The introduction and application of audio amplifier

With the continuous changes in the audio market demand, the audio amplifier design structure has made great progress. When selecting the best audio amplifier IC for a specific application, it is important to have a full understanding of the types of audio amplifiers available and their characteristics.

The audio amplifier increases the amplitude of a small signal to a useful level while retaining the details of the small signal, which is called linearity. The better the linearity of the amplifier, the more true the output signal can represent the input signal.

As the audio market's requirements for amplifier performance continue to change, audio amplifier circuit structures have made great progress. Therefore, the designer must understand the available audio amplifiers and their characteristics. This is the only way to ensure that the best audio amplifier is selected. In this article, we introduce the most important features of the various audio amplifiers available today: Class A, Class B, Class AB, Class D, Class G, Class DG, and Class H.

Class A amplifier

The simplest audio amplifier is a class A amplifier. Class A amplifiers have a conducting output transistor (Figure 1), independent of the output signal waveform. Class A audio amplifiers have the best linearity but lower efficiency. This type of amplifier is used for applications that require high linearity and the available power meeting the requirements.

Class B amplifier

Class B amplifier adopts push-pull structure. The output of the Class B amplifier uses forward and reverse transistors. To reproduce the input signal, each transistor is only turned on during the half-cycle (180°) of the signal waveform (Figure 2). This allows the amplifier to have zero current when idle and is, therefore, more efficient than Class A amplifiers.

Class B amplifiers are a compromise: efficiency increases and audio quality decreases. This is because there is a crossover point where both transistors change from the on the state to the off state. As we all know, Class B audio amplifiers have crossover distortion when processing low-level signals and are not suitable for low-power applications.

Class AB amplifier

Between the class A and class B amplifier structures are the AB audio amplifier. Class AB amplifiers have the sound quality of a class A amplifier and at the same time have the efficiency of a class B amplifier. This performance is achieved using the following method: Biasing the two transistors to a signal output whose conduction is close to zero, that is, the point where the Class B amplifier begins to exhibit nonlinearity (Figure 3). For small signals, both transistors are turned on, which is equivalent to a class A amplifier; for large-signal deviations, only one transistor is turned on in each half of the waveform period, so it works like a class B amplifier.

Class AB speaker amplifiers have a high signal-to-noise ratio (SNR), low THD+N, and a typical efficiency of up to 65%, making it an ideal choice for high-fidelity speaker drivers. Class AB amplifiers such as MAX98309 and MAX98310 are widely used in portable media players, digital cameras, tablet computers, and e-book readers that must ensure high fidelity. Some headphone amplifiers use the class AB amplifier structure of the BTL configuration. For example, the MAX97220A headphone amplifier has excellent low THD+N performance over the entire audio frequency, while outputting up to 125mW of power. MAX97220A is one of the most widely used class AB headphone amplifiers in the world today. 

Class D amplifier

The popularity of handheld mobile audio devices such as smartphones, MP3 players, and portable docking stations has pushed the power consumption issue to the forefront. Now, battery life must be extended by reducing power consumption. Class D amplifiers use pulse width modulation (PWM) to generate a rail-to-rail digital output signal and use the change in duty cycle to represent the analog input signal (Figure 4). Since the output transistors are either completely on or completely off during operation, the efficiency of these amplifiers is higher (often as high as 90% or higher). This method completely avoids the use of the linear region of the transistor, and the linear region of the transistor is the reason for the lower efficiency of other types of amplifiers. The fidelity of modern class D amplifiers is also comparable to that of class AB amplifiers. Because Class D switching amplifiers have higher efficiency, such as MAX98304 and MAX98400A, they are widely used in portable applications. 

Class G amplifier

Class G amplifiers are similar to Class AB amplifiers but use two or more supply voltages. When the Class G amplifier works at a low signal level, a low power supply voltage is selected. When the signal level rises, these amplifiers automatically select the corresponding supply voltage (Figure 5). Class G amplifiers use the highest power supply voltage only when necessary, while Class AB amplifiers always use the highest power supply voltage, so the efficiency of Class G amplifiers is higher than that of Class AB amplifiers.

There is a common problem in portable audio applications: the supply voltage available for speaker amplifiers is limited. Class G power amplifiers use a charge pump to boost the power supply, thereby solving the problem of the power supply voltage. For example, the MAX9730 speaker amplifier is optimized for traditional dynamic speakers, while the MAX9788 speaker amplifier is designed for ceramic speakers. 

Class DG amplifier

Class DG amplifiers use PWM to generate a digital output signal with full swing and variable duty cycle. In this respect, the class DG amplifier is the same as the class D amplifier. However, class DG amplifiers also use a multi-level output stage to detect the amplitude of the output signal (Figure 6); then switch the power rail as needed to provide the required power with higher efficiency. Class DG amplifiers, such as MAX98308, use the same dual-supply concept as Class D amplifiers to achieve higher efficiency. For more examples, please refer to Maxim's Class DG amplifier.

Class H amplifier

Class H amplifier modulates its power supply voltage to minimize the voltage drop of the output stage. Implementation methods range from using multiple discrete voltages to infinitely adjustable power supplies. Although similar to the technology used by the Class G amplifier to reduce the power consumption of the output device, the Class H amplifier structure does not require multiple power supplies (Figure 7).

Class H amplifiers are generally more complex than other audio amplifier designs. These amplifiers require precise control circuits to predict and control the power supply voltage. Audio codec ICs, such as MAX98090 and MAX98091, integrate a class AB headphone amplifier and a Class H power supply structure to achieve extremely low power consumption and provide a complete audio solution. For more examples, please refer to Maxim's Class H amplifiers.

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