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A Class D amplifier is a switching power amplifier: its output transistors rapidly turn on and off, and a modulator shapes those switching pulses to represent the audio signal. Many designs use a low-pass filter to recover the audio and reduce high-frequency switching energy. “Class D” describes how the output stage operates—not whether the amplifier takes a digital input.
How a Class D amplifier works
In a common pulse-width modulation (PWM) design, a circuit compares the audio input with a triangle or sawtooth waveform. The comparison produces pulses whose width changes with the input signal. MOSFETs switch the output node between the supply rails, and an output filter can smooth the resulting waveform into audio for the speaker. Analog Devices gives 250 kHz to 1.5 MHz as a representative switching-frequency range in its fundamentals article; that range is specific to that explanation, not a universal requirement. Analog Devices’ Class D amplifier fundamentals
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PWM is common, but not the only modulation approach. Designs may use other methods, including pulse-density or sigma-delta techniques, and may incorporate feedback to improve performance. The exact signal path and output requirements depend on the amplifier design. Analog Devices’ Class D amplifier fundamentals Analog Devices on what Class D amplifiers are and why they are used
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Does “Class D” mean the amplifier is digital?
No. The class label refers to the output stage’s switching operation, not the format of the input. An amplifier can accept an analog audio signal and use a modulator to convert it into switching pulses. As Analog Devices author Eric Gaalaas explains, “Since most audio signals are not pulse trains, a modulator must be included to convert the audio input into pulses.” Analog Devices on what Class D amplifiers are and why they are used
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Why many Class D designs use an output filter
A traditional filtered design places an LC low-pass network between the switching stage and the speaker. It passes the audio-band component while attenuating high-frequency switching energy. Some designs use filterless modulation instead, so an external LC filter is not universal. “Filterless” also does not mean that every speaker load or wiring arrangement is acceptable: follow the specific amplifier’s documentation for permitted loads and installation requirements. Analog Devices’ Class D amplifier fundamentals Analog Devices on what Class D amplifiers are and why they are used
Efficiency: an advantage, not a guarantee
Switches can spend much of their time fully on or fully off, which can reduce dissipation compared with operating output devices continuously in their linear region. But real amplifiers still lose power through transistor on-resistance, switching transitions and controller overhead. Efficiency also varies with output level and design, so “Class D” alone does not establish a particular efficiency rating. Analog Devices’ Class D amplifier fundamentals Analog Devices on what Class D amplifiers are and why they are used
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Specific figures illustrate why operating conditions matter. In its comparison involving the AD1994, Analog Devices reports 90% output-stage efficiency at the onset of clipping, but 78% at a 1 W average load in an example with a 10 W maximum load. These are figures for that article’s specific comparison, not ratings for Class D amplifiers generally. The Audio Engineering Society’s Pro Audio Reference says practical Class D amplifiers can approach 90% efficiency; that general statement is not a promise for an individual model. Analog Devices on what Class D amplifiers are and why they are used Audio Engineering Society Pro Audio Reference
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What to check when choosing an amplifier
The class label is not enough to determine whether an amplifier suits a system or how it will sound. Compare the device’s specifications and installation requirements against the intended speaker, supply and use.
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- Power and load: Check the rated output power at the speaker impedance you plan to use, along with the amplifier’s permitted load range.
- Performance: Compare distortion and noise specifications, and look for information about feedback and protection features.
- Installation: Check whether the design requires an output filter and what it specifies for electromagnetic-interference (EMI) behavior, wiring and speaker loads.
- Thermal and supply needs: Confirm the required supply and cooling provisions. Efficiency and heat depend on the design and operating level.
At low output levels, system overhead can make Class AB competitive in some applications; Class D has a stronger dissipation advantage at higher output levels, according to Analog Devices. That trade-off does not make one class universally better: suitability depends on the complete system and the conditions in which it operates. Analog Devices on what Class D amplifiers are and why they are used Analog Devices’ Class D amplifier fundamentals
Quick Recap
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