What is Efficiency with Regards to an Amplifier?

In amplifiers, efficiency is the measure of they amount that a power source is usefully applied to an amplifier's output. The classes of amplifiers can be seen below:

• Class A amplifiers are said to be very inefficient, providing in the range of 10% to 20% efficiency with a maximum efficiency of just 25% in coupling of the output directly. Their efficiency can be raised to a maximum of 50% by using inductive coupling on the output.

• Class B amplifiers, on the other hand, have a very high efficiency. Class B are impractical for such things as audio work due to high levels of distortion.

• Class AB modern amplifiers are typically between 35% and 55% efficiency with a potentially theoretical maximum of 78+%. With regards to practical design a trade-off between class A and class B results in a class AB amplifier design.

• Class C are typically said to be highly efficient amplifiers.

• Class D commercially available switching amplifiers have been said to have efficiencies as large as 90%.

• Class E are again typically said to be highly efficient amplifiers.

• Class F are also typically said to be highly efficient amplifiers.

• Class G amplifiers are described as more efficient than class AB amplifiers.

• Class H amplifiers are based on class G, but have an infinitely variable supply rail.

The more efficient the amplifier the cooler the temperature it operates at this can lead to not needing any cooling fans. This is because it is the loss in efficiency of the amplifier which produces high temperatures as a by-product of the energy that is lost throughout the conversion of power. With the more efficient amplifiers there is much less energy loss meaning there is less heat generated.

In broadcast transmitters and cellular base stations radio frequency (RF) Power Amplifiers are used. They are constructed using specialist design techniques that are used to greatly improve efficiency. Envelope Tracking designs on power amplifiers (PAs) are used in order to achieve efficiencies reaching 60%. This is done by modulating the amplifier's supply voltage in-line with the envelope of the signal.