Side-by-side comparison
| Parameter | Class C | Class D Amplifier |
|---|---|---|
| Conduction angle | Less than 180° (typically 90–150°) | Varies (PWM switching; not sine-wave conduction) |
| Operating principle | Tuned tank circuit recovers fundamental frequency | PWM switching; LC filter reconstructs audio signal |
| Efficiency (η) | Up to 90–95% (at low conduction angles) | 85–95% (PAM8403: ~90% at 1 W into 4 Ω) |
| Linearity | Highly non-linear; not suitable for AM audio | High linearity with feedback; THD < 0.1% (TPA3116) |
| Output filter required | High-Q LC tank (tuned to carrier frequency) | LC low-pass filter (cutoff ~20 kHz) |
| Frequency range | RF: 1 MHz–1 GHz+ | Audio: 20 Hz–20 kHz signal; carrier 200 kHz–1 MHz |
| Distortion | Very high harmonic distortion without tuned load | Low with proper filter; audible artifacts if EMI is bad |
| Application | FM transmitters, CB radios, satellite uplinks | Bluetooth speakers, soundbars, car audio (TPA3116, PAM8403) |
| Drive signal | Sine wave at carrier frequency | PWM signal from comparator or digital modulator |
| Device examples | MRF300AN RF transistor, BLF188XR (GaN RF) | TPA3116D2, PAM8403, IRS2092 |
Key differences
Class C amplifiers conduct for less than 180° — the transistor is biased below cutoff and fires only on the peaks of the drive signal. The missing portion of the waveform is irrelevant because the high-Q tuned tank stores energy and delivers a clean sine wave at the output. This makes class C completely unusable for audio (which is wideband) and ideal for single-frequency RF carriers. Class D abandons sine-wave conduction entirely — the output transistors are switches (MOSFETs in TPA3116) toggling at 200–500 kHz in PWM mode. The duty cycle encodes the audio amplitude; a 20 kHz LC filter strips the carrier and passes only audio. TPA3116D2 achieves 98% efficiency at 25 W into 4 Ω — impossible for class C at audio frequencies. Class D's main issue is EMI from the high-frequency switching; class C's main issue is narrow bandwidth, confined to its tank resonance.
When to use Class C
Use class C when amplifying a fixed RF carrier frequency — FM broadcast (88–108 MHz), CB (27 MHz), or satellite uplink. An MRF300AN in a class C stage with a tuned 100 MHz tank delivers 300 W RF output at ~90% efficiency; the tank eliminates all harmonics beyond the fundamental.
When to use Class D Amplifier
Use class D in any battery-powered or efficiency-critical audio application where size and heat dissipation matter. A PAM8403 class D amplifier IC delivers 3 W + 3 W stereo into 4 Ω from a 5 V USB supply with no heatsink needed, consuming less than 15 mA at idle.
Recommendation
Choose class D for any audio amplification task — it's efficient, integrable, and produces low distortion with the built-in LC filter. Choose class C only when amplifying a single RF carrier frequency where a tuned tank filter can be used. These two classes solve completely different problems and are never interchangeable.
Exam tip: University papers ask students to explain why class C cannot amplify audio signals directly — the answer must cite both the non-linearity (only peak conduction) and the fact that a tuned tank is narrowband; an audio signal's bandwidth of 20 kHz cannot be recovered from a single resonant frequency.
Interview tip: Core electronics interviewers ask how class D achieves high efficiency despite using active transistors — the expected answer is that MOSFETs operate as switches (fully ON or fully OFF), so they spend minimal time in the resistive linear region where V × I is non-zero and power is wasted.