Audio amplifiers have long been used to control the sound amplification, equalization, filtering, special effects, and other signal processing of the audio signal. Power amplification is an important requirement in many different areas of the audio industry, such as high quality equipment used for monitoring during mastering and mixdown, large and powerful sound reinforcement systems used for live performances, and consumer level audio reproduction equipment. Audio amplifiers typically include an audio input, which is connected to some type of audio source, and an audio output, which is connected to audio speakers. To generate the current signals necessary to drive the speakers, audio amplifiers typically include an output stage specifically designed for that purpose. The output stage usually includes a pair of power transistors that are coupled to one another and a power source, which provides power to the power transistors so they can generate the necessary output currents. The audio signal is generated by an audio source and then amplified and filtered by the power audio amplifier. Common examples of the audio source include a musical instrument, such as a guitar, a receiver and tuner, and a microphone. Audio amplifiers are available in a wide assortment of design technologies, power amplification capabilities, frequency responses, enclosures, and price ranges; each offered with particular end applications. The heart of a modern power amplifier is an amplifier or series of amplifiers for supplying the current necessary to drive the loudspeakers. The power amplifier may consist of a transformer, vacuum tubes, BJT power transistors, MOSFET power transistors or other similar devices. There are electronic or solid state power audio amplifiers, and there are vacuum tube power audio amplifiers. Audio amplifiers using vacuum tubes have a characteristic sound which musicians and audiophiles find pleasing. Audio amplifiers employing thermionic valves, most commonly known as vacuum tubes,remain highly prized by audiophiles for their sonic characteristics. Such characteristics include a "warmth" and "coloring" of musical sound that cannot be provided by audio amplifiers employing only solid state amplification devices. In vacuum tube amplifiers, the input signal becomes distorted, especially when the input is overdriven. The distortion seems to result from clipping and rounding of the input waveform so that the resulting sound is softened. Solid state amplifiers have become the standard in virtually every audio amplification application ranging from automotive sound to stereo hi-fi and home theater surround sound to professional sound reinforcement applications. While solid state amplifiers provide compact, low cost and high performance, there remain some audiophiles who long for the days of the vacuum tube amplifier and its characteristically warmer sound. Solid state audio amplifiers do not amplify signals in the same way as tubes and thus do not exhibit the distinctive tube sound. Solid state devices produce a clean signal which to some sounds unpleasing, sharp and harsh. This seems to be the result of sharp clipping when the amplifier is overdriven. Despite the availability of solid state amplifiers, vacuum tube amplifiers are still popular because of their substantially linear amplification characteristics.

Electronic amplifier circuits are often classified in various classes. There are class A output stages, which include power transistors that operate continuously, and class B output stages, which include power transistors that operate only 50% of the time. There are also class AB output stages, which include power transistors that operate somewhere between 50% of the time and continuously, and class C output stages that include power transistors that operate less than 50% of the time. The output drive transistors of class A amplifier circuits conduct DC current even with no audio signal, and the entire output voltage swing is of a single polarity. Class B audio amplifier output stages typically include two bipolar power transistors directly coupled to one another and a power supply. The power supply is connected to the power transistors and supplies the transistors with the power necessary to generate the required output currents. Class B amplifiers are necessarily more efficient, because both complementary output drive transistors are never on at the same time. Class AB amplifiers maintain a small bias current through complementary output drive transistors, so that the output voltage swing is centered slightly above (or below) ground voltage. Although inefficient from a power standpoint, class A amplifiers provide a minimum amount of waveform distortion, and are thus widely used in audio systems. A class B amplifier is more efficient at delivering power to a load than a class A amplifier, but adds significantly more distortion to the output signal than a class A amplifier. Class A or class AB operation of power amplifiers is desirable for audio signal amplification because it eliminates or reduces crossover distortion and improves the linearity of a power output stage. Due to their high fidelity, class A and class AB amplifiers are also used in conventional portable applications such as portable compact disc (CD) players, portable tape players and notebook and subnotebook computers. Class B amplifiers have a linear region of operation that ideally passes through the turn off point of the amplifier, and are biased to operate from the turn off point. Most of the A class amplifiers, the B class amplifiers and the AB class amplifiers have used push-pull emitter followers, as the final constructions of their power amplifying stages.

In recent years, digital signal processing techniques have become prevalent in many electronic systems. The fidelity provided by digital techniques has increased dramatically with the switching speed of digital circuits. Digital techniques for audio signal processing now extend to the driving of the audio output amplifiers. A new class of amplifier circuits has now become popular in many audio applications, namely "class D" amplifiers. Class D amplifiers, also known as switching amplifiers, are amplifiers that switch at a high frequency. In a digital audio system, an analog to digital converter (A/D converter) is used at the first stage of a digital audio system, where the audio signal is changed to a digital signal. A digital to analog converter (D/A converter) is used at the final output stage in order to convert the digital signal to an analog signal. The output signal from the D/A converter contains higher frequency harmonic components than the output signal from the A/D converter. Class D amplifiers use active power circuit elements, such as switches which are alternately driven to saturation and cut-off at a high switching speed, generating a rectangular waveform at its output. A class D amplifier is operated in switch mode, where its output stage produces a rectangular wave that is filtered before delivery to a load, such as a loudspeaker. The rectangular wave varies according to an input analog signal, and when the rectangular wave is filtered at the output, the resulting waveform is an amplified version of the input analog signal. Class D audio power amplifiers typically use two main techniques: pulse width modulation (PWM), and pulse density modulation (PDM). Pulse width modulation (PWM), sometimes referred to as pulse duration modulation, is a signal processing technique in which a sample value of an input information signal is represented by some property of a resultant pulse other than an amplitude value. Pulse width modulation (PWM) drives the output stage at a constant carrier frequency and modulates the duty cycle of its pulses. In an analog approach, a differential analog comparator has the input analog signal at one input and a triangle waveform at another input. The triangle waveform oscillates at the carrier frequency, and the output stage is switched every time the triangle waveform crosses the instantaneous value of the input analog signal. This switching results in output pulses having pulse widths that correspond to the value of the input analog signal. Pulse density modulation (PDM), sometimes referred to as "delta-sigma modulation," uses a decision block that is clocked at a constant frequency. On every clock edge, the block decides whether the output stage should remain in the present state or change to the opposite state.

Class D amplifier has the advantage of high efficiency, cooler operation and, ultimately, improved sound quality. Class D amplifiers are often used for audio amplification because of their power efficiency. They provide substantially full output power, while minimizing internal power consumption. In the class D amplifier, the output stage transistors are switched either completely on or completely off. Amplifier topologies that operate in a partially on state, such as Class A and AB, act like resistors and produce heat, thereby wasting energy. Class D amplifiers are substantially more efficient than non-switching linear amplifiers. Class A amplifiers have a linear region of operation, and are biased to operate from the center of the linear region of operation. When the input signal has no amplitude, a current that corresponds with the center of the linear region flows through the amplifier. When the amplitude of the input signal increases, the current increases, and when the amplitude decreases, the current decreases. As a result, a class A amplifier consumes power throughout the entire cycle of the input signal. The use of less energy and the generation of less heat are important considerations in multi-channel home theater amplifiers, for example, where five or more audio channels are typically found. Higher efficiency and less waste heat allow the class D amplifier to utilize a smaller power supply and to be offered in a more compact package than a comparable linear amplifier. Digital audio has found increasingly wide applications due to the success of digital storage media and the advances of digital signal processing technology. A digital PWM power amplifier for audio amplification is preferred, since it eliminates the need for an extra digital-to-analog converter, associated sampling and hold circuitry, and an analog low-pass output filter with a very sharp cutoff as are needed in a conventional digital-analog PWM amplifier. The switching scheme makes Class-D amplifiers more efficient and smaller in size, with less wasted heat energy and a smaller power supply. Thus, against a background of demands for miniaturization and low power consumption of audio devices in recent years, a growing number of audio devices use digital amplifiers.