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Negative resistance



 
 
Negative resistance is a property of some electric circuits where an increase in the current
Current

Current may refer to:* Current affairs* Electric current* Current ** Ocean current* Current , geometrical current in differential topology...
 entering a port, results in a decreased voltage
Voltage

Electrical tension is the potential difference between two points of an electrical or electronic circuit, expressed in volts. It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor....
 across the same port. This is in contrast to a simple ohmic resistor
Resistor

|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable resistor...
, which exhibits an increase in voltage under the same conditions. Negative resistors are theoretical and do not exist as a discrete component. However some types of diode
Diode

In electronics, a diode is a two-terminal device .Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property....
s can be built that exhibit negative resistance in some part of their operating range.










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Negative resistance is a property of some electric circuits where an increase in the current
Current

Current may refer to:* Current affairs* Electric current* Current ** Ocean current* Current , geometrical current in differential topology...
 entering a port, results in a decreased voltage
Voltage

Electrical tension is the potential difference between two points of an electrical or electronic circuit, expressed in volts. It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor....
 across the same port. This is in contrast to a simple ohmic resistor
Resistor

|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable resistor...
, which exhibits an increase in voltage under the same conditions. Negative resistors are theoretical and do not exist as a discrete component. However some types of diode
Diode

In electronics, a diode is a two-terminal device .Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property....
s can be built that exhibit negative resistance in some part of their operating range.

Negative Differential Resistance


Properties

Fig. 2 shows a graph of a negative resistor, showing the negative slope. In contrast to this, a resistor will have a positive slope. Tunnel diode
Tunnel diode

A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanics effects....
s and Gunn diode
Gunn diode

A Gunn diode, also known as a transferred electron device , is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of Doping semiconductor material, whereas most diodes consist of both P and N-doped regions....
s exhibit a negative resistance region in their IV (current - voltage) curve. They have two terminals like a resistor; but are not linear devices. A curve tracer can be used to display the properties of these devices. Unijunction transistor
Unijunction transistor

A unijunction transistor is an Electronics semiconductor device that has only one junction. The UJT has three terminals: an emitter and two bases ....
s also have negative resistance properties when a circuit is built using other components. For negative impedance to be present there must be active components in the circuit providing a source of energy. This is because current through a negative resistance implies a source of energy just as current through positive resistance implies that energy is being dissipated. A resistor produces voltage that is proportional to the current through it according to Ohm's law
Ohm's law

Ohm's law applies to electrical circuits; it states that the electric current through a conductor between two points is directly Proportionality to the potential difference or voltage across the two points, and inversely proportional to the Electrical resistance between them....
. The IV curve of a theoretical negative resistor has a negative slope and passes through the origin of the coordinate system. This is to be compared with devices such as the tunnel diode where the negative slope portion of the curve does not pass through the origin. The curve can only enter the 2nd and 4th quadrants if energy is being supplied, current through a negative resistance implies a source of energy just as current through positive resistance implies that energy is being dissipated. Clearly, there is no source of energy in a two terminal diode.

History


It was noticed early on that arc discharge devices and some vacuum tube
Vacuum tube

In electronics, a vacuum tube, electron tube , thermionic valve, or just valve is a device used to amplifier, switch, otherwise modify, or create an Electricity signal by controlling the movement of electrons in a low-pressure space....
 devices such as the dynatron
Dynatron

For the brand, see Dynatron Radio LtdThe dedicated dynatron vacuum tube was invented by Albert Hull in 1918. It has three electrodes: a thermionic cathode, a perforated anode, and a supplementary anode or plate, and its characteristic curves have a region exhibiting negative resistance, which is the property desired....
 exhibit negative resistance effects. Practical and economic devices only became available with solid state technology
Solid state (electronics)

Solid-state electronic components, devices, and systems are based entirely on the semiconductor, such as transistors, microprocessor chips, and the bubble memory....
. The negative impedance converter is due to John G. Linvill
John G. Linvill

John G. Linvill is an United States professor of Electrical engineering at Stanford University, known for his pioneering work in higher education, integrated circuits and semiconductors, and for development of the Optacon reading machine for the blind....
 (1953) and the tunnel diode is due to Leo Esaki
Leo Esaki

Leona Esaki also known as Leo Esaki is a Japanese physicist who shared the Nobel Prize in Physics in 1973 with Ivar Giaever and Brian David Josephson for his discovery of the phenomenon of electron tunneling....
 (1958).

Implementations


Diodes
Tunnel diode
Tunnel diode

A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanics effects....
s are heavily doped semiconductor junctions that have an "N" shaped transfer curve. A vacuum tube
Vacuum tube

In electronics, a vacuum tube, electron tube , thermionic valve, or just valve is a device used to amplifier, switch, otherwise modify, or create an Electricity signal by controlling the movement of electrons in a low-pressure space....
 can also be made to exhibit negative resistance. . Other negative resistance diodes have been built that have an "S" shaped transfer curve. When biased so that the operating point is in the negative resistance region, these devices can be used as an Amplifier
Amplifier

Generally, an amplifier or simply amp, is any machine that changes, usually increases, the amplitude of a Signal . The "signal" is usually voltage or current....
. These devices can also be biased so that they will switch between two states very quickly, as the applied voltage changes.

Opamps

The negative resistance circuit shown in Figure 3 is an opamp implementation of the negative impedance converter (see below). The two resistors R1 and the op amp constitute a negative feedback non-inverting amplifier with gain A = 2. In the case , the input resistance (for an ideal opamp) is given by;

The input port of the circuit can be connected into another network as if it were a negative resistance component.

In the general case can be selected to produce negative capacitances or negative inductances.

Applications


Oscillators


All feedback oscillators imply the presence of negative resistance. There are many such topologies, including the Dynatron oscillator
Dynatron oscillator

A dynatron oscillator is an Electronics circuit that uses negative resistance to keep an LC circuit oscillating.In theory, if an ideal capacitor is connected in parallel with an ideal inductor, they form a resonant circuit that, once it begins oscillating, will oscillate forever as the energy is transferred back and forth between the capaci...
, Colpitts oscillator
Colpitts oscillator

A Colpitts electronic oscillator, named after its inventor Edwin H. Colpitts, is one of a number of designs for electronic oscillator circuits using the combination of an inductance with a capacitor for frequency determination, thus also called LC oscillator....
, Hartley oscillator
Hartley oscillator

The Hartley oscillator is an inductorcapacitor electronic oscillator that derives its feedback from a tapped coil in parallel with a capacitor ....
, Wien bridge oscillator
Wien bridge oscillator

A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies. The circuit is based on an electrical network originally developed by Max Wien in 1891....
, and some types of relaxation oscillator
Relaxation oscillator

A relaxation oscillator is an electronic oscillator in which a capacitor is charged gradually and then discharged rapidly. It is usually implemented with a resistor or current source, a capacitor, and a "threshold" device such as a neon lamp, diac, unijunction transistor, or Gunn diode....
s. If the feedback loop is broken and the input impedance examined it will be found to include negative resistance. Negative resistance characteristics of Gunn diode
Gunn diode

A Gunn diode, also known as a transferred electron device , is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of Doping semiconductor material, whereas most diodes consist of both P and N-doped regions....
s are often used in microwave
Microwave

Microwaves are electromagnetic radiation with wavelengths ranging from 1 mm to 1 m, or frequency between 0.3 hertz and 300 GHz....
 frequencies as well.

Amplifiers


A device exhibiting negative resistance can be used to amplify a signal and this is an especially useful technique at microwave
Microwave

Microwaves are electromagnetic radiation with wavelengths ranging from 1 mm to 1 m, or frequency between 0.3 hertz and 300 GHz....
 frequencies. Such devices do not present as pure negative resistance at these frequencies (in the case of the tunnel diode a large parallel capacitance is also present) and a matching filter is usually required. The reactive components of the device's equivalent circuit can be absorbed into the filter design so the circuit can be represented as a pure resistance followed by a bandpass filter. The output of this arrangement is fed into one port
Two-port network

A two-port network is an electrical circuit or device with two pairs of terminals . Two terminals constitute a port if they satisfy the essential requirement known as the port condition: the same current must enter and leave a port....
 of a three-port circulator
Circulator

A circulator is a passive electronic component with three or more ports in which the ports can be accessed in such a way that when a signal is fed into any port it is transferred to the next port only, the first port being counted as following the last in numeric order....
. The other two ports constitute the input and output of the amplifier with the direction of circulation as shown in the diagram. Treating R0 as being positive, the reflection coefficient
Reflection coefficient

The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuity is considered....
s at the two ends of the filter are given by;

and,

Since the filter has no resistive elements, there is no dissipation and the magnitudes of the two reflection coefficients must be equal,

The input power entering the cirulator is directed at the matching filter, is reflected at both the input and output of the filter and a portion finally arrives at the load. This portion is given by;

For a well matched filter, the reflection coefficients will be very small in the passband and very little power will reach the load. On the other hand if R0 is replaced by a negative resistance such that,

then,

and,

Now the reflection coefficients are very large and more power is reaching the load than was injected in the input port. The net result of terminating one port in a negative resistance is amplification between the remaining two ports.

Mixers and frequency converters


The highly non-linear characteristics of tunnel diodes makes them useful as frequency mixers. The conversion gain of a tunnel diode mixer can be as high as 20 dB if it is biased to operate in the negative resistance region.

Antenna design


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.

Bi-stable operation


Switching circuits. Negative resistance is also useful in certain switching and comparator
Comparator

In electronics, a comparator is a device which compares two voltages or Electric currents and switches its output to indicate which is larger....
 circuits, such as the op-amp non-inverting Schmitt trigger
Schmitt trigger

In electronics, a Schmitt trigger is a comparator electrical network that incorporates positive feedback.When the input is higher than a certain chosen threshold, the output is high; when the input is below another chosen threshold, the output is low; when the input is between the two, the output retains its value....
. Specialized diode
Diode

In electronics, a diode is a two-terminal device .Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property....
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
Frequency multiplier

A frequency multiplier is commonly used in a radio receiver or radio transmitter to multiply the base frequency of the oscillator by a predetermined number....
 at gigahertz frequencies. This is sometimes used in certain frequency synthesiser designs.

Circuits containing negative differential resistance can be described using the Liénard system of equation closely related to the Van der Pol oscillator
Van der Pol oscillator

In dynamics , the Van der Pol oscillator is a type of Conservative force oscillator with nonlinearity damping. It evolves in time according to the second order differential equation:...
.

Impedance cancellation

Negative impedances can be used to cancel the effects of positive impedances, for example, by eliminating the internal resistance of a voltage source or making the internal resistance of a current source infinite. This property is used in telephony line repeaters and in circuits such as the Howland current source, Deboo integrator and load cancellers.

Further reading

  • Negatron yields real natural frequency, Aleksandr Belousov, USA, EDN, 08/1993 (practical application of the equivalent Negatron circuit related to Instrumentation and Measurement knowledge domain)
  • E.W. Herold, "Negative Resistance and Devices for Obtaining It," Proceedings of the Institute of Radio Engineers, Volume 23, Number 10, October 1935.


See also


  • Negative impedance converter
    Negative impedance converter

    The negative impedance converter is a configuration of an operational amplifier which acts as a negative load. This is achieved by introducing a phase shift of 180? between the voltage and the current for any signal generator....


External links

  • - condensed version of article originally published in Amateur Radio, November 1995
  • - nice material from Answers.com
  • - a formal but well-written electronic book