Class C Amplifier
A Class C amplifier biases its single active device below cutoff so it conducts for less than half the input cycle — often just 30-40% — producing a train of current pulses that would be unusably distorted as audio but that a resonant LC tank filters back into a clean sine wave, trading away linearity entirely in exchange for efficiency above 80%, which is why Class C exists almost exclusively in narrowband RF transmitters rather than anywhere audio fidelity matters.
The conduction angle is the direct trade for efficiency, and it is deliberately narrow. Where a Class A device conducts for 360 degrees and Class AB conducts for somewhat more than 180, a Class C device is biased past cutoff so it only turns on for a fraction of the cycle near the input's peak, often 60-120 degrees total; because the device is off for most of the waveform, it dissipates almost no power during that dead time, which is the whole mechanism behind Class C's 80-90%+ theoretical efficiency and why it was the default choice for vacuum-tube AM and shortwave transmitters decades before solid-state RF existed.
A resonant tank does the linearity work the transistor refuses to do. The raw drain or collector current out of a Class C stage is a series of narrow pulses, not a sine wave, and would be hopelessly distorted if fed straight into a speaker or antenna; a parallel LC tank tuned to the carrier frequency acts as a flywheel, absorbing energy from each current pulse and ringing at its resonant frequency to reconstruct a clean sinusoid at the output, which only works because the tank's bandwidth is narrow enough to reject the pulse train's harmonics — Class C is fundamentally a narrowband technique for exactly this reason.
It cannot amplify amplitude information faithfully, so modern radios work around it. Because the output amplitude saturates with drive level rather than tracking it linearly, Class C stages distort AM and any amplitude-modulated signal; this is why Class C survives today mainly in constant-envelope applications — FM and PM transmitters, CW and FSK radio links, and the final power stage of some amateur-radio and broadcast transmitters — while modern cellular and Wi-Fi power amplifiers that must preserve amplitude information use Class AB or linearization techniques like Doherty amplifiers instead.
The device itself sees brutal peak stress in exchange for near-zero average dissipation. Because all the current flows in a brief pulse rather than spread across the cycle, peak collector or drain current in a Class C stage can run many times higher than the DC average, which demands devices with adequate peak-current and breakdown-voltage margin — RF power transistors and tubes built for Class C service, like those historically used in AM broadcast transmitters from companies such as RCA and Continental Electronics, were specified around that peak-pulse rating rather than average power alone.
Read a Class C amplifier through a *flywheel* lens rather than a *linear amplification* lens: the transistor's job is reduced to delivering sharp kicks of energy at the right moment, and the resonant tank's job is to store and smooth that energy into a usable sine wave, which is why Class C buys efficiency numbers no linear class can match but only within the narrow bandwidth its tank is tuned to serve.