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Confusion regarding negative resistance circuits

by resonant.freq
Tags: circuits, confusion, negative, resistance
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Nov2-11, 06:22 PM
P: 3
Hello Everyone!

I have recently been attempting to understand negative resistance circuits(on a practical level at least to start). Ive pulled up a few websites giving actual circuits to demonstrate the principle, but still cant quite understand it. Every circuit example so far shows either a pulsed or ac power source and is basicly an rlc circuit that oscillates at a frequency dependant on the values of the capacitor and inductor. such as > (example only). In my mind in a circuit like that its not all that clear exactly how and if negative resistance actually shows its self.

To cut a long story short what I would love to know is there an actual dc only(not oscillating) circuit that can be built to either... Increase the apparent voltage applied to an inductive component(coil) AND/OR decrease the apparent resistance of that coil(thus allowing more current to flow than you would expect given the power source, resistance, and ohms law).

Thanks for any help on this subject as I am still coming to terms understanding it.

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Nov4-11, 08:30 AM
P: 33
Negative resistance is just a property shown by some elements like UJT(in normal ac circuits),GUNN DIODE(in microwave circuits) etc.All of them exhibit that property in a range of ac voltage.Hence as far as I know,there is no such dc circuit.
Nov4-11, 09:13 AM
Sci Advisor
HW Helper
P: 7,284
You can't have a "negative resistor" that works with DC. That would mean that if you applied a voltage in one direction, the current would flow in the opposite direction.

OK, you could do that if your "resistor" included a battery or some other energy source, but that's not what we are talking about.

The negative resistance is the dynamic resistance, or the slope of the graph of voltage against current. In other words, if you increase the voltage slightly, the current still flows in the "forwards" direction, but it gets smaller not bigger.

This only happens for a particular range of voltages across the component. There are semiconductors that are specially designed to do that (e.g. tunnel diodes), but your web page is using the fact that some "ordinary" transistors show the same effect if you use them way outside of the conditions they were designed for.

Nov4-11, 04:39 PM
P: 3
Confusion regarding negative resistance circuits

Excellent ! 'that it applies only in ac circuits' makes perfect sence... I suppose its just so assumed that it was neglected in anything id read. ... I would still wonder why they chose the term negative ''resistance''...and not another term.... never the less thanks for clearing that up.

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