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New for March 2014! Some things are best served cold. Even when your "thing" is only a 3.2 beer that your hotel charged six bucks for plus they forced you to buy a cookie that you threw away. What was Brian Wilson smoking when he penned Salt Lake City?
On the other hand, this page provides less than 3.2% of the desired material on cold S-parameters...
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By cold, we refer to active devices that are not powered up. This can be an individual device, or an amplifier, or module, or anything active that is operated passively. Below we provide two situations reasons when you might want cold S-parameters.
Device modeling
An equivalent circuit model of a FET has many elements. Some of them, such as gate resistance are difficult to resolve when the transistor turned on. It turns out that some elements can be determined accurately from cold measurements. Perhaps one of the advantages of a cold measurement is that the device becomes reciprocal.
More to come!
Balanced amplifier
One very good reason that engineers employ balanced amplifiers is that the reflection coefficients S11 and S22 ideally cancel out and the overall amplifier provides a good impedance match. Sometimes an amplifier is turned off when its channel is not needed in order to save power in a system. At this point you need cold S-parameters, and you join the rest of us in frustration when you find out that MMIC suppliers usually don't provide these data.
Here's a new page on balanced amplifier VSWR which analyzes just how well you can match a 10:1 VSWR using quadrature couplers. Well worth a read!