Could some electronics guru shed some light on this ? Are there any active impedance matching devices , i.e., impedance matching circuit made entirely of active devices, as opposed to those made of reactive components only. Google search throws out a number of research papaers, but as is often the case in these matters, the authors are very vague.
I think an active power factor-correction circuit would fit that description in a broad sense, you're actively altering the load impedance to achieve a desired effect, though the goal is maximum efficiency not maximum power transfer (you're certainly not trying to match the line's very low source impedance 1:1)
Impedance matching is well defined when the source and load are linear and Thevenin equivalent. In that case, maximum power transfer occurs when the impedances are conjugate.
If there's an external source of power, power does not need to be conserved, and there can't be such a straightforward condition.
You could even make one of the impedances negative and achieve arbitrary signal gain!
When someone talks about active devices as "impedance matching", it's informally, as a block which has a high or low input impedance, a useful output impedance, and not too much voltage or current gain in and of itself. Example: voltage follower, common base amplifier, etc. It's still gain -- an impedance transformation without V and I changing by the same ratios, is an overall power gain (or loss..).
Indeed, there are many ways to look at it. Negative impedance converters ar e well known. Whether you use them for matching or for something else, they can only be active:
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They exist in many other, less obvious forms. Some extremely wideband ampli fiers use the negative Miller capacitance. If that's not matching per se, i t's at least a way to compensate some impedance component that can't be bro adband-matched.
Generally speaking, in two-port networks the active part gives you directio nality. Or it can be seen as an extra degree of freedom for realizing an im pedance function. Of course, it doesn't beat other constraints like the Kra mers?Kronig relations.
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