Class D Audio Power Amplifier Detailed Explanation

Audio power amplifiers play a vital role in every system that produces audible sound. For decades, amplifiers have been produced using analog (Class A and Class AB) designs, which are highly inefficient and result in products that are large, bulky, and fairly expensive.

Today, the innovation cycle for analog audio power conversion has matured to the point where it can be achieved with virtually no technical difficulty, which is where Class D audio power amplifiers come in. Class D power amplifier technologies are just beginning to advance, and these technologies have great potential to provide higher efficiency and audio performance, making audio products more reliable, higher quality, smaller in size, and lower in cost.

What is a Class D power amplifier?

The Class D amplifier was first invented by British scientist Alec Reeves in 1950. In recent years, the demand for Class D power amplifiers has been rising and it has become increasingly popular in the music field for its high efficiency. In simple terms, a Class D power amplifier is an electronic amplifier, also known as a power switching amplifier, that works on pulse width modulation, which converts the input signal into a stream of pulses.

The output transistor stage of a Class D power amplifier operates as an electronic switch and does not have a linear gain like other amplifiers. A Class D power amplifier starts working by taking an incoming analog input signal and generating a PWM or PDM. It then converts the input signal into a stream of pulses. This is an amplifier that primarily amplifies MOSFETs and transistors, so it can be said that a typical Class D amplifier consists of two output MOSFETs, a pulse width modulator, and an external low-pass filter to recover the amplified audio.

Unlike the linear power regulation used in Class AB power amplifiers, which results in energy losses in the power transistors, Class D amplifiers use switching transistors that only operate in two phases, "on" or "off". There is almost no energy loss in the transistor, and almost all the power is transferred to the transducer. Therefore, compared to Class A, Class B and Class AB amplifiers, D audio amplifiers can have efficiencies as high as 90-95%, while AB amplifiers have a maximum efficiency of only 60-65%. The efficiency and distortion comparison is shown in the figure below:

Working principle of Class D power amplifier

As we all know, the Class D amplifier works with Pulse Width Modulation (PWM) or Pulse Density Modulation (PDM). So, by using pulse width or pulse density, it creates an equal analog output stage for the speaker instead of the traditional digital-to-analog conversion. The device it amplifies is usually a MOSFET or transistor, which operates as an electronic switch.

The Class D amplifier generates a series of rectangular pulses of fixed amplitude at the beginning of operation, which vary in area and spacing, or number per unit time. In addition, the amplitude changes of the analog audio input stream are also represented by these pulses, and it is also possible to synchronize the modulator clock with the incoming digital audio input signal, so there is no need to convert the digital audio signal to analog signal. The output stage of the modulator controls the operation of the output transistors by alternately turning them on and off. It is important to ensure that the pair of transistors does not conduct at the same time. If it conducts together, a short circuit between the power rails through the transistors will occur. The pulse width modulation waveform diagram is shown below:

Since the transistors are either fully "on" or fully "off", they spend very little time in the linear region and during this time they dissipate very little power, which is a major factor in their high efficiency. This means that it is more efficient than Class A, Class B, Class AB, and Class C amplifiers.

In addition, a basic low-pass filter consisting of an inductor and capacitor provides a low-frequency conducting path for the audio signal while leaving the high-frequency pulses behind. Sometimes, the output stage filter is omitted in cost-sensitive applications.

Advantages of Class D Amplifiers

High efficiency; The biggest advantage of Class D amplifiers is their high efficiency. It also provides low heat dissipation, incredible performance, provides excellent output, and can be used for a variety of purposes. And Class D amplifiers weigh less than A, B, or AB amplifiers. For portable sound reinforcement structure equipment and bass amplifiers, this is a crucial issue.

Wide application range: Class D power amplifiers have a wide range of applications and a low minimum connected load. In addition, the battery conversion rate remains unchanged regardless of the load capacitance.

Clear sound; Class D amplifier lacks high and medium frequencies, and can make relative frequency adjustments. The sound is very clear and the sound and image positioning is quite accurate.

Can be mass produced; Class D power amplifiers are very suitable for mass production. As long as the components are placed correctly, the consistency of the product can be guaranteed, eliminating the need for debugging during the manufacturing process, and it is safe and reliable.

Multifunctional; Class D amplifier can directly perform group control, remote control, monitoring and other operations without any other equipment.

Power saving; Class D amplifiers are more power efficient compared to other amplifiers like Class A, Class B, and Class AB. A well constructed Class D amplifier will never produce less than 90% efficiency while using a real speaker as a load as an AB amplifier. Due to the high efficiency of Class D amplifiers, they require smaller heat sinks and power supplies. Therefore, Class D amplifiers are much lighter than analog amplifiers, which is also the key advantage of Class D amplifiers.

Disadvantages of Class D Amplifiers

There is no specific switch; Class D power amplifiers do not have specific switches, and if the power transistors and other components installed are not properly matched, the quality of the entire product will fail.

Potential power fluctuations; during the initial connection and final shutdown of a Class D amplifier, the potential of the power transistors close to ground can fluctuate, causing increased noise.

Blind zone: Class D power amplifier output circuits may contain blind zones.

Some people don't like the sound of a Class D amplifier because it sounds very digital. Of course, this varies from person to person.

Although the Class D amplifier has the above disadvantages, it is the trend of audio amplifiers now due to its small size and high efficiency. The advantages are many, and the disadvantages will not cause big problems in use and can be ignored.

Why choose Class D amplifier for home stereo system?

When looking for the best audio amplifier for your home stereo system, you will find that most of the amplifiers are made of Class D amplifiers. It is almost universally used in home theater stereo systems. Because of its simple design, stable performance, and much smaller size compared to other amplifiers. Another reason is that it is best suited for wireless surround sound speakers or Bluetooth surround systems. If you do not want to hear any sound distortion from your audio system, such as clicks, pops, or hisses, then choosing a Class D power amplifier will be the right decision. Class D amplifiers sound more decent and are known for their high efficiency.

Modern Class D amplifiers are designed with wireless systems so anyone can stream their favorite music wirelessly via Bluetooth, WIFI, and Airplay, and can also wirelessly control the connection of the whole house audio through the amplifier. Modern Class D amplifiers enhance the performance of speakers and subwoofers, play loud and clear, and ensure the quality of recordings. Considering all the facts and the comparison with other amplifiers, Class D amplifiers are best for home stereo systems.

Summarize

Class D power amplifiers are a type of audio amplifier in which power management devices such as MOSFETs are used as electronic switches. Since these amplifying devices operate as perfect switching devices, no power and no time is wasted during phase transitions in zero input conditions. The demands on Class D amplifiers, such as minimum power consumption, allow Class D to be used in a wide variety of applications compared to other topologies such as Class A, Class AB, and Class B.

The reality is that today, Class D amplifier technology is increasingly being used for large-scale public broadcasting, speech reinforcement, and to effectively replace traditionally bulky and inefficient power amplifiers in live sound reinforcement systems.