TABLE OF CONTENTS Throughout the last few chapters, it has been taken for granted that any nonlinearity in the transfer characteristic of a power amplifier constitutes a problem of some kind, but so far no attempts have been made to quantify the problem. It has also been noted that the impact of nonlinearity is strongly dependent on the actual application, most specifically the modulation system in use, coupled with the relevant regulatory specifications. This chapter attempts to quantify the effects of a nonlinear power transfer characteristic, specifically the spectral distortion that is an inevitable consequence of PA nonlinearity.
In previous chapters, it was shown that a power transfer characteristic could often be derived for a particular amplifier type or mode, given reasonable assumptions about the RF load and harmonic terminations. Such transfer curves were derived using simplified models, and could also be derived using CAD simulation. Clearly, in cases where the modulation system simply varies the amplitude of a CW carrier ( Amplitude Modulation, or AM ), it would seem to be a logical extension, or application, of these power transfer characteristics to map the input amplitude variations onto the output plane. The resulting distorted waveform can then be analyzed for its frequency components, which constitute the spectral distortion caused by the amplitude nonlinearity of the amplifier. This is the principal strategy used in this chapter, and it will be shown that much useful information about the spectral distortion characteristics can be derived in this manner.
Surprisingly, for many years this has not...
