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Circuit dreamer (talk | contribs) I have included two links pointing to stories about the philosophy of dynamic and negative resistance creating. |
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* Peter D. Hooper, G. McHale, and M. I. Newton, "Negative differential resistance in MIM devices from vacuum to atmospheric pressure", ''Proc. SPIE Int. Soc. Opt. Eng.'', 2780, 38 (1996) |
* Peter D. Hooper, G. McHale, and M. I. Newton, "Negative differential resistance in MIM devices from vacuum to atmospheric pressure", ''Proc. SPIE Int. Soc. Opt. Eng.'', 2780, 38 (1996) |
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*[http://www.circuit-fantasia.com/circuit_stories/inventing_circuits/dynamic_resistance/dynamic_resistance.htm How do We Create Dynamic Resistance?] |
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*[http://www.circuit-fantasia.com/circuit_stories/inventing_circuits/decreased_resistance/decreased_resistance.htm How do We Make Decreased, Zero and Negative Resistance?] |
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[[Category:Electronics terms]] |
[[Category:Electronics terms]] |
Revision as of 17:55, 21 June 2006
Negative resistance or Negative differential resistance is a property of electrical circuit elements composed of certain materials in which, over certain voltage ranges, current is a decreasing function of the voltage. This range of voltages is known as a negative resistance region.
Some writers prefer to reserve the term negative resistance for situations in which the negatively-sloping portion of the load line passes through the origin, so that a positive absolute value of voltage is associated with a negative absolute value of current. Such a circuit supplies energy and can be used as a form of amplifier. However, the use of the term negative resistance to encompass negative differential resistance is common.
Static and dynamic resistance
In electrical circuits, static resistance is the ratio of the voltage across a circuit element to the current through it. However, the ratio of the voltage to the current may vary with either voltage or current. The ratio of the change in voltage to the change in current is known as dynamic resistance.
It is 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 the slope of the resistance curve which is negative.
Examples of negative differential resistance
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, acts as an amplifier. See also Gunn diode. Another example is the medium within a gas discharge lamp which, as current increases, ionizes to a greater extent, thereby carrying more current. If such a lamp were allowed to draw power without limit, it would instantly burn itself out. Limiting the possible current is one of the roles of the ballast in a fluorescent lamp.
In compliance with the law of conservation of energy, a plot of the negative differential resistance region of a passive component cannot pass through the origin.
Negative impedance
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.
Mechanical examples
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 (or a key on a computer keyboard), 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 is that by driving them "firmly" it is possible to traverse the negative resistance region continuously, but bistable switching action occurs if the system is driven "loosely."
Negative resistance circuits
Many circuit topologies are capable of producing negative resistance. The simplest case requires an amplifier with voltage gain greater than one. If a resistor R is connected from input to output, the input current, , for a given input voltage is:
Where is the output voltage. This assumes an ideal amplifier with infinite input impedance and zero output impedance. If the voltage gain, , of the amplifier is defined as:
The input resistance, is:
The input resistance is negative for values of .
In the case of a non-ideal amplifier, negative resistance is still possible as long as the amplifier input impedance is sufficiently high. The net resistance is reduced to:
where is the amplifier input impedance and is the amplifier output impedance.
Interesting examples of the use of negative resistances in analogue computing can be found in the works of Gabriel Kron. While a scientist for General Electric, [1] Kron used negative resistors (circuits like those described above) for the US Navy's "Network Analyser" in the 1930s. [2] For example, this paper refers to the use of active negative resistances with network analysers, and also shows how these can be replaced by inductors and capacitors in AC simulations.
References
- Shoukai Wang and D.D.L. Chung, "Apparent negative electrical resistance in carbon fiber composites," Composites, Part B, Vol. 30, 1999, p. 579-590.
- Peter D. Hooper, G. McHale, and M. I. Newton, "Negative differential resistance in MIM devices from vacuum to atmospheric pressure", Proc. SPIE Int. Soc. Opt. Eng., 2780, 38 (1996)
- How do We Create Dynamic Resistance?
- How do We Make Decreased, Zero and Negative Resistance?