CLASS D VS CLASS AB AMPLIFIERS
Ever since the beginning of recorded audio, amplifiers have been used to drive loudspeakers. The first form of amplifiers was the Triode Vacuum Tube that is sought after even today for their warmer sound signature and characteristic tone. The second iteration of the amplifier was in the form of the the Triode Vacuum Amplifier which was used to make the first AM radio. These amplifiers have since evolved to be more efficient and produce lower amounts of distortion.
There are a few different kinds of amplifiers usually denoted by one or two letters of the alphabet. The most commonly used amplifiers used in consumer home audio today are Class A, AB, D, G and H. These classes don’t just form a grading system based on quality but are based on the amplifier’s topology and core functioning.
The two most important types of amplifiers used today out of the ones mentioned above are the Class AB and Class D amplifiers. Both these classes of amplifiers have their own pros and cons, thereby lending them to different applications. Before we can dissect them to get to their cores, we would need to first understand what an amplifier truly is.
An amplifier is an electronic component that amplifies low-power audio signals to be high enough to drive the diaphragms in headphones or even loudspeakers. The amplifier usually receives an input signal that is only about a few hundred micro watts and amplifies the same to be a few watts depending on the application. Every amplifier though will have its own characteristics that make them different. We find that out of all their characteristics, two of them are the most vital. Those characteristics are Total Harmonic Distortion(THD) and Efficiency. When we take a closer look at the Class AB and Class D amplifiers, we find that this is where they are different. 
The Class AB amplifier, as one might deduce, is a combination of Class A and Class B amplifiers that preceded it. While the Class AB amplifier was efficient (~50%-70%), its claim to fame was its extremely low THD even at higher output levels. The Class AB amplifier was also known to run quite hot and therefore required large heat sinks and good ventilation to ensure a longer life span of the amplifier. The Class AB amplifier is also known for being fairly large and consumes a lot of power as well. Even though this amplifier has a combination of the Class A and Class B amplifier, its topology is fairly simple and relatively lives in the analog domain.
The Class D amplifier was once thought to be the pinnacle of amplifier technology due to its remarkably high efficiency of >90% which was a lot higher compared to the amplifiers that came before it. The Class D amplifier doesn’t generate much heat or develop high levels of distortion. Due to these properties, the Class D amplifier was found to be best suited for the automative audio industry. Class D amplifiers are also known to be small as well. This amplifier is far more complex compared to the Class AB amplifier in that it uses Pulse Width Modulation to switch two transistors (MOSFETs) which gives the Class D amplifier its characteristic efficiency stat.
