Class C BJT Amplifiers

Discrete Semiconductor Devices and Circuits

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  • Question 1

    A common class of operation used in radio-frequency (RF) amplifier circuits is class-C. Explain what this means, contrasting it against the class-A and class-B operations common in audio-frequency amplifier circuits.

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  • Question 2

    Contrast class-A, class-B, and class-C amplifier operations, explaining what defines each class. Then, rank these three classes in order of least power efficiency to greatest power efficiency.

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  • Question 3

    Shown here is a schematic diagram for a class-C RF (radio frequency) amplifier circuit:

    This circuit will look very strange if you are accustomed to analyzing audio-frequency and DC amplifier circuits. Note some of the distinct differences between this amplifier and an amplifier used to boost audio signals. Also, explain what “class-C” operation means, and how this amplifier is able to output a continuous sine wave despite the transistor’s behavior in class-C mode.

    Finally, write an equation that predicts this amplifier’s operating frequency, based on certain component values which you identify.

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