Conduction angle recognition: In Class C operation of a transistor power amplifier, the collector current flows for what portion of the input signal cycle?

Introduction / Context:
Amplifier classes are categorized by conduction angle—how long the active device conducts current during one cycle of a sinusoidal input. Class C is commonly used in RF transmitters with tuned loads to obtain high efficiency at the cost of linearity.

Given Data / Assumptions:

• Sinusoidal drive applied to a biased transistor stage.
• Standard class definitions: A (360°), B (180°), AB (>180° but <360°), C (<180°).

Concept / Approach:

In Class C, the bias point is set below cutoff so the device conducts only around the peaks of the input waveform, for a conduction angle strictly less than 180°. A resonant load network reconstructs a sinusoidal output from these narrow current pulses.

Step-by-Step Solution:

Verification / Alternative check:

Typical Class C conduction ranges between 60° and 120°, depending on the desired efficiency and distortion acceptable before the tank circuit filters harmonics.

Why Other Options Are Wrong:

• 360° corresponds to Class A.
• 180° corresponds to ideal Class B.
• More than 180° corresponds to Class AB.
• “Exactly 120°” is not a definition, only an example.

Common Pitfalls:

• Confusing the conduction angle definition with the harmonic content at the output of a tuned circuit.

Final Answer:

less than 180° of the input wave