Audio Amplifier Classes

Amplifier classes describe how the active devices in a power amplifier conduct over each cycle of the input signal. The classification turns on the conduction angle — the fraction of the input cycle during which an output device passes current — and on whether the device acts as a continuously variable element or as a switch. These choices set the fundamental trade-off between linearity, which keeps distortion low, and efficiency, which determines how much supply power is wasted as heat.

In a Class A amplifier the output device conducts for the entire cycle. This gives very low distortion but poor efficiency, because the device dissipates power even with no signal. A Class B stage uses two devices, each conducting for half the cycle, which improves efficiency but introduces crossover distortion at the handover point. Class AB is the common compromise: each device conducts for slightly more than half the cycle, nearly eliminating crossover distortion while remaining far more efficient than Class A. Class C conducts for less than half the cycle and is highly efficient but very non-linear, so it is used for radio-frequency rather than audio amplification.

Switching designs take a different approach. A Class D amplifier rapidly switches its output devices fully on or off and uses pulse-width modulation followed by a low-pass filter to reconstruct the audio, achieving high efficiency and little heat, which suits compact and battery-powered equipment. Class E is a specialised high-efficiency switching topology used mainly in radio-frequency power applications rather than for general audio.

Frequently asked questions

Why is Class A amplification so inefficient?

The output device conducts current throughout the whole signal cycle, even with no input, so it constantly dissipates power as heat regardless of the signal level.

What is crossover distortion?

It is the distortion that occurs in Class B amplifiers at the moment one output device hands over to the other near the zero-crossing of the waveform. Class AB biasing largely removes it.

Why are Class D amplifiers so efficient?

Their output devices act as switches that are either fully on or fully off, so they dissipate little power. The audio is recovered by filtering the switched waveform.