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Negative resistance

Negative resistance or negative differential resistance is a property of electrical circuit elements composed of certain materials in which, over certain voltage Voltage

Voltage is the difference of electrical potential [i] between two points of an electrical network [i] ... 

 ranges, current Current

Current may refer to: * Current affairs [i] ... 

 is a decreasing function of 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 must contain an energy source, and can be used as a form of amplifier.

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Negative resistance or negative differential resistance is a property of electrical circuit elements composed of certain materials in which, over certain voltage Voltage

Voltage is the difference of electrical potential [i] between two points of an electrical network [i] ... 

 ranges, current Current

Current may refer to:
  • Current affairs [i]

... 

 is a decreasing function of 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 must contain an energy source, and can be used as a form of amplifier. However, the use of the term negative resistance to encompass negative differential resistance is more common.

Absolute negative resistances without an external energy source cannot exist as they would violate the law of conservation of energy Conservation of energy

[i] in an [[isolated system]... 

.

Static and dynamic resistance

In electrical circuits, static resistance Electrical resistance

Electrical resistance is a measure of the degree to which an object opposes the passage of an electric current [i]... 

 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 Tunnel diode

A tunnel diode [i] or Esaki diode is a type of semiconductor [i] diode which is capable of very fa... 

. Such a device, when biased into its negative differential resistance region, acts as an amplifier. See also Gunn diode Gunn diode

A Gunn diode, also known as a transferred electron device is a form of diode [i] used in high-freq ... 

. 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 Fluorescent lamp

A fluorescent lamp is a gas-discharge lamp [i] that uses electricity [i] to excite mercury [i] vapor [i] ... 

.

In compliance with the law of conservation of energy Conservation of energy

[i] in an [[isolated system]... 

, 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 a key on a computer keyboard Computer keyboard

A computer keyboard is a peripheral [i] modeled after the typewriter keyboard [i]. ... 

, taking the key position and upward force to be analogous to voltage and current, respectively. As the 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 "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."

Absolute negative resistance circuits


Many circuit topologies are capable of producing absolute negative resistance . The simplest case requires an amplifier with voltage gain Gain

In electronics [i], gain is usually taken as the mean ratio [i] of the signal output [i] of ... 

 greater than one. If a resistor Resistor

|- align = "center"
|
|width = "25"|
... 

 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 Analog computer

An analog computer is a form of computer [i] that uses electrical [i] or mechanical [i]... 

 can be found in the works of Gabriel Kron. While a scientist for General Electric General Electric

The General Electric Company, or GE is a multinational [i] American [i] ... 

, Kron used negative resistors for the US Navy United States Navy

The United States Navy is the branch of the United States armed forces [i] responsible for conducting naval [i] ... 

's "Network Analyser" in the 1930s. For example, 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.

Deborah Chung's 'apparent negative resistance'





In July 1998, Deborah Chung and Shoukai Wang of the University of Buffalo University at Buffalo, The State University of New York

University at Buffalo, The State University of New York is located in Buffalo [i] and ... 

 presented the results of an experiment that showed an apparent absolute negative resistance in bare carbon fiber Carbon fiber

Carbon fiber can refer to carbon [i] filament thread, or to felt or woven cloth made from those carbon f ... 

s held together by pressure.

In the experiment, two bundles of carbon fibers are arranged in a cross Cross

A cross is a geometrical figure consisting of two lines or bars intersecting each other at a 90 angle, ... 

 shape, with the ends of each bundle shorted with copper foil and silver paint . A current is driven through one branch, and a voltage is measured across the other branch. In the paper, the voltage divided by current is referred to as an "apparent resistance".

The paper describes how the apparent contact resistance of the interface changes from positive to negative when the fibers are compressed. The current-voltage characteristic of the measured "negative resistance" is then a straight line of negative slope through the origin. The apparent negative resistance was also observed in metal wires , but was not observed for a single fiber crossing another single fiber. The paper claims that this phenomenon is useful because the forward flow and backflow of electrons in the same piece of material can be reproducibly controlled by external forces.

It was initially reported on July 9, 1998 by the University as a breakthrough in room temperature superconductor research, in the press release Superconduction At Room Temperature: Negative Electrical Resistance Seen In Carbon Composites, claiming that the discoveries "have enabled carbon-fiber materials to superconduct at room temperature", because of measurements of "zero apparent resistance" at certain pressures. This was quickly seized upon by the free energy community as a working example of a device that supplies energy with no apparent source, claiming it to be a true, absolute negative resistance, and was reported in the popular press as a breakthrough. The original press release was later pulled from UB's website, on July 16, 1998, and replaced with one which stated "her findings do not indicate that the combination is itself a superconductor."

Chung's paper itself says:

Applications


Electrical negative resistance is often used to design oscillators. Many topologies are possible, such as the Colpitts oscillator Colpitts oscillator

A Colpitts oscillator, named after its inventor Edwin H. Colpitts [i], is one of a number of designs for ... 

, Hartley oscillator Hartley oscillator

The Hartley oscillator is an L [i]C [i] electronic oscillator [i] that derives its fe ... 

, Wien bridge oscillator Wien bridge oscillator

A Wien bridge oscillator is a type of electronic oscillator [i] that generates sine wave [i]s without ha ... 

, and some types of relaxation oscillators. Negative resistance characteristics of Gunn diode Gunn diode

A Gunn diode, also known as a transferred electron device is a form of diode [i] used in high-freq ... 

s are often used in microwave Microwave

Microwaves are electromagnetic waves [i] with wavelength [i]s longer than thos ... 

 frequencies as well.

Negative resistance is also useful in certain switching and comparator Comparator

In electronics [i], a comparator is a device which compares two voltage [i]s or current [i]s an ... 

 circuits, such as the Schmitt trigger Schmitt trigger

In electronics [i], a Schmitt trigger is a comparator [i] circuit [i] that inco ... 

. Specialized diode Diode

In electronics [i], a diode is a component [i] that restricts the direction of mov... 

s, such as the step recovery diode also exhibit negative resistance. In this case, a very sharp pulse can be generated that produces a broad spectrum of harmonics. This can be used as a frequency multiplier at gigahertz frequencies. This is sometimes used in certain frequency synthesiser Frequency synthesiser

A frequency synthesizer is an electronic [i] system for generating any of a range of frequencies [i] ... 

 designs.

References


  • 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
  • Negative impedance converter Negative impedance converter

    The negative impedance converter is a configuration of an operational amplifier [i] which acts as a nega ... 

     - is dedicated to INIC.

External links


  • - reveals in three consecutive steps the basic idea behind VNIC.
  • shows how a transistor may act as a negative resistor






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