Overview

Load-pull is something of a euphemism when applied to PA design. The term originated in the world of oscillators, where a key specification is the amount of frequency change, or pulling the oscillator displays as the output load is tuned from its nominal value. In PA design, the term is used to describe an empirical process by which the matching requirements of a PA device are determined, using some form of variable impedance tuning device.

It might be thought that load-pull techniques, along with the slide rule and the slotted line, would be teetering on the brink of extinction due to the availability and almost universal intrusion of the CAD approach into RF circuit design. Judging, however, by the continuing evolution and availability of more complex and versatile commercial load-pull hardware, it would seem that this is a preserve which has successfully survived the CAD revolution. This chapter is therefore intended as an update on current load-pull techniques and capabilities, rather than an extended justification for its continuing use.

We pick up the story, in some respects, from Chapter 2, which describes the basic loadline approach to Class A RFPA design. As long ago as 1983, this author (Chapter 2, [1]) showed that the oval-shaped load-pull power contours which were frequently measured using fundamental load-pull equipment could be predicted using loadline principles. The use of a simple fundamental tuner was appropriate for Class A design, where up to the onset of gain compression the harmonic impedances can be expected...