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| {{article issues|original research=February 2009|primarysources=February 2009|tone=February 2009}} | |||
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| ''Negative differential resistance'' (NDR) or ''differential negative resistance'' (DNR) is a property of ] elements composed of certain materials in which, over certain ] ranges, ] is a decreasing function of voltage. This range of voltages is known as a negative resistance region. | |||
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| The IV curve of an ohmic (static) resistor is sloped from left to right. The only way to slope it from right to left in a limited region is to "dynamize" sufficiently the ohmic resistor in this region. In this way, the problem of obtaining a negative resistance is reduced to the problem of creating a ] . | |||
| ]]In electrical circuits, static ] is the ratio of the voltage across a ] 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. | |||
| There are two techniques for obtaining dynamic (negative) resistance - ''by varying the resistance'' and ''by varying the voltage'' . The first produces ''negative differential resistance'', while the second gives ]. | |||
| === Resistance variation === | |||
| ] | |||
| This is historically the first and more natural way of creating negative resistance. In electronics, there are a few two-terminal electronic components having negative differential resistance. Some of them have an S-shaped IV curve while other components have an N-shaped IV curve. Electronically-active ] such as ] can also show marked negative differential resistance. | |||
| ==== S-shaped constant-voltage dynamic resistance ==== | |||
| By dynamically decreasing the resistance of an ordinary ohmic resistor , three degrees of dynamic resistance may be obtained (Fig. 3a): ''decreased'' (section 1-2), ''zeroed'' (section 2-3) and ''S-negative differential resistance'' (section 3-4). As the section 2-3 represents a voltage-stable dynamic resistor (for example, a ]), a conclusion may be derived: | |||
| ''An S-shaped negative differential resistor is actually an "over-acting" voltage-stable dynamic resistor.'' | |||
| An example of an electronic component exhibiting a negative differential resistance region is the medium within a ] 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 ] in a ]. | |||
| ==== N-shaped constant-current dynamic resistance ==== | |||
| ] | |||
| Dually, by dynamically increasing the resistance of an ordinary ohmic resistor (fig. 3b), three other degrees of dynamic resistance may be obtained: ''increased'' (section 1-2), ''infinite'' (section 2-3) and ''N-negative differential resistance'' (section 3-4). As the section 2-3 represents a current-stable dynamic resistor (for example, a ] or the collector-emitter part of a ]), another conclusion may be derived: | |||
| ''An N-shaped negative differential resistor is actually an "over-acting" current-stable dynamic resistor.'' | |||
| An example of an electronic component exhibiting an N-shaped negative differential resistance region is the ]. Such a device, when biased into its negative differential resistance region, acts as an amplifier. See also ]. | |||
| '''''Negative differential resistor is an "over-acting" dynamic resistor (a dynamic resistor with extremely varying resistance).''''' | |||
| In compliance with the ], a plot of the negative differential resistance region of a passive component cannot pass through the origin. | |||
| === Absolute negative resistance === | |||
| The negative differential resistor is not a true negative resistor as it does not contain a source; it is just a part of a true negative resistor. In order to get an absolute negative resistor, an additional constant voltage source has to be connected in series: | |||
| <center>''A negative differential resistor + constant voltage source = absolute negative resistor''</center> | |||
| Actually, the combination of the two components constitutes the varying voltage source needed. By applying this approach, a tunnel diode amplifier is built (see applications). | |||
| == Applications == | |||
| ==== Amplification ==== | |||
| '''Basic idea.''' An ] is nothing else than controlled ]. According to this paradoxical idea, an ] consists of two components: a controlled regulating element and a power source. In electronics, the classic 3-terminal regulating element (tube, ] etc.) acts as an electrically controlled resistor with separate input and output ports. The voltage (current) applied across (through) the input port controls the resistance between the two terminals of the output port.] The odd 2-terminal regulating element (for example, a ]) acts as an electrically controlled resistor, which input and output are the same. The voltage (current) applied across (through) the two terminals of the element controls the resistance between the same two terminals. In order to do that, the 2-terminal regulating element is actually an "over-acting" dynamic resistor (that is, a negative resistor). | |||
| '''Tunnel diode amplifier.''' In order to build such a ], four components have to be connected in series (Fig. 10): a constant-voltage power supply V, an input voltage source V<sub>IN</sub>, a "positive" resistor R and a negative differential resistor NDR (for example, a tunnel diode). Actually, the two resistors constitute a "dynamic" ] supplied by a varying composed voltage source (V + V<sub>IN</sub>). When the input voltage varies slightly, the negative differential resistor reacts vigorously to this "intervention"; it changes considerably its resistance according to the input voltage, which makes the voltage divider change noticeably its ratio. As a result, the voltage drops across the "positive" and negative resistors vary considerably; therefore, some of them may be used as an output voltage. In this arrangement, the differential negative resistor is not an amplifier; it is just a part of an amplifier (the differential negative resistor is just a 2-terminal regulating element). The combination of the differential negative resistor acting as a regulating element and the power supply constitutes true amplifier: | |||
| <center>''Negative differential resistor + power supply = negative resistance amplifier''</center> | |||
| ==== Non-electrical 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 "]" or a "positive action" or a "click." A popular example is the well-known pen clicker. Good examples are also the keys on a ] and on a ], taking the key position and upward force to be analogous to voltage and current, respectively. As a key is pressed downward, it initially presents a firm and increasing upward force. Beyond a critical point, a zone is entered in which the upward force decreases, which feels like a "sudden" yielding. This is often referred to as a "]" mechanism. There are several ] that give such collapse action, such as ] switches. A general characteristic of negative resistance systems is that by driving them "firmly" it is possible to traverse the negative resistance region continuously (linear applications), but ] switching action occurs if the system is driven "loosely" (bi-stable applications). | |||
| ==Components with negative differential resistance== | |||
| *] (]) | |||
| * ]* | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] with significant ] operated in the ] mode | |||
| ==Circuits with negative differential resistance== | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ]. | |||
| == External links == | |||
| * 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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