Negative differential resistance: Difference between revisions
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#redirect [[Negative_resistance]] |
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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 [[dynamic resistance]] [http://www.circuit-fantasia.com/circuit_stories/inventing_circuits/dynamic_resistance/dynamic_resistance.htm]. |
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[[Image:Circuit2 1000.jpg|thumb|280px|left|Fig. 3a: Obtaining a dynamic resistance by varying the ohmic resistance]][[Image:Circuit3 1000.jpg|thumb|280px|right|Fig. 3b: Obtaining a dynamic resistance by varying the voltage]]In electrical circuits, static [[electrical resistance|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. |
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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. |
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There are two techniques for obtaining dynamic (negative) resistance - ''by varying the resistance'' [http://www.circuit-fantasia.com/circuit_stories/inventing_circuits/dynamic_resistance/dynamic_resistance.htm#step2] and ''by varying the voltage'' [http://www.circuit-fantasia.com/circuit_stories/inventing_circuits/dynamic_resistance/dynamic_resistance.htm#step3]. The first produces ''negative differential resistance'', while the second gives ''absolute negative resistance''. |
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=== Resistance variation === |
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[[Image:S iv curves5 1000.jpg|thumb|400px|<center>Fig. 3a: An S-shaped IV curve of a negative resistor based on a constant-voltage dynamic resistor</center>]] |
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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 [[conductive polymers]] such as [[Melanin]] can also show marked negative differential resistance. |
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==== S-shaped constant-voltage dynamic resistance ==== |
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By dynamically decreasing the resistance of an ordinary ohmic resistor [http://www.circuit-fantasia.com/circuit_stories/inventing_circuits/r_decreased_resistance/r_decreased_resistance.htm], 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 [[zener diode]]), a conclusion may be derived: |
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''An S-shaped negative differential resistor is actually an "over-acting" voltage-stable dynamic resistor.'' |
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An example of an electronic component exhibiting a negative differential resistance region 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 (electrical)|ballast]] in a [[fluorescent lamp]]. |
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==== N-shaped constant-current dynamic resistance ==== |
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[[Image:N iv curves5 1000.jpg|thumb|400px|<center>Fig. 3b: An N-shaped IV curve of a negative resistor based on a constant-current dynamic resistor</center>]] |
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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 [[wikt:Barreter|barreter]] or the collector-emitter part of a [[transistor]]), another conclusion may be derived: |
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''An N-shaped negative differential resistor is actually an "over-acting" current-stable dynamic resistor.'' |
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An example of an electronic component exhibiting an N-shaped 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]]. |
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'''''Negative differential resistor is an "over-acting" dynamic resistor (a dynamic resistor with extremely varying resistance).''''' |
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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. |
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=== Absolute negative resistance === |
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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: |
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<center>''A negative differential resistor + constant voltage source = absolute negative resistor''</center> |
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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). |
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Revision as of 16:39, 24 January 2009
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 dynamic resistance [1].
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.
There are two techniques for obtaining dynamic (negative) resistance - by varying the resistance [2] and by varying the voltage [3]. The first produces negative differential resistance, while the second gives absolute negative resistance.
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 conductive polymers such as Melanin can also show marked negative differential resistance.
S-shaped constant-voltage dynamic resistance
By dynamically decreasing the resistance of an ordinary ohmic resistor [4], 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 zener diode), 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 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.
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 barreter or the collector-emitter part of a transistor), 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 tunnel diode. Such a device, when biased into its negative differential resistance region, acts as an amplifier. See also Gunn diode.
Negative differential resistor is an "over-acting" dynamic resistor (a dynamic resistor with extremely varying resistance).
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.
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:
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).