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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchA class-D amplifier represents audio as a high-frequency pulse train, switches power devices to reproduce that pulse pattern, and delivers the audio component to the speaker. In a conventional design, an LC low-pass filter attenuates switching energy. To size even a first-pass design, specify the supply, speaker load, output power, bandwidth, switching conditions, thermal limits, distortion target, and EMI constraints; there is no single filter or efficiency figure that applies to every amplifier.
How does a class-D amplifier work?
Unlike a linear amplifier, which varies transistor conduction to reproduce the audio waveform, a class-D amplifier uses its output transistors primarily as switches. A modulator encodes the audio in a high-frequency pulse train, usually by varying pulse width or duty cycle. The switching stage connects the supply to the output in response to those pulses; filtering and the speaker load recover the audio-frequency component.
A conventional signal path is:
Audio input → PWM/modulator → gate driver and switching stage → LC low-pass filter → speaker
Supply decoupling and protection support the power stage. Some architectures also use feedback. The exact circuit depends on whether the output is a half-bridge or bridge-tied load (BTL), whether the input is analog or digital, and whether the controller and power switches are integrated. In a BTL amplifier, two outputs drive opposite sides of the speaker, increasing differential output swing for a given supply.
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For a practical overview of switching operation and amplifier selection, see Texas Instruments’ class-D amplifier selection guide and Analog Devices’ explanation of class-D operation.
What must be specified before sizing a basic design?
First define the conditions the amplifier must meet. A power calculation is only meaningful when its load and operating assumptions are clear.
- Supply voltage and its expected range.
- Speaker’s nominal and minimum impedance, plus a realistic impedance model if available.
- Desired continuous and peak output power.
- Audio bandwidth and acceptable distortion and noise.
- Switching frequency or the settings permitted by the selected device.
- Thermal conditions, including cooling and ambient temperature.
- EMI/EMC constraints and the intended speaker wiring and enclosure.
For an ideal resistive load driven by a sine wave, the load-side estimates are:
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- VRMS = √(P × R)
- IRMS = √(P ÷ R)
- Equivalently, P = VRMS2 ÷ R and IRMS = VRMS ÷ R.
These equations estimate ideal voltage and current at the load. They do not establish the required supply voltage or prove that a particular amplifier can deliver the target power. Supply headroom, modulation limits, switch voltage drops, dead time, reactive speaker impedance, filter losses, temperature, and clipping all affect the real result.
Does a class-D amplifier need an output filter?
No single answer applies to every architecture. A conventional filtered amplifier uses a low-pass network—commonly a second-order LC filter—to pass audio while attenuating switching-frequency energy. Some integrated class-D devices use filterless modulation and can operate without external LC parts. “Filterless” does not mean free of switching emissions or speaker-load considerations: the complete product still needs system-level EMI assessment.
The filter is part of the amplifier’s operating environment, not a generic afterthought. Its response and losses affect audio performance, switching attenuation, current, EMI, size, and cost.
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How do I design an LC filter for a class-D amplifier?
There is no reliable universal inductor-and-capacitor pair. Choose the filter for the actual output stage and load, then verify both audio response and switching attenuation.
- Establish the operating conditions. Identify the amplifier or modulator, switching frequency, supply, output topology, and intended speaker impedance range.
- Set the response and EMI goals. Define the audio band to pass and the switching energy that must be attenuated, along with applicable system constraints.
- Evaluate a realistic load. A speaker is not a perfect resistor; impedance varies with frequency and may be reactive. Check behavior across the expected load rather than only at nominal ohms.
- Select components against electrical and physical constraints. Inductor current capability, winding resistance, core behavior, capacitor voltage rating, losses, package size, and cost all matter.
- Verify the resulting design. Check audio response, filter current and loss, thermal behavior, distortion, and EMI in the actual implementation.
Analog Devices’ output-filter optimization article describes particular MAX4295/MAX4297 conditions and component choices; those are examples for that application, not universal values. Texas Instruments also discusses how a higher switching frequency can permit smaller filter inductors in a particular comparison, while leaving system tradeoffs to evaluate in its LC-filter article.
What determines efficiency and output-stage loss?
Two important loss categories are conduction loss, while switches carry load current, and switching or gate-drive loss during transitions. Lower MOSFET on-resistance can reduce conduction loss, but devices with lower resistance may have greater gate capacitance. That capacitance increases drive energy, particularly as switching frequency and gate-drive voltage rise. The best tradeoff depends on the power stage and operating conditions.
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Efficiency therefore changes with output power and design; a single generic percentage is not a sound design assumption. For context, Analog Devices gives a 90% idealized output-stage efficiency figure at its clipping-onset comparison point. Separately, a 2002 application note reports efficiency exceeding 85% for a specific MAX4295/MAX4297 example driving a BTL 4-ohm load from a +2.7 V to +5.5 V supply under the conditions described there. Neither figure guarantees the efficiency of another amplifier or build.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is dead time in a class-D amplifier?
Dead time is the brief break-before-make interval between complementary switches in one output leg. It prevents both devices from conducting simultaneously, which could create a damaging supply-to-return shoot-through path. But dead time also changes pulse timing: too much can increase distortion, while too little risks switch overlap. Choose driver/controller timing in conjunction with the MOSFET switching behavior rather than treating dead time as an isolated setting. Analog Devices discusses this timing and its distortion and EMI implications in its class-D design overview.
How do layout and feedback affect performance?
Keep switching-current paths compact
Fast switching edges and current pulses can create conducted and radiated EMI. Keep high-frequency current loops small, place the output filter close to the amplifier, and keep outgoing and return paths near each other. Speaker wiring is also part of the current loop, so its routing can affect emissions. A filterless architecture removes external filter parts, not the need to assess EMI in the finished system.
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Account for the speaker and control loop
Because speaker impedance is frequency-dependent and may be reactive, nominal impedance alone may not predict filter behavior. Feedback can improve distortion and supply rejection in some architectures, but introduces loop-stability work. Audio response, switching attenuation, conduction and switching losses, dead-time safety, filter size, EMI/EMC, and cost are interdependent design choices.
What a first-pass calculation cannot validate
A schematic and ideal load equations do not demonstrate safe or validated performance. Use the selected device’s current datasheet and layout guidance, then verify the actual design’s thermal behavior, distortion, and EMI under its intended supply and load conditions. Component values and operating limits should be tied to that specific design, not borrowed from an unrelated example.
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