Question Bank (3 marks) · Q9
Q.Which of the Class A, Class B and Class C power amplifiers have maximum efficiency? Mention the approximate power efficiency of these amplifiers.
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Class C is the most efficient; the approximate efficiencies are Class A about 25% (up to 50%), Class B about 78.5%, and Class C about 85-90%.
The collector (conversion) efficiency of a power amplifier is the ratio of AC signal power delivered to the load to the DC power drawn from the supply. A transistor that conducts for a smaller part of the cycle wastes less DC power as heat, so efficiency increases as the conduction angle decreases:
- Class A — conducts for the full 360°, drawing DC current even with no signal, so much power is dissipated in the transistor. Efficiency is low, about 25% for a series-fed (directly loaded) stage and up to about 50% for a transformer-coupled stage. …
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