Peak to average power ratio and its effect on the efficiency of the rf power amplifier

This is a brief post. Peak to average power ratio of a signal is defined by the ratio of the square of the absolute value of the signal to the average of the square of the absolute value of the signal. This ratio has a significant impact on the efficiency of the output RF power amplifier.

In order to understand this effect one has to consider the characteristic curve of the RF power amplifier output power with respect to the input power. Elsewhere in this blog is a clear definition of this curve. The most important point on the curve (for this discussion) is the 1 dB compression point. Typically this is the optimum point to operate the power amplifier. This point has its associated input driver power.

Now if a signal with really low PAPR is used to drive this amplifier it can be driven at its 1 dB compression point fairly robustly with very little distortion. However if the the input signal has a large PAPR then when the signal continues to increase in amplitude because of its high peak power, the amplifier will distort. In order to eliminate this, the user must back off the operating point of the amplifier to some lower input power to accommodate the large PAPR signal. This means that the amplifier is no longer operating at its optimum high efficiency. ( For a discussion of RF Amplifier efficiencies please search the blog with that as a key word and examine the paper on RF Power amplifier design or visit the Signal Processing Group Inc website). It is this effect that causes high PAPR signals to be less efficient when using a RF power amplifier in a transmitter.

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