TABLE OF CONTENTS Any RFPA will show some dynamic deviations from its static characteristics. Such deviations have become known, for better or for worse, as memory effects. These effects are very troublesome for the process of predistortion, as will be discussed in Chapter 14. Memory effects are an additional source of nonlinear behavior that is typically not accounted for in PA models, and represent a source of error in attempts to simulate the distortion characteristics of any PA. Memory effects can be traced to three main causes:
Dynamic thermal effects;
Unintentional modulation on supply rails;
Semiconductor trapping effects.
The second of these is probably the most common cause of asymmetrical IM sidebands, and can be potentially cured by more attentive design of the biasing networks to the PA stage. Inasmuch as a whole chapter is devoted to this subject (Chapter 11), this effect will not be discussed here. Suffice it to say that a circuit simulator should be able to predict such memory effects, provided that the designer can be bothered to include sufficient details of the bias circuitry, as well as the RF circuit itself, in the simulation file. [3.]
Trapping effects is another unsatisfactory generic term, which at worst can be used to categorize just about any anomalous effect which is observed in a semiconductor. The external manifestation is usually most apparent when observing the current in a device whose input voltage is switched from below cutoff to a value which places it into...
