The main function of the power amplifier driver is to further amplify the pre-processed signal and convert it into an analog signal to drive the entire speaker system. Since the AV amplifier is a transistor amplification method, the amplification module has the characteristics of small volume, low heat generation, and large current output compared with the tube amplifier. According to the different working modes of the output stage, it can be divided into A, B, AB, D, G and H, etc. Among them, Class AB and Class D amplifiers are commonly used in home theater amplifiers. These types of amplification methods have their own characteristics, and each of the subsequent amplification methods can be improved according to the shortcomings of the original types. For example, Class A has excellent linear performance with low distortion but high power consumption. Class B improves the power consumption problem, but it leads to the problem of large distortion. AB, G, and H balance the low distortion and low power consumption, while the D class improves the traditional analog amplification by digital amplification.
Class AB amplification is a commonly used method of AV amplifiers. It combines the characteristics of both Class A and Class B. The efficiency is higher than that of Class A amplifiers, and the distortion is smaller than that of Class B amplifiers. In terms of operating characteristics, the Class AB amplifier introduces a non-linear operating point that turns the two transistors on when the signal is close to zero and turns to the B mode when the signal is large. Here are some of the features of Class A and Class B amplifier circuits. Class A amplification occurs at the earliest time, and its working characteristic is that the output stage transistor is always on. The advantages of this type of amplifier are excellent linear output, low signal distortion, and the disadvantage of low performance, only 20% to 30%, and the rest are converted into heat, which requires a large heat sink. Therefore, Class A amplifiers are very large and heavy. The Class B amplification method was developed to overcome the lack of efficiency of the Class A method. The working characteristic is that two transistors are used, and the transistors of each output stage are turned on only for a half cycle of the signal waveform, and are used for positive and negative periods, respectively. The class B amplification method is characterized by a very good effect on the performance, low heat generation, but the disadvantages are also obvious, crossover distortion occurs, the linearity is not excellent, and the distortion is severe.
The class D amplification method is different from any of the amplification methods described above, and its working principle is to use the principle of PWM pulse modulation to express the amplitude of the analog signal by the density variation of the pulse digital frequency waveform. The switching frequency of the class D amplifier is much higher than the highest frequency of the original signal. After filtering by the LPF low-pass filter of the latter stage, the average value of the output waveform is consistent with the actual audio signal. Class D amplification actually adds the working principle of digital amplification. The output stage transistor is fully or completely disconnected during operation, and the Class D amplifier operates at up to 90% or more. The disadvantage of the class D amplification method is also very obvious, that is, the distortion is large, because in the process of modulation amplification, it will inevitably be different from the original signal. However, as the level of production of Class D amplifiers is getting higher and higher, the distortion problem that occurs during the modulation process is greatly reduced. Therefore, many AV amplifiers and post-stage power amplifiers use Class D amplification modules.
The overall structure of the Class G amplification method is the same as that of the AB class. Its working characteristic is to use two or more power supplies for low-signal low-level output. Since the power is supplied using a lower voltage power supply, the amplifier automatically switches to the appropriate supply voltage as the signal level increases. The Class H amplification method is developed on the basis of the G class. The operational characteristics are to predict and control the power supply voltage through an additional control circuit to minimize the voltage of the output stage without using multiple power supplies. The efficiency of these two types of amplification is significantly higher than that of class AB, but the disadvantage is that the design is more complicated, and also because of cost, it is not common in AV amplifiers.
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