120 degrees. Class C amplifiers would operate only with a tuned or resonant circuit, which provides a full cycle of operation for the tuned or resonant frequency.

The reduced conduction angle improves the efficiency that can reach around 90% but causes a lot of distortion. Due to huge amounts of distortion, the Class C configurations are not widely used in audio applications. Class C amplifiers are usually used in the RF (radio frequency) circuits like RF oscillator, RF amplifier, etc. In such applications, the distortion has little effect on the output and is controlled by a tuned load on the amplifiers.

input and output waveform of class c amplifier
Has the highest efficiency compared to class A, AB, and B.Not suitable for audio applications
Best result in RF applicationsLots of noise
Suitable physical sizeVery difficult to obtain ideal inductors and coupling transformers


So, which one is the best amplifier to use? Well, it depends on what do you want to work on. Here's the summary of the pros and cons of each amplifier. Hope it helps!

Low efficiencyHigher efficiency than AHigher efficiency than BThe most efficient of four
High audio qualityHas crossover distortion -> lower audio qualityNo crossover distortionHuge amounts of distortion

If you're that kind of visual-learner person, this is for you:

waveform of different classes of amplifiers