介绍D类功放的好文章

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一篇详细介绍D类功放的文章，其中比较AB类功放和D类功放的基本原理。
【文件名】:0811@52RD_How Class D audio amplifiers work.doc
【格　式】:doc
【大　小】:373K
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How Class D audio amplifiers work


A Class D audio amplifier is basically a switching amplifier or PWM amplifier. There are a number of different classes of amplifiers. We will take a look at the definitions for the main classifications as an introduction:

Class A – In a Class A amplifier, the output devices are continuously conducting for the entire cycle, or in other words there is always bias current flowing in the output devices. This topology has the least distortion and is the most linear, but at the same time is the least efficient at about 20%. The design is typically not complementary with a high and low side output devices.

Class B – This type of amplifier operates in the opposite way to Class A amplifiers. The output devices only conduct for half the sinusoidal cycle (one conducts in the positive region, and one conducts in the negative region), or in other words, if there is no input signal then there is no current flow in the output devices. This class of amplifier is obviously more efficient than Class A, at about 50%, but has some issue with linearity at the crossover point, due to the time it takes to turn one device off and turn the other device on.

Class AB – This type of amplifier is a combination of the above two types, and is currently one of the most common types of power amplifier in existence. Here both devices are allowed to conduct at the same time, but just a small amount near the crossover point. Hence each device is conducting for more than half a cycle but less than the whole cycle, so the inherent non-linearity of Class B designs is overcome, without the inefficiencies of a Class A design. Efficiencies for Class AB amplifiers is about 50%.

Class D – This class of amplifier is a switching or PWM amplifier as mentioned above. This class of amplifier is the main focus of this application note. In this type of amplifier, the switches are either fully on or fully off, significantly reducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.

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