Active impedance matching ??

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)

To what end? What's being matched?

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..).

Tim

Sure. One common example is diode-terminated ECL.

Cheers

Phil Hobbs

an emitter follower m

The OP was rather vague about what he wanted exactly. 'Matching' means different things in different contexts.

I've used feedback around amplifiers to obtain a quieter

Jeroen Belleman

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:

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.

Cheers, Nikolai