Negative resistance

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 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.

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."

Absolute negative resistance circuits

Many circuit topologies are capable of producing absolute negative resistance (which requires that an energy source be included). 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, $i_i$ , for a given input voltage $v_i$ is:

$i_i = \frac{v_i - v_o}{R}$

Where $v_o$ is the output voltage. This assumes an ideal amplifier with infinite input impedance and zero output impedance. If the voltage gain, $A_v$ , of the amplifier is defined as:

$A_v = \frac{v_o}{v_i}$

The input resistance, $R_i$ is:

$R_i = \frac{v_i}{i_i} = \frac{R}{1-A_v}$

The input resistance is negative for values of $A_v > 1$ .

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:

$R_i = \frac{1}{\frac{1}{Z_{i}} + \frac{1-A_v}{R + Z_{o}}}$

where $Z_i$ is the amplifier input impedance and $Z_o$ 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, Kron used negative resistors (circuits like those described above) for the US Navy's "Network Analyser" in the 1930s. [http://www.cheniere.org/misc/kron.htm 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.

Deborah Chung's negative resistance

In July 1998, Deborah Chung and Shoukai Wang of the University of Buffalo presented the results of an experiment that showed an apparent absolute negative resistance in bare carbon fibers held together by pressure.Apparent negative electrical resistance in carbon fiber composites — Shoukai Wang, D.D.L. Chung — Composite Materials Research Laboratory, State University of New York at Buffalo — Received 8 April 1998; accepted 31 March 1999

In the experiment, carbon fibers are arranged in a cross shape, with the ends of each fiber shorted with copper foil and silver paint to their neighbors (at A, B, C, and D in the image). 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 interpreted as a resistance, though a real resistance would require both the current and voltage to be measured at the same points. 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 (silver-coated copper), 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 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", because of measurements of zero voltage at certain pressures.Copy of the original press release available from Zero Point Technologies article The Zero Point Interaction 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 resistanceDr. Deborah Chung's Negative Resistor — The Tom Bearden website — "There is no question at all about it being a true negative resistor."On Extracting Electromagnetic Energy From The Vacuum — Thomas E. BeardenThe Chung's Negative Resistance experiment — JLN Labs, and was reported in the popular press as a breakthrough. 'Negative resistance' surprises material scientists — PhysicsWeb, 10 July 1998 The original press release was later pulled from UB's website, and replaced with one which stated "her findings do not indicate that the combination is itself a superconductor."Editor's note from UB Professor Looking To Identify Mechanism Behind Observation Of Negative Electrical ResistanceResearch Focusing On Mechanism Behind Observation of Negative Electrical Resistance — University of Buffalo news — latest revision of the press release

Chung's paper itself says:

It does not claim that the device is a source of energy.

Applications

Electrical negative resistance is often used to design oscillators. Many topologies are possible, such as the Colpitts oscillator, Hartley oscillator, Wien bridge oscillator, and some types of relaxation oscillators. Negative resistance characteristics of Gunn diodes are often used in microwave frequencies as well.

Negative resistance is also useful in certain switching and comparator circuits, such as the Schmitt trigger. Specialized diodes, 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 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 (1996)
How do We Create Dynamic Resistance?
How do We Make Decreased, Zero and Negative Resistance?

Electronics terms

Negativ differentiel modstand | Elektrischer_Widerstand#Differenzieller_Widerstand | Résistance négative | negatieve weerstand

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