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.
Encyclopedia
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.
Absolute negative resistances without an external energy source cannot exist as they would violate the law of
conservation of energy.
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 a key on a
computer keyboard, 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 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, Kron used negative resistors for the
US Navy'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 presented the results of an experiment that showed an
apparent absolute negative resistance in bare
carbon fibers held together by pressure.
In the experiment, two bundles of carbon fibers are arranged in a
cross 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,
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
- Negative impedance converter - 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
