Abstract
This paper tries to get a response to the following question: When can a narrowband power amplifier (PA) be
considered to be memoryless and when can it not be considered memoryless? To this end, a thorough and consistent analysis of
the notions and definitions related with the above topic is carried out. In the considerations presented, two models of the
narrowband PA are exploited interchangeably: the black box model widely used in the literature and a model developed here,
which is based on the Volterra series. These two models complement each other. In this paper, the conditions for a linear
or nonlinear narrowband PA to be memoryless or approximately memoryless or possessing memory are derived and illustrated.
They are formulated in terms of the signal delay as well as in terms of the amplitude-to-phase (AM/PM) conversion of the
amplifier. Furthermore, the two possible interpretations of the amplitude-to-amplitude (AM/AM) and AM/PM conversions are
given a mathematical framework. That is these conversions are presented through some operations. One set of these operations
allows to treat the AM/AM and AM/PM conversions as distortions of the modulating signals. Or equivalently as
distortions of a given signal constellation when it passes through the PA. Finally, it is proved that the Saleh’s and Ghorbani’s
models of the AM/AM and AM/PM conversions occurring in the PAs, which were published in the literature, are not memoryless
ones.
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