What is Power Backoff in an Amplifier?

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- everything RF

Dec 12, 2019

When a power amplifier (PA) is fed with a multi-tone signal at its input, it amplifies the signal and in the process generates unwanted intermodulation (IM) products. 

When the amplifier is operating in the linear region, the intermodulation products are generated at various frequency multiples of the input signals which do not interfere with the main signals. 

However, as the Power Amplifier approaches the saturation point and enters the non-linear region of operation, the interference increases – since the amplifier is operating in the non-linear region, the interference products can start to interfere with the main signals and thus lowers the quality/linearity of the output signal.

In applications where we want the amplifier to operate only in the linear region, we must operate the amplifier at a power level that is lower than the saturated level. We do this to ensure that even if the input power increases slightly the amplifier remains in the linear region and does not move over to the saturated region of operation.

Power Backoff in an amplifier is a power level below the saturation point at which the amplifier will continue to operate in the linear region even if there is a slight increase in the input power level. Usually, power amplifiers operate close to the saturation point as that is where efficiency is maximum. However, at this point, a small increase in input power can push the amplifier from the linear mode to the saturated mode. Thus to ensure it operates in the linear region we lower the power level from point of maximum efficiency to ensure that it operates in the linear region if there is a slight increase in power. The amount by which we lower the power level is called Power Backoff.

Another technique to ensure that a Power Amplifier operates linearly is called Digital Pre-Distortion. Click here to learn more about Digital Pre-Distortion.