An amplifier class defines the conduction angle and biasing of the active output devices over a full 360-degree signal cycle, directly dictating the trade-off between power efficiency and signal linearity. In a real circuit, your chosen amp class dictates the physical size of your heat sinks, the current rating of your power supply, and the baseline total harmonic distortion (THD) before any feedback loops are applied. Builders frequently confuse amplifier class (the internal transistor topology like Class A, AB, or D) with amplifier category (preamp vs. power amp), and most commonly mistake Class D for "digital"—when it is actually a high-frequency analog switching topology.
The Core Amplifier Classes: Efficiency vs. Linearity
Every amplifier topology forces a compromise between how much of the input signal is faithfully reproduced (linearity) and how much of the power supply energy is converted into useful output rather than waste heat (efficiency). Here is how the primary audio and RF classes stack up in 2026:
| Class | Conduction Angle | Theoretical Max Efficiency | Typical Real-World Efficiency | Primary Trade-off |
|---|---|---|---|---|
| Class A | 360° (Always on) | 50% (Transformer-coupled) | 15% - 30% | Zero crossover distortion, but massive heat generation. |
| Class B | 180° (Push-pull) | 78.5% | 50% - 60% | High efficiency, but severe crossover distortion at zero-crossing. |
| Class AB | >180° to <360° | 78.5% | 45% - 65% | The analog sweet spot; slight bias current eliminates crossover distortion. |
| Class D | Switching (PWM) | 100% | 85% - 95% | Near-perfect efficiency, but requires output LC filtering and EMI management. |
| Class G/H | AB with rail switching | Varies | 65% - 80% | Complex power supply design to dynamically track signal voltage. |
To understand why pure Class B is rarely used in audio, imagine two lanes of traffic merging at a toll booth where drivers hesitate before accelerating; this hesitation causes a rhythmic gap in the flow. In a Class B push-pull circuit, this "hesitation" happens every time the signal crosses zero volts, resulting in harsh crossover distortion that ruins audio fidelity. Class AB solves this by applying a small bias current, keeping both "lanes" slightly engaged at all times.
Worked Example: Thermal Design for a 50W Output Stage
The most profound impact of amplifier class selection is on your thermal management budget. Let us calculate the heat sink requirements for a 50W RMS amplifier driving an 8Ω load, comparing a traditional Class AB design against a modern Class D design.
Class AB Thermal Calculation (Assuming 50% Efficiency)
- Output Power: 50W
- Total Power Drawn from PSU: 100W
- Power Dissipated as Heat: 50W
- Max Junction Temp ($T_J$): 150°C
- Ambient Temp ($T_A$): 25°C
- Junction-to-Case Thermal Resistance ($\theta_{JC}$): 1.5°C/W
- Required Heat Sink ($\theta_{SA}$): $[(150 - 25) / 50] - 1.5 = 1.0°C/W$
Result: A 1.0°C/W heat sink is a massive, heavy extruded aluminum block costing upwards of $30-$50 and requiring forced-air cooling in a confined chassis.
Class D Thermal Calculation (Assuming 90% Efficiency)
- Output Power: 50W
- Total Power Drawn from PSU: 55.5W
- Power Dissipated as Heat: 5.5W
- Required Heat Sink ($\theta_{SA}$): $[(150 - 25) / 5.5] - 1.5 = 21.2°C/W$
Result: A 21.2°C/W thermal budget is easily handled by a $2 surface-mount stamped metal tab, or even just the copper pour on the PCB itself with thermal vias.
This single efficiency gap is why portable Bluetooth speakers and modern AV receivers have almost entirely abandoned Class AB for their main output stages.
Where You Meet This in Practice
You will encounter these topologies across different domains, each optimized for specific physical constraints:
- High-End Audiophile Hi-Fi: Class A and Class AB still dominate here. Builders prioritize the complete absence of switching noise and the "warm" even-harmonic distortion profile of Class A, accepting the massive heat output as a feature of the design.
- Portable and Battery-Powered Audio: Class D is mandatory. When running off 18650 Li-ion cells, you cannot afford to waste 50% of your battery capacity as heat.
- Active Subwoofers: Class D is the standard. Bass frequencies require massive voltage swings and high current, but human ears are less sensitive to THD at 40Hz than at 4kHz, making the high efficiency of Class D ideal.
- RF Transmitters: Class C and Class E are common in radio frequency applications where linearity is either handled by upstream modulation schemes or is irrelevant (like CW/Morse code), allowing for extreme efficiency at high frequencies.
Decision Tree: Picking Your Amp Class and IC
Use this decision matrix to select the correct topology and a specific, proven integrated circuit for your next build.
| If Your Build Requires... | Choose Class... | Recommended IC / Topology | Key Design Constraint |
|---|---|---|---|
| Battery-powered portable speaker (10W - 50W) | Class D | TI TPA3116D2 | Requires LC output filter and tight PCB layout. |
| High-power home theater / PA system (100W+) | Class D | STMicro TDA7498E | Needs robust 36V+ PSU and EMI shielding. |
| Audiophile desktop / living room amp (<68W) | Class AB | TI LM3886 (or clone) | Requires large heat sink and dual-rail linear PSU. |
| Low-power headphone amp (<1W) | Class A / AB | TI OPA1622 or discrete JLH | Focus on low noise floor and PSRR. |
| RF narrowband transmitter | Class C | Discrete LDMOS / BJTs | Requires precise LC tank tuning for harmonics. |
PCB Layout Rules for Switching Amplifiers (Class D)
Because Class D is the default choice for 95% of modern DIY builds, you must understand its layout pitfalls. A poorly routed Class D board will emit enough electromagnetic interference (EMI) to brick nearby ESP32 microcontrollers or cause audible whine in adjacent preamp stages.
- Minimize the High di/dt Loop: The loop formed by the decoupling capacitors, the IC power pins, and the internal MOSFETs must be as physically small as possible. Use wide, short traces and place 100nF ceramic capacitors within 2mm of the IC VCC pins.
- Use Shielded Inductors: The output LC filter inductors radiate magnetic fields. Use shielded drum-core or toroidal inductors, and keep them away from sensitive analog input traces.
- Ferrite Beads on Outputs: Place a ferrite bead (e.g., BLM21PG221SN1D) in series with each speaker output, followed by a small ceramic capacitor to ground. This creates a low-pass Pi filter that prevents the 400kHz+ switching carrier from radiating down the speaker wires acting as antennas.
- Solid Ground Plane: Do not route signal traces under the inductors or the switching nodes. Use a continuous ground plane on the layer directly beneath the output stage.
For deeper technical specifications on filter component selection, refer to the Texas Instruments TPA3116D2 Datasheet, which provides excellent reference designs for the output LC network.
FAQ: Amplifier Class Misconceptions
Does Class D inherently sound worse than Class AB?
No. Early Class D designs from the early 2000s suffered from high dead-time distortion and poor power supply rejection. Modern Class D ICs utilize advanced modulation schemes (like Phase-Shifted Full-Bridge or proprietary spread-spectrum PWM) and operate at switching frequencies above 600kHz. When paired with a clean, low-impedance power supply, modern Class D achieves THD+N figures below 0.01%, which is indistinguishable from Class AB in blind listening tests. For a detailed breakdown of modern switching audio topology, see the Analog Devices guide on Class-D Audio Amplifiers.
What is the difference between Class D and Class G/H?
Class D uses high-frequency PWM switching to regulate power delivery to the load. Class G and Class H are still linear analog amplifiers (based on Class AB), but they use multiple power supply rails. A Class G amp switches between a low-voltage and high-voltage rail depending on the signal peak, while Class H uses a modulator to continuously track the signal envelope, dynamically adjusting the rail voltage to minimize the voltage drop across the output transistors.
Can I use a switching power supply (SMPS) with a Class AB amplifier?
Yes, but you must oversize it. Class AB amplifiers draw current in large, dynamic bursts correlated with the audio signal peaks. An SMPS designed for a 50W continuous load will likely trigger its over-current protection or suffer severe voltage sag when hit with a 100W transient bass kick. If using an SMPS with Class AB, ensure it has a high peak-current rating and add large bulk capacitance (10,000µF+) on the amplifier board.
The Default Recommendation: If you are designing a general-purpose audio amplifier for home, automotive, or portable use in 2026, default to Class D using the TI TPA3116D2. It offers the best balance of high efficiency, low BOM cost, excellent thermal performance, and proven reliability, freeing you from the burden of managing massive heat sinks and heavy linear transformers.
