Why is a Class C amplifier so efficient?

Efficiency drops when a transistor has both high voltage and high current flowing through it simultaneously, because that generates heat. A Class C amplifier is biased deep into the 'off' state. It only turns on and conducts current for a brief instant when the input sine wave reaches its absolute peak. For the majority of the cycle, the transistor is completely off, drawing zero current and burning zero heat. This allows theoretical efficiencies approaching 80%, far higher than Class A or Class AB.

If it only outputs pulses, how does it transmit a sine wave?

The output of a Class C transistor is a severely distorted train of sharp current spikes. If you transmitted that directly, it would contain massive amounts of harmonic interference. However, a Class C amplifier must be connected to a high-Q resonant LC tank circuit (a 'flywheel'). When the sharp pulse hits the tank, the tank rings at its resonant frequency. The tank absorbs the pulse and 'fills in the gaps' during the time the transistor is off, smoothing the sharp pulse back into a pure, clean sine wave.

Why can't Class C amplify complex signals like 5G or Wi-Fi?

Class C is completely non-linear. Because it requires the input signal to overcome a deep cut-off bias just to turn on, it destroys any amplitude variations. If you feed it a 5G OFDM signal with varying peaks and valleys, the low-amplitude parts won't turn the transistor on at all, and the data will be lost. Class C can only be used for constant-envelope signals (like FM radio) or as the specifically designed 'Peaking Amplifier' in a Doherty architecture where it is intentionally meant to turn on only at maximum power.

Active Components

Class C Amplifier

An engineer designing a 100-kilowatt FM radio transmitter needs massive efficiency; burning 50 kilowatts of heat is not an option. They abandon linear Class AB amplifiers and switch to Class C. They bias the massive transmitter tubes far below pinch-off. When the RF signal enters, the tubes remain completely off for most of the sine wave, only snapping on for a brief fraction of a second at the absolute peak of the wave to deliver a violent pulse of current. Because the tubes are off most of the time, they generate very little heat, achieving incredible efficiency. To fix the horrifically distorted, pulsed output, the engineer routes the signal through a massive resonant tank circuit. The tank acts like a heavy pendulum; when hit with the sharp pulse, it swings smoothly, reconstructing the pure 100 MHz sine wave required for broadcast. Class C trades absolute linearity for brute-force thermal survival.

Category: Active Components

Conduction Angle: < 180 degrees (Pulsed)

Primary Trade-off: High Efficiency vs. Severe Non-Linearity

Amplifier Class Comparison

Amplifier Class	Conduction Angle	Bias Point	Theoretical Efficiency	Linearity
Class A	360° (Always On)	Middle of Active Region	50%	Perfect (Highest)
Class AB	>180° but <360°	Slightly above Cut-off	~65%	Good
Class B	Exactly 180°	Exactly at Cut-off	78.5%	Moderate
Class C	< 180° (Often ~90°)	Deep below Cut-off	80% - 85%	Terrible (None)

Conduction Angle (θ_c) and Efficiency:
As θ_c approaches 0°, theoretical efficiency approaches 100%, but output power drops to 0 Watts.
Engineers must balance the bias point. A typical Class C amplifier is biased to conduct for about 120°. This provides a massive efficiency boost over Class B while still allowing enough current to flow to generate useful output power.

Doherty Peaking Bias:
In a Doherty PA, the peaking amplifier is explicitly biased in Class C. It remains asleep (drawing zero DC power) until the input voltage swing exceeds its deep pinch-off threshold, at which point it wakes up to inject current into the load.

Common Questions

Frequently Asked Questions

Why can't you use Class C for AM radio?

AM radio (Amplitude Modulation) stores information in the varying voltage levels of the signal. Because a Class C amplifier is biased deep below cut-off, it completely ignores the low-voltage parts of the signal. The output amplitude does not linearly track the input amplitude, so the AM audio envelope is destroyed. It can only be used for constant-envelope signals like FM (Frequency Modulation) or Phase Modulation.

How does the resonant tank "fix" the signal?

The "flywheel effect." The sharp current pulse generated by the Class C transistor is mathematically composed of a fundamental frequency and a massive number of harmonics. A high-Q resonant LC tank acts as a very narrow bandpass filter. It provides a high impedance to the fundamental frequency (allowing it to develop voltage) but acts as a short circuit to all the harmonics, filtering them out and leaving only the clean sine wave.

Are Class C amplifiers used in 5G?

Yes, but not as standalone amplifiers. A 5G signal is heavily amplitude-modulated (OFDM), so a standalone Class C would destroy it. However, the Doherty PA architecture uses a Class C amplifier as its "Peaking" stage. The main "Carrier" amplifier (Class AB) handles the complex signal, while the Class C peaking stage only turns on during massive, rare power spikes to assist the main amplifier.

Related Terms

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Active Design

Amplifier Conduction Angle Simulator

Adjust the gate bias voltage of a GaN HEMT and instantly watch the conduction angle drop. See exactly how pushing the transistor into deep Class C operation trades output power capability for massive gains in drain efficiency.

Simulate Conduction Angle