Explore the workings of the current-mode Class C amplifier, its high efficiency, and its extensive use in RF applications.
Introduction to Current-Mode Class C Amplifier
The current-mode Class C amplifier is a variant of the power amplifier known for its high efficiency and extensive use in radio frequency (RF) applications. Unlike other classes of amplifiers, it is specifically designed to operate in a non-linear region, making it uniquely suited for tasks involving RF and continuous wave operations.
Working Principle
At the heart of a current-mode Class C amplifier lies the active device, typically a bipolar junction transistor (BJT) or field-effect transistor (FET), which is biased in a way that it conducts current for less than half of the input cycle.
- Input Stage: The input signal is applied to the base (in BJT) or gate (in FET) of the transistor. The biasing is set so that the transistor is ‘off’ for more than half of the input cycle.
- Conduction Stage: When the input signal exceeds the bias, the transistor ‘turns on’ and starts conducting. This period is less than half of the input cycle.
- Output Stage: The output is taken from the collector (in BJT) or drain (in FET). Due to the class C operation, the output signal is a series of pulses. These pulses are then reshaped into a sinusoidal wave using a tuned LC circuit.
Key Features
- High Efficiency: As the active device is ‘on’ for less than half the input cycle, power dissipation is minimal, leading to efficiencies of up to 80% to 90%.
- Non-Linear: The non-linear operation of the Class C amplifier results in significant distortion. However, in applications like RF signal transmission where the exact waveform shape is not critical, this is not a significant drawback.
- RF Applications: The Class C amplifier’s inherent pulse output and its ability to work with a tuned LC circuit make it ideal for RF and continuous wave operations.
Conclusion
In conclusion, the current-mode Class C amplifier is a highly efficient, non-linear power amplifier uniquely suited to RF and continuous wave applications. Despite its distortion characteristics, it remains a preferred choice for systems where high efficiency and frequency response are more critical than fidelity of the signal shape.
