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Circuit elements composed of certain materials exhibit the property that, over certain voltage ranges current is a decreasing function of the voltage. This range of voltages is known as a negative resistance region.
It may be more correct to say that a circuit element has a negative differential resistance region than to say that it exhibits negative resistance because even in this region the static resistance of the circuit element is positive, while it is more precisely the slope of the resistance curve which is negative.
An example of an electronic component exhibiting the negative differential resistance region is the tunnel diode. Such a device, when biased into its negative differential resistance region, will act as an amplifier.
In conformance with the known law of conservation of energy, a plot of the negative differential resistance region of a component cannot normally pass through the origin.
Some work by Professor Deborah Chung at the University of Buffalo has discovered a composite configuration of carbon nanotubes which appears to exhibit anomalous results which resemble a static negative resistor. However, the physical interpretation of this observation is still controversial.
Another concept of negative resistance exists in the domain of radio frequency antenna design. This is also known as negative impedance. It is not uncommon for an antenna containing multiple driven elements to exhibit apparent negative impedance in one or more of the driven elements.
There are many mechanical systems that exhibit ranges of negative differential resistance. In fact, this is a common design element in systems that are designed to have "detents" or a "positive action" or a "click." A good example is an ordinary toggle switch, which, as the handle is moved from "off" to "on, initially presents a firm and increasing downward force. As the center point is passed, a zone is entered in which the downward force decreases, which feels like a "sudden" yielding. This is often referred to as a "collapse action" mechanism. A general characteristic of negative resistance systems it is possible to traverse the negative resistance region if driven "firmly," but act as bistable switching devices if driven "loosely."