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How to produce high quality audio

Posted: 03 Sep 2015     Print Version  Bookmark and Share

Keywords:Class D  amplifiers  audio applications  NMOS  transistor 

In high-quality audio applications, an analogue signal is elaborated to get to the output of a signal with the greatest power at the least possible distortion. There are several basic types of amplifier designs, each having advantages and disadvantages, although Class D amplifiers have emerged because of their low power consumption and low distortion.

The amplifier is the core of a sound system. Good amplifiers introduce little distortion to the original signal, as shown in figure 1.

Every electronic amplifier contains one or more active elements. Figure 2 shows an NMOS transistor-based amplifier. In this configuration, the amplifier uses a common-source stage.

Figure 1: An amplifier increases a signal's amplitude.

Figure 2: Common Source Amplifier using an NMOS transistor.

Depending on the working mode of the transistor, there are different classes of amplifiers that you can use. I've listed some below.

Type A: These amplifiers feature low distortion because the working point of the transistor is in the middle of the line load, at the centre of the linear zone. But Type A amplifiers have high current consumption that generates considerable heat that must be dissipated. That requires a large heat sink. Audiophiles prefer this type for its low distortion.

Type B: The working point of the transistor is at the boundary between the linear and the interdiction zone. This means a very low current consumption and high power out generation, but there is a high distortion due to the crossing of the line load with the interdiction zone.

Type AB: The advantage of this typology of amplifier is the high power out of class B type without its distortion. The current consumption is low, even when there is no signal, so the heat production is low. The AB output stages are often used as final stages of HI-FI audio systems.

Type C: The class C amplifiers have high efficiency, but the weakness is a very high distortion of the output signal. This is why these amplifiers are not utilised in audio applications.

These four kinds of amplifiers present strengths and weaknesses that hardly make them employable in high quality audio applications with high standards for audio quality, output power, low distortion, and heat production. Figure 3 shows how each of the amplifiers work.

Figure 3: Amplifier characteristics for output stage, type: A, AB , B ,C. Source: Confronto Tra Diverse Tipologie Di Stadi Di Ustica, page 7.

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