Which class of amplifier provides the highest efficiency?

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Multiple Choice

Which class of amplifier provides the highest efficiency?

Efficiency in RF power amplifiers comes from limiting when the active device conducts. Class C keeps the transistor on for less than half of the input cycle, often only a small fraction. That means most of the cycle the device is off, so it dissipates little power as heat while the tuned output network shapes and delivers energy to the load in short, high-amplitude bursts. The result is very high average efficiency, especially when the circuit is tuned for a single frequency. However, because the amplification is highly nonlinear, the output is distorted and the technique is only suitable for narrow-band, fixed-frequency applications. In contrast, Class A conducts through the entire cycle, so the device always has current and voltage across it, which limits efficiency to roughly 50% in a practical single-ended design. Class B conducts for about half the cycle and can be more efficient than Class A, but not as high as Class C. Class AB sits between A and B in conduction and efficiency, but still generally does not reach the high efficiency possible with Class C in RF uses.

Efficiency in RF power amplifiers comes from limiting when the active device conducts. Class C keeps the transistor on for less than half of the input cycle, often only a small fraction. That means most of the cycle the device is off, so it dissipates little power as heat while the tuned output network shapes and delivers energy to the load in short, high-amplitude bursts. The result is very high average efficiency, especially when the circuit is tuned for a single frequency. However, because the amplification is highly nonlinear, the output is distorted and the technique is only suitable for narrow-band, fixed-frequency applications.

In contrast, Class A conducts through the entire cycle, so the device always has current and voltage across it, which limits efficiency to roughly 50% in a practical single-ended design. Class B conducts for about half the cycle and can be more efficient than Class A, but not as high as Class C. Class AB sits between A and B in conduction and efficiency, but still generally does not reach the high efficiency possible with Class C in RF uses.

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