Literature Survey : Carvalho and Pedro, "Large and Small-signal IMD Behavior of Power Amplifiers"

In this article, Carvalho and Pedro examine the "so-called large signal IMD sweet spots," that arise in amplifiers that exhibit gain expansion in large signal operation.


From "IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999," page 2364

By combining the Volterra series small-signal model of the FET amplifer with a "two sinusoidal input describing function," based on that proposed by Gelb (1968), a model was developed that accurately predicts the nonlinear behavior of the FET amplifer for a wide dynamic range of input signals.

IMD sweets spots are shown to be due to phase cancellation of IMD components arising from the small-signal part of the LS model by those generated by the nonlinear describing function.

At the heart of the analysis is that the small signal and large signal nonlinearities are fundamentally different mechanisms. Therefore, they presented a model whereby the small-signal term becomes negligible as the amplitude of the input increases and the large-signal describing term dominates.