Class D audio amplifier: What, why, and how
Time:2023-09-29
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Class D audio amplifiers have become increasingly famous in recent years. This article will introduce the content, reasons, and methods of Class D audio amplifiers. This article will also introduce the background of audio amplifiers, the advantages of Class D amplifiers, and some comparisons with other amplifiers.
Class D audio amplifiers have become increasingly famous in recent years. This article will introduce the content, reasons, and methods of Class D audio amplifiers. This article will also introduce the background of audio amplifiers, the advantages of Class D amplifiers, and some comparisons with other amplifiers.
Class D amplifiers were first proposed in 1958 and have become increasingly popular in recent years. What is a Class D amplifier? How do they compare to other types of amplifiers? Why is Class D interested in audio? What does it take to create a "good" audio D-class amplifier? What are the characteristics of ADI‘s Class D amplifier products?
Audio amplifier background
The goal of an audio amplifier is to faithfully, efficiently, and low distortion reproduce the input audio signal at the required volume and power levels at the sound output component. The audio frequency range is approximately 20 Hz to 20 kHz, so the amplifier must have good frequency response within this range (frequency response will be lower when driving frequency limited speakers such as bass or tweeters). The power capability varies greatly depending on the application, from milliwatts in headphones, to a few watts in TV or computer audio, to tens of watts in "mini" home stereos and car audio, to more powerful home and commercial hundreds of watts or even higher sound systems - making theaters or auditoriums full of sound.
The simple analog implementation of an audio amplifier uses a linear mode transistor to create an output voltage that is a scaled copy of the input voltage. The forward voltage gain is usually very high (at least 40 dB). If the forward gain is part of the feedback loop, the overall loop gain will also be high. Feedback is often used because high loop gain can improve performance by suppressing distortion caused by nonlinearity in the forward path and reducing power noise by increasing power supply rejection (PSR).
Advantages of Class D amplifiers
In traditional transistor amplifiers, the output stage includes transistors that provide instantaneous continuous output current. Many possible implementations of audio systems include Class A, Class AB, and Class B. Compared to D-class designs, even in efficient linear output stages, the output stage consumes a lot of power. This difference gives Class D a significant advantage in many applications, as lower power consumption generates less heat, saves circuit board space and costs, and extends the battery life of portable systems.
Linear amplifier, Class D amplifier, and power consumption
The linear amplifier output stage is directly connected to the speaker (in some cases through a capacitor). If bipolar junction transistors (BJTs) are used in the output stage, they typically operate in linear mode and have a large collector emitter voltage. The output stage can also be implemented using MOS transistors, as shown in Figure 1.
All linear output stages consume power because?? The process of generating VOUT inevitably leads to non zero IDS and VDS in at least one output transistor. The power consumption largely depends on the method used to bias the output transistor.
Class A topology uses one of the transistors as a DC current source, which can provide the audio current required by the speaker. Class A output stage can provide good sound quality, but the power consumption is too high because large DC bias current usually flows through the output stage transistor (we do not want this current), rather than being transmitted to the speaker (we need this current). Want it).
Class B topology eliminates DC bias current and significantly reduces power consumption. Its output transistor is individually controlled in a push-pull manner, allowing the MH device to provide positive current to the speaker, while the ML absorbs negative current. This reduces the power consumption of the output stage, only the signal current is conducted through the transistor. However, the sound quality of Class B circuits is poor because nonlinear behavior (crossover distortion) occurs when the output current passes through 0 and the transistor changes between the on and off states.
Class AB is a hybrid compromise solution between Class A and Class B, using some DC bias current, but much less than a pure Class A design. A small DC bias current is sufficient to prevent cross distortion and achieve good sound quality. Although the power consumption is between the limits of Class A and Class B, it is usually closer to Class B. Some control similar to Class B circuits is required to allow Class AB circuits to provide or absorb large output currents.
Unfortunately, even well-designed Class AB amplifiers have significant power consumption, as the intermediate frequency output voltage is usually far from the positive or negative power rails. Therefore, a large drain source voltage drop will result in significant instantaneous power consumption of IDS X VDS.
For power levels below 1 W, wasting power may be more difficult than generating heat. If powered by a battery, the linear output stage will deplete the battery faster than the D-class design. In the above example, the power current consumed by the D class output stage is 2.8 times lower than that of the B class and 23.6 times lower than that of the A class, resulting in significant differences in battery life used in products such as mobile phones, PDAs, and MP3 players.
For simplicity, the analysis so far has only focused on the amplifier output stage. However, when considering all power sources in the amplifier system, linear amplifiers have advantages over Class D amplifiers at low output power levels. The reason is that the power required to generate and modulate switch waveforms may be significant at low levels. Therefore, the system wide static power consumption of carefully designed mid to low power Class AB amplifiers can compete with Class D amplifiers. However, for a higher output power range, Class D power consumption is undoubtedly superior.
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