The S-parameters of a power amplifier under excited conditions represent critical information having a bearing on efficiency, output power, stability, and often the economic viability of the design. The output reflection coefficient, in particular, is of considerable interest. One way of indirectly getting this information is through load-pull measurements in which output power is measured as a function of load impedance. While this load-pull measurement does access the practical impact of output reflection coefficient (among other things), it is expensive and time-consuming to perform. In some cases, primarily in power devices operating well away from compression or in somewhat matched amplifiers, a more direct, quasi-linear measurement of S-parameters while the amplifier is operating under normal drive may be useful. Such measurements, termed hot S-parameters, can provide some information on the degree of mismatch in-system, potential operational stability, and the effects of this amplifier's performance on subsequent stages or antennae. The purpose of this note is to explore what hot measurements can do and what are the practical measurement constraints.
Load-Pull: A Different But Related Measurement
A typical load-pull system consists of the device or amplifier structure under test, electronic or mechanical tuners (whose impedances have been measured previously in many different states), one or more power meters, and associated hardware. As is always the case, output power will be a function of load match. Unlike the small signal case, the relationship will be non-trivial in the large signal limit due to load line limitations along with other issues (even while still quasi-linear). Once the device is operating non-linearly, the relationships become even more complicated.
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