Class C Bias Point
Understanding the Class C Bias Point
The Class C Bias Point is an extreme operating state designed strictly for maximum RF power efficiency. Unlike linear amplifiers (Class A or AB) that attempt to replicate the input waveform, a Class C amplifier intentionally mutilates the signal. The DC gate or base voltage is biased deeply below the transistor's turn-on threshold (pinch-off). Because of this deep negative bias, the transistor is held entirely in the off-state for the majority of the time.
When a massive RF input signal is applied, only the absolute highest peaks of the positive sine wave carry enough voltage to overcome the deep bias and turn the transistor on. Consequently, the transistor conducts current in short, violent pulses, acting for less than 180 degrees of the entire RF cycle (a conduction angle θ < π). The narrower these current pulses become, the higher the efficiency climbs, theoretically approaching 100%. However, these sharp pulses are rich in harmonic distortion, rendering the raw output completely unrecognizable from the input sine wave.
Resonant Tank Circuits
Because the transistor in Class C generates a train of narrow, distorted current pulses, it cannot be directly connected to an antenna. It must be paired with a high-Q resonant "tank" circuit (a tuned inductor-capacitor network) on the output. The sharp current pulse acts like a hammer striking a bell; it "rings" the resonant tank, and the tank's momentum reconstructs a smooth, continuous sine wave at the fundamental frequency while aggressively filtering out all the harmonic distortion.
Conduction Angle: 0 ≤ θ < 180°
Maximum Efficiency:
η = [ θ - sin(θ) ] / [ 4 × sin(θ/2) - 2θ × cos(θ/2) ]
As the conduction angle (θ) approaches 0 (infinitely narrow pulse), η approaches 100%.
Comparison
| Class Type | Conduction Angle | Max Efficiency | Signal Linearity |
|---|---|---|---|
| Class A | 360° | 50% | Perfect |
| Class AB | 180° - 360° | 50% - 78% | Good (Used for 5G/LTE) |
| Class C | < 180° (Pulses) | Approaches 100% | Terrible (Non-linear) |
Frequently Asked Questions
If Class C is so efficient, why don't cell phones use it?
Class C amplifiers completely destroy any amplitude information in the signal. Modern cellular communications (like 4G LTE, 5G, and Wi-Fi) use complex modulations like QAM that encode data in both the phase and the amplitude of the wave. If you run a QAM signal through a Class C amp, the data is instantly corrupted. Class C can only be used for constant-envelope signals where amplitude doesn't matter.
What types of systems actually use Class C bias points?
Class C is heavily used in high-power FM radio broadcasting, Continuous Wave (CW) radar jammers, and legacy analog television transmitters. Because FM radio only encodes audio via frequency changes (the amplitude never changes), the severe amplitude clipping of a Class C amplifier does no damage to the audio data.
What is the trade-off for approaching 100% efficiency in Class C?
To get the efficiency to approach 100%, you must bias the transistor so deeply that the conduction pulse is infinitely narrow. A narrow pulse contains very little total energy. Therefore, as efficiency goes up, the total RF output power plummets. You need a massive, expensive transistor to generate a meaningful amount of output power if you run the conduction angle too narrow.