Class D Amplifiers: How Switching Beats Heat

Class D amplifiers do not amplify continuously they switch at hundreds of kilohertz and let a low-pass filter reconstruct the audio. Losses collapse because transistors are either fully on or fully off. Efficiencies of 90 %+ put a 100 W amplifier in a palm-sized box.
> At a glance: 8 minute guide · part 7 of 10 in the amplifiers complete guide track · includes a worked example and a quick-reference table.

## Modulation and the filter

Here is the working theory in one pass. The input is compared against a triangle wave, producing PWM whose duty tracks the audio. After the switching stage, an LC filter strips the carrier, leaving amplified audio. Filter design and load interaction set the high-frequency behaviour speakers are not resistors.

| Aspect | Class AB | Class D |
| — | — | — |
| Efficiency (typical) | 50–65 % | 85–93 % |
| Idle dissipation | Moderate | Milliwatts–small |
| Size for 100 W | Heatsink-dominated | Palm-sized modules |
| Distortion (modern parts) | Very low | Comparable |
| Design complexity | Analog craft | Controller + layout rules |

## Dead time and distortion

Both switches cannot conduct simultaneously (shoot-through). The protective dead time between transitions is the main distortion generator. Modern controller ICs shrink it to nanoseconds. EMI is the second tax layout discipline like any switching power design.

## Why modern Class D wins

Post-filter feedback and integrated controller chips lifted performance into territory AB once owned: THD under 0. 01 %, idle currents of milliamps. Thermal budgets that fit inside powered speakers and car audio without heroic sinks.

## How to apply this in your build

Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. Choose a controller/module with post-filter feedback for serious audio
2. Follow the layout and inductor recommendations exactly
3. Verify stability into the real speaker load
4. Filter and snub the supply rail for EMI compliance

### Worked example

A 2×50 W Class D board idles at ~30 mA from 24 V under a watt. The AB amplifier it replaced idled at 8 W doing the same job. A battery-life multiplication in portable rigs. Run the numbers yourself with the 555 Timer Astable Mode and the result should agree to within rounding.

> Practical note from the bench. Class D demands switching-supply layout instincts our board notes treat it exactly like a buck converter that happens to carry audio.

## Common mistakes to avoid

– Ignoring the specified output inductor and “any toroid” substituting
– Loose layout around the switching node radiating havoc
– Expecting filter-free boards to behave into capacitive loads

## Key takeaways

– Modulation and the filter the foundation of this guide; revisit it if any measurement here surprises you.
– Dead time and distortion the foundation of this guide; revisit it if any measurement here surprises you.
– Why modern Class D wins the foundation of this guide; revisit it if any measurement here surprises you.

## Who this guide is for

Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the amplifiers complete guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.

## What you need before starting

Nothing exotic: the parts or tools named in the guide, a multimeter. The 555 Timer Astable Mode / Electrical Power Calculator open in a tab. Choose a controller/module with post-filter feedback for serious audio before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.

### Quick reference card

| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Modulation and the filter |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Common mistakes to avoid |

## How this fits the amplifiers complete guide track

This guide is one stop in the structured learning path. Start from the [amplifiers complete guide](/tutorial/amplifiers-complete-guide) pillar page for the full map, or continue with [Class AB stages](/tutorial/class-ab-push-pull-output) and [buck converter principles](/tutorial/buck-converter-basics). For the arithmetic, open the [555 Timer Astable Mode](/tools/timer-555-astable) or [Electrical Power Calculator](/tools/electrical-power).

## Frequently asked questions

Does Class D sound digital?
The output is analog after filtering; modern implementations measure transparently. Old reputation came from early parts and layout sins.

Can I build my own from discretes?
Educationally yes; practically, integrated controllers deliver performance no bench-built discrete bridge matches on deadline.

Is there a calculator for this?
Yes the [555 Timer Astable Mode](/tools/timer-555-astable) and [Electrical Power Calculator](/tools/electrical-power) tools run the formulas from this guide instantly, client-side, with no signup.

## Continue the learning path

– The complete amplifiers & audio guide: [Amplifiers & Audio complete guide](/tutorial/amplifiers-complete-guide)
– Read next: [inverting op-amp: gain, virtual ground and design](/tutorial/inverting-op-amp-configuration)
– Also in this track: [non-inverting op-amp: high-impedance gain stage](/tutorial/non-inverting-op-amp-configuration)
– Continue with: [the op-amp voltage follower: small circuit, big leverage](/tutorial/op-amp-voltage-follower)
– Calculate as you go: [op-amp gain calculator](/tools/op-amp-gain) · [power dissipation tool](/tools/electrical-power) · [gain to dB converter](/tools/opamp-gain)
– Bookmark this page against the day a measurement surprises you. Most readers return to the table and the mistake list first, and that is the correct order.

## Measurement discipline

When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.

Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.
## Formulas and checks from this guide

Verification checklist for this track: measure bias at idle before signal testing, check heatsink temperature under load. Confirm gain across frequency rather than at 1 kHz alone. Amplifiers forgive nothing at their edges.

Bookmark this page against your next build in the track. The checklist above is the same one used across 19 guides in this series.

## Hard-won notes

Across the emitter resistors at idle, no signal. Compare each output device.

Thermal drift in bias. Check the bias tracker is bolted to the heatsink and re-set per the service values.