Understanding the Categories of Amplifiers: Classes A, B, AB, C, and D Explained

An amplifier category (or class) defines the fraction of the input signal cycle during which the active output devices conduct current, directly dictating the circuit's efficiency, linearity, and heat dissipation. In a real circuit or installation, selecting the category changes the biasing network topology, the required heat sink mass, the power supply current rating, and the complexity of the output filtering. People commonly confuse amplifier classes (which describe conduction angles and switching behavior) with amplifier topologies (like differential or instrumentation setups), and frequently mistake "Class D" as standing for "Digital" rather than switched-mode analog.

The Core Categories of Amplifiers at a Glance

The categories of amplifiers are primarily divided by their conduction angle—how many degrees of the 360-degree input waveform the output transistors are actively passing current. Below is a reference matrix detailing the electrical characteristics and common silicon implementations for each major class.

According to foundational texts on amplifier classes from Electronics Tutorials, the trade-off is always between linearity (low Total Harmonic Distortion, or THD) and efficiency. Class A keeps the transistor permanently biased "on," eliminating crossover distortion but wasting massive amounts of power as heat. Class B turns the device completely off at the zero-crossing, creating severe crossover distortion. Class AB is the pragmatic compromise, applying just enough bias current to keep both devices slightly on during the zero-crossing.

Worked Example: Thermal Dissipation in Class AB vs. Class D

To understand why the categories of amplifiers matter on the workbench, let us calculate the thermal requirements for driving an 8-ohm bookshelf speaker to 50W RMS continuous output. We will compare a traditional Class AB chip amp (like the LM3886) against a modern Class D module (like the TPA3116D2).

Class AB Thermal Calculation

Assume the Class AB amplifier is operating at 55% practical efficiency at full continuous sine-wave output.

• Output Power ($P_{OUT}$): 50W
• Total Power Drawn ($P_{IN}$): $50W / 0.55 = 90.9W$
• Power Dissipated as Heat ($P_{D}$): $90.9W - 50W = 40.9W$

Class D Thermal Calculation

Assume the Class D amplifier operates at 90% practical efficiency (accounting for switching losses and the LC output filter).

• Output Power ($P_{OUT}$): 50W
• Total Power Drawn ($P_{IN}$): $50W / 0.90 = 55.5W$
• Power Dissipated as Heat ($P_{D}$): $55.5W - 50W = 5.5W$

Using the same 110°C allowable rise, the required $\theta_{JA}$ is $110°C / 5.5W = \mathbf{20°C/W}$. A standard 4-layer PCB with a thermal via array under the IC's exposed pad easily achieves a thermal resistance of 15°C/W to 20°C/W. No external finned heatsink is required. This single calculation explains why Class D has entirely conquered the portable and compact audio market.

Where You Meet These Categories in Practice

The theoretical categories of amplifiers map directly to specific hardware implementations you will encounter in commercial products and DIY builds.

Class AB: Studio Monitors and High-End Receivers

When absolute signal purity is required and physical size is not a constraint, Class AB dominates. The TI LM3886 and OPA541 are legendary Class AB power ICs used in active studio monitors. The primary design challenge here is managing the quiescent current. If the $V_{BE}$ multiplier bias circuit drifts due to temperature, the amp will either thermal runaway (bias too high) or introduce crossover distortion (bias too low). Proper thermal coupling of the bias transistor to the output devices is mandatory.

Class D: Subwoofers, PA Systems, and Portable Audio

Class D amplifiers use a high-frequency PWM carrier (typically 300kHz to 600kHz) to switch the output MOSFETs fully on or fully off, minimizing the time spent in the high-dissipation linear region. As detailed in All About Circuits' Class D primer, the catch is the output filter. You must design a precise LC low-pass filter (usually a 10µH inductor and 1µF capacitor) to strip the switching carrier before it reaches the speaker. If you omit this filter, the high-frequency energy will fry tweeters and cause severe EMI interference with nearby RF equipment.

Class C: Ham Radio and RF Transmitters

You will almost never see Class C in audio because the output waveform is heavily clipped, resulting in massive harmonic distortion. However, in RF applications like a 144 MHz ham radio transmitter, the output is fed into a high-Q LC resonant tank circuit. The tank circuit "rings" and reconstructs the missing portions of the sine wave. Because linearity is irrelevant for Continuous Wave (CW) or Frequency Modulation (FM) signals, the 80%+ efficiency of Class C makes it the standard for RF final amplifier stages.

Class G: Tracked-Supply Audio Amplifiers

Class G is essentially a Class AB amplifier with multiple power supply rails. When the audio signal is quiet, it runs off a low-voltage rail (e.g., ±15V) to maintain high efficiency. When a transient peak hits, fast diodes or MOSFETs switch the output stage to a high-voltage rail (e.g., ±50V) to prevent clipping. This architecture is common in high-power professional audio amplifiers where average power levels are low, but peak headroom must be massive.

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