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ffrige

negative impedances

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Hi. I ran into the problem of subtracting two impedances. I thought I could solve it simply by putting the first in series with the negation of the second (using a negative impedance converter), but something is not working. Is it really possible, for example, to put a resistance in series with its negative equivalent and then read zero across the two with a simple ohmmeter? Or is the circuit more complicated? Thanks!

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It is a little more complicated than that and the cure is dependent upon the characteristics of your circuit.
It is a well discussed topic and I am not an expert on it. Using the search words, "compensate negative impedance", I find 83 pages of articles on google.com . Many are pdf files with schematics. I would start there.

MP

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Yes to the last one. The circuit is more complicated. The term "negative resistance" is used to describe the behavior (oops, most of the folks here are in Europe, so that's behaviour) of the device as the voltage (or current) varies within the range so described. Instead of voltage and current increasing or decreasing together (linearly or otherwise), and increase in one is the cause or result of a decrease in the other.

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Guest Alun

A neggitive impedance reduces its resistance as the current increases instead of remaining constant, this means that if the current is increased the voltage across it won't increase proportionally as ohms law normally states and in some cases can cause destruction of the componants.

Examples of negitive impedances are:
Electrical arcs this includes discharge lamps (neon, flourescent, sodium ect.).
Transistors, eg. output stages of class AB amplifiers.
Negitive coefficient thermistors.
Diacs.
Zener diodes.
Diodes.
LEDs.

The first two have the desructive properites I described before the current is limited with a ballast inductor or resistor with discharge lamps and purly with resistors in amplifier output stages. Results from experiment I've conducted indicate you can (despite what some people say) power LEDs from a constant voltage source but it has to be exactly the right voltage for the forward current you require and this is totally differant for each  LED even if they have the same make/model.

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There are also negative resistance Op-Amp circuits.  Voltage increases inversely(?) I believe with current.  I believe it looks like a negative slope.  If you are really interested.  I will take the time to verify it for you, just ask.

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