Class C Amplifiers

What Is a Class C Amplifier?

Class C amplifiers are fundamentally different from the amplifier classes commonly used in home audio. Unlike Class A, B, or AB designs, a Class C amplifier conducts current for less than 180° of the input signal.​
This extremely short conduction angle results in very high efficiency but introduces severe distortion, making Class C unsuitable for reproducing music.​
Instead, these amplifiers are primarily used in radio-frequency (RF) applications, where resonant circuits reconstruct the waveform.​

How Class C Works
The output transistor is heavily biased so that it switches on only during a small portion of the signal cycle.​
A tuned LC circuit then filters and reconstructs the desired waveform.​
Without this tuned circuit, the output would be highly distorted.​

Advantages
• Extremely high efficiency (up to 90%)​
• Very little heat generation​
• Compact design​
• Excellent for RF transmission​

Disadvantages
• Extremely high distortion​
• Not suitable for audio reproduction​
• Requires tuned output circuits​

Typical Applications
• Radio transmitters​
• RF power amplifiers​
• Communication systems​
• Oscillators​

Class D Amplifiers
What Is a Class D Amplifier?
Class D is one of the most important developments in amplifier technology over the past few decades. Despite the name, the letter D does not stand for "Digital."​
A Class D amplifier is an analogue switching amplifier that uses pulse-width modulation (PWM), pulse-density modulation (PDM), or similar techniques to control its output devices.​
Instead of operating continuously like Class A or AB, the output transistors switch fully on or off hundreds of thousands—or even millions—of times per second.​
This dramatically reduces power losses.​

How Class D Works
The incoming analogue audio signal is converted into a high-frequency switching waveform.​
The output MOSFETs rapidly switch between fully conducting and fully non-conducting states.​
Because transistors dissipate very little power while fully on or fully off, efficiency is exceptionally high.​
A low-pass output filter removes the switching frequency and reconstructs the original analogue audio waveform for the loudspeaker.​

Why Class D Became So Popular
Earlier Class D amplifiers were criticised for:​
• Audible switching noise​
• Limited bandwidth​
• Higher distortion​
• Poor damping factor​
Modern technology has transformed the situation.​
Today's premium Class D amplifiers achieve distortion figures below 0.001%, excellent linearity, and outstanding signal-to-noise ratios. Many now compete directly with the finest Class AB amplifiers.​

Advantages
• Outstanding efficiency (90–98%)​
• Very low heat generation​
• Compact and lightweight​
• Smaller power supplies​
• Lower electricity consumption​
• High output power​
• Excellent reliability​

Disadvantages
• Output filter design is critical​
• Poor implementations may introduce switching artefacts​
• Sound quality varies greatly depending on the design​

Typical Applications
Class D technology is now used in:​
• Stereo amplifiers​
• AV receivers​
• Active loudspeakers​
• Studio monitors​
• Bluetooth speakers​
• Portable audio devices​
• Car audio systems​
• Professional PA amplifiers​
• Subwoofer amplifiers​

Famous Class D Amplifiers
Examples include:​
• NAD M33​
• NAD C298​
• Lyngdorf TDAI-3400​
• Bel Canto e.One REF601M​
• Topping PA5 II​
• Fosi Audio V3​
• SMSL AO300​
• Hypex NC502MP-based amplifiers​
• Purifi 1ET400A-based amplifiers​
• Orchard Audio Starkrimson (GaN)​

Sound Characteristics
Modern Class D amplifiers are often described as:​
• Neutral​
• Highly detailed​
• Fast and precise​
• Tight, controlled bass​
• Low background noise​
• Excellent dynamics​
Many listeners can no longer distinguish premium Class D amplifiers from high-end Class AB designs in controlled listening tests.​

Class E Amplifiers
What Is a Class E Amplifier?
Class E amplifiers are another switching amplifier topology designed primarily for radio-frequency applications rather than audio.​
They use carefully timed switching and resonant circuits to minimise power losses.​

Advantages
• Extremely high efficiency​
• Very low switching losses​
• Compact design​
• Excellent for RF applications​

Disadvantages
• Unsuitable for high-fidelity audio​
• Requires carefully designed resonant networks​
• Limited audio applications​

Typical Applications
• RF transmitters​
• Wireless charging systems​
• Industrial electronics​
• Scientific equipment​

Class F Amplifiers
What Is a Class F Amplifier?
Class F amplifiers improve efficiency by shaping voltage and current waveforms using harmonic tuning.​
Special resonant circuits suppress unwanted harmonics and maximise efficiency.​
Like Class C and E, Class F amplifiers are intended for RF systems rather than music playback.​

Advantages
• Extremely high efficiency​
• Reduced switching losses​
• High output power capability​

Disadvantages
• Complex implementation​
• Not suitable for Hi-Fi audio​
• Requires harmonic tuning networks​

Applications
• Broadcast transmitters​
• Microwave communications​
• Radar systems​
• RF laboratory equipment​



Why Class D Is the Future of Consumer Audio
Over the last twenty years, Class D technology has experienced remarkable progress thanks to improvements in semiconductor devices, digital control techniques, and feedback systems.​
Modern Class D amplifiers now offer:​
• Exceptional measured performance​
• Compact form factors​
• Lower manufacturing costs​
• Reduced energy consumption​
• High power output​
• Excellent reliability​
As a result, Class D has become the preferred choice for many manufacturers of Hi-Fi equipment, powered speakers, home theatre receivers, and professional audio systems.​
While Class A and Class AB continue to hold a special place among audiophiles, the gap in sound quality has narrowed significantly. Today's best Class D amplifiers demonstrate that high efficiency and outstanding musical performance can coexist in a single design.