Amplifier Class


The power amplifier is called a power amplifier. It is generally referred to as a basic device in the audio system. It is commonly called a “amplifier”. Its task is to amplify the weak electrical signal from the signal source to drive the speaker to make a sound. It can also refer to other devices that perform power amplification.

According to the different conductivity modes of the power amplifier tube in the power amplifier, it can be divided into Class A power amplifier (also known as Class A), Class B power amplifier (also known as Class B), Class A and Class B power amplifier (also known as Class AB) and Class D power amplifier (also known as D class).

Class A Amplifier

A pure class A power amplifier, also known as a Class A power amplifier (Class A), is a complete linear amplification form of the amplifier. When a pure Class A power amplifier is operating, the positive and negative channels of the transistor are normally open with or without a signal, which means more power is dissipated as heat, but the distortion rate is extremely low.

Pure Class A power amplifiers are rare in automotive audio applications, such as the Italian Sinfoni Advanced Series. This is because the efficiency of a pure class A power amplifier is very low, usually only 20-30%, but audiophiles are relishing its sound.

Class B Amplifier

Class B power amplifiers, also known as Class B power amplifiers (Class B), are also known as linear amplifiers, but they work in a completely different way than pure Class A power amplifiers. When class B power is put into operation, the positive and negative channels of the transistor are usually in the off state unless there is signal input, that is, only the positive phase channel works when the positive phase signal comes, and the negative phase channel turns off.

The two channels never work at the same time, so there is no power loss in the part where there is no signal. However, when the positive and negative channels are turned on and off, crossover distortion is often generated, especially in the case of low level, so the class B power amplifier is not a true high fidelity power amplifier. In practical applications, in fact, many car audio amplifiers in the early days were Class B amplifiers because of their high efficiency.

Class AB Amplifiers

Class AB power amplifiers, also known as Class AB power amplifiers (Class AB), are a design that is compatible with Class A and Class B amplifiers. When there is no signal or the signal is very small, the positive and negative channels of the transistor are always open, and the power is lost, but there is no serious class A power amplifier. When the signal is positive, the negative phase channel is normally open before the signal becomes strong, but the signal is strong and the negative channel is closed.

When the signal is a negative phase, the positive and negative channels work just the opposite. The disadvantage of the class AB power amplifier is that it will produce a little bit of crossover distortion, but it is superior to the class A and class B power amplifiers in terms of its efficiency ratio and fidelity. The class AB power amplifier is also the most widely used in car audio.

Class D Amplifier

Class D amplifiers, unlike the A, B or AB amplifiers described above, operate on the basis of switching transistors that can be fully turned on or completely turned off in a very short period of time. The two transistors do not turn on at the same time, so there is very little heat generated. This type of amplifier is extremely efficient (about 90%), ideally 100%, compared to only 78.5% for Class AB amplifiers. On the other hand, however, the switching mode of operation also increases the distortion of the output signal.

Class D amplifier circuits are divided into three levels: input switching stage, power amplification stage, and output filtering stage. Class D amplifiers can operate in pulse-width modulated (PWM) mode when switched. The PWM can convert the audio input signal into a high-frequency switching signal, and compare the audio signal with the high-frequency triangular wave through a comparator. When the voltage of the inverting terminal is higher than the voltage of the non-inverting terminal, the output is low;

When the inverting terminal voltage is lower than the non-inverting terminal voltage, the output is high. In a class D amplifier, the output of the comparator is connected to a power amplifier circuit that uses a metal oxide field effect transistor (MOSFET) instead of a bipolar transistor (BJT) because the former has a faster response time and is therefore suitable In high frequency mode of operation.

Class D amplifiers require two MOSFETs that can operate fully on or off in a very short period of time. When a MOSFET is fully turned on, its tube voltage drop is low; when the MOSFET is completely turned off, the current through the tube is zero.

The switching speed of the two MOSFETs operating in the on and off states is very fast, so the efficiency is extremely high, and the generated heat is very low, so the class D amplifier does not require a large heat sink.

Class D amplifiers have many other notations, such as Class T, which are all variants of Class D amplifiers. In practical applications, until the 1980s, this type of switching power amplifier was rapidly developed due to the emergence of MOSFETs.

In the actual development process, although there is high efficiency, it also has high distortion, high noise and poor damping factor. With the development of technology, such defects will be less and less, and it is estimated that the future Class D power will be more widely used in the field of car audio.

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