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Maxim > Design Support > Technical Documents > Application Notes > Audio Circuits > APP 3977
Maxim > Design Support > Technical Documents > Application Notes > Automotive > APP 3977
Keywords: Audio, Amplifier, Audio Amplifier, Stereo, Stereo Class D, Mono, Mono Class D, High
Efficiency, Class D, PWM, Spread Spectrum, EMI, Low EMI, Notebook, Notebook Audio, Microsoft Vista
Audio, Vista Audio, TV, FPTV, FPD, Cellphone, MP3, PMP
APPLICATION NOTE 3977
Class D Amplifiers: Fundamentals of Operation
and Recent Developments
Jan 31, 2007
Abstract: A Class D amplifier's high efficiency makes it ideal for portable and compact high-power
applications. Traditional Class D amplifiers require an external lowpass filter to extract the audio signal
from the pulse-width-modulated (PWM) output waveform. Many modern Class D amplifiers, however,
utilize advanced modulation techniques that, in various applications, both eliminate the need for external
filtering and reduce electromagnetic interference (EMI). Eliminating external filters not only reduces
board-space requirements, but can also significantly reduce the cost of many portable/compact systems.
Introduction
Most audio system design engineers are well aware of the power-
efficiency advantages of Class D amplifiers over linear audio-
amplifier classes such as Class A, B, and AB. In linear amplifiers
such as Class AB, significant amounts of power are lost due to
biasing elements and the linear operation of the output transistors.
Because the transistors of a Class D amplifier are simply used as
switches to steer current through the load, minimal power is lost
due to the output stage. Any power losses associated with a Class
D amplifier are primarily attributed to output transistor on-resistances, switching losses, and quiescent
current overhead. Most power lost in an amplifier is dissipated as heat. Because heatsink requirements
can be greatly reduced or eliminated in Class D amplifiers, they are ideal for compact high-power
applications.
In the past, the power-efficiency advantage of classical PWM-based Class D amplifiers has been
overshadowed by external filter component cost, EMI/EMC compliance, and poor THD+N performance
when compared to linear amplifiers. However, most current-generation Class D amplifiers utilize
advanced modulation and feedback techniques to mitigate these issues.
The Basics of Class D Amplifiers
While there are a variety of modulator topologies used in modern Class D amplifiers, the most basic
topology utilizes pulse-width modulation (PWM) with a triangle-wave (or sawtooth) oscillator. Figure 1
shows a simplified block diagram of a PWM-based, half-bridge Class D amplifier. It consists of a pulse-
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