Home      Discussion      Topics      Dictionary      Almanac
Signup       Login
Positive feedback

Positive feedback

Discussion
Ask a question about 'Positive feedback'
Start a new discussion about 'Positive feedback'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia
Positive feedback is a process in which the effects of a small disturbance on (a perturbation of) a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is said to exhibit negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

. These concepts were first recognized as broadly applicable by Norbert Wiener
Norbert Wiener
Norbert Wiener was an American mathematician.A famous child prodigy, Wiener later became an early researcher in stochastic and noise processes, contributing work relevant to electronic engineering, electronic communication, and control systems.Wiener is regarded as the originator of cybernetics, a...

 in his 1948 work on cybernetics
Cybernetics
Cybernetics is the interdisciplinary study of the structure of regulatory systems. Cybernetics is closely related to information theory, control theory and systems theory, at least in its first-order form...

.

Positive feedback is generally unstable and leads to divergence from equilibrium, often with exponential growth. When there is more positive feedback than there are stabilizing forces, systems will typically accelerate towards a non-linear region, which may stabilise the system or (depending on the system) even damage or destroy it. Positive feedback may end with the system 'latched' into a new stable state. Positive feedback is controlled by signals in the system being filtered
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...

, damped
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...

 or limited
Maxima and minima
In mathematics, the maximum and minimum of a function, known collectively as extrema , are the largest and smallest value that the function takes at a point either within a given neighborhood or on the function domain in its entirety .More generally, the...

, or it can cancelled or reduced by adding negative feedback.

Positive feedback is used in digital electronics to force voltages away from intermediate voltages into '0' and '1' states. On the other hand, thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

 is a positive feedback that can destroy semiconductor junctions. Positive feedback in chemical reactions can increase the rate of reactions, and in some cases can lead to explosions. Positive feedback in mechanical design causes tipping-point
Tipping point (physics)
A tipping point is the point at which an object is displaced from a state of stable equilibrium into a new equilibrium state qualitatively dissimilar from the first.- In electric power :...

, or 'over-centre', mechanisms to snap into position, for example in switches and locking pliers
Locking pliers
Locking pliers, Mole grips or Vise-Grips are pliers that can be locked into position, using an over-center action. One side of the handle includes a bolt that is used to adjust the spacing of the jaws, the other side of the handle often includes a lever to push the two sides of the handles apart...

. Out of control, it can cause bridges to collapse
Tacoma Narrows Bridge (1940)
The 1940 Tacoma Narrows Bridge was the first incarnation of the Tacoma Narrows Bridge, a suspension bridge in the U.S. state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It opened to traffic on July 1, 1940, and dramatically collapsed...

. Positive feedback in economic systems can cause boom-then-bust cycles.

In the term "positive feedback", "positive" refers to the mathematical sign of the direction of change rather than the desirability of the outcome. To avoid this confusion, it is sometimes better to use other terms such as self-reinforcing feedback. In social and financial systems, positive feedback effects may also be referred to as 'virtuous' or 'vicious' circles
Virtuous circle and vicious circle
A virtuous circle and a vicious circle are economic terms. They refer to a complex of events that reinforces itself through a feedback loop. A virtuous circle has favorable results, while a vicious circle has detrimental results...

.

Overview


The key feature of positive feedback is that small disturbances are amplified. When positive feedback is present, there is some causal loop where a small change creates an effect that causes an even bigger change, like a ball rolling down an increasingly steep hill.

When a change in a variable occurs in a system which exhibits positive feedback, the system responds by changing that variable even more in the same direction.

The end result of positive feedback is to amplify
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

 so that small perturbations may result in big changes. For example, if a chemical reaction causes the release of heat
Exothermic reaction
An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...

, and the reaction itself happens faster
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...

 at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

 can occur and very quickly lead to a chemical explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

. Equally, if a PA system
Public address
A public address system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.Simple PA systems are often used in...

 microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

 picks up sounds from its own loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

s and these sounds are re-amplified enough, the effect can be loud squealing or howling noises from the loudspeakers.

Formally, a system in equilibrium in which there is positive feedback to any change from its current state is said to be in an unstable equilibrium. The magnitude of the forces which act to move such a system away from its set point
Set point
Set point or setpoint might mean one of:* Set point , a tennis term meaning one player is one point away from winning a set* Set point , a term referring to any one of a number of quantities Set point or setpoint might mean one of:* Set point (tennis), a tennis term meaning one player is one point...

 are an increasing function of the "distance" from the set point.

In the real world, positive feedback loops are always controlled eventually by negative feedback or limiting effects of some sort. Acoustic feedback causes a PA system to reach its maximum volume and so it cannot amplify any further; a flask may crack and the chemical reactants spray into the air or spill onto the floor, where they spread out and cool. Negative feedback effects within the same system can also modulate the effect of positive feedback so that it may add to the responsiveness of the system but does not necessarily lead to a runaway process. Chaotic
Chaos
- Mythology, philosophy, and religion :* Chaos , the concept in classical creation myths** a chasm or abyss, especially in biblical usage, see Abyss** prima materia, the primordial state from which the cosmos originated...

 systems exhibit positive feedback at the small scale, but non linearities prevent the systems from expanding infinitely, but do so without reaching equilibrium.

In Biology


]
In biology, a number of examples of positive feedback systems may be found in physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

, some of which are well described in works such as Arthur Guyton
Arthur Guyton
Arthur Clifton Guyton was an American physiologist. He was born in Oxford, Mississippi, to Dr. Billy S. Guyton, a highly respected eye, ear, nose, and throat specialist, who later became Dean of the University of Mississippi Medical School, and Kate Smallwood Guyton, a mathematics and physics...

's 'Textbook of Medical Physiology'.
  • One example is the onset of contractions
    Contraction (childbirth)
    -Throughout menstrual cycle:The uterus frequently contracts throughout the entire menstrual cycle, and these contractions have been termed endometrial waves or contractile waves. These appear to involve only the sub-endometrial layer of the myometrium...

     in childbirth
    Childbirth
    Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...

    , known as the Ferguson reflex
    Ferguson reflex
    The Ferguson reflex is an example of positive feedback and the female body's response to pressure application in the cervix or vaginal walls.Upon application of pressure, oxytocin is released and uterine contractions are stimulated , until the baby is delivered...

    . When a contraction occurs, the hormone oxytocin
    Oxytocin
    Oxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...

     is released into the body, which stimulates further contractions. This results in contractions increasing in amplitude and frequency.

  • Another example is the process of blood clotting
    Coagulation
    Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...

    . The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.

  • Lactation
    Lactation
    Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...

     also involves positive feedback in that the more the baby sucks, the more milk is produced, via a surge in prolactin secretion.

  • Estrogen
    Estrogen
    Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

     that functions during the follicular phase of menstruation is also an example of positive feedback.

  • The generation of nerve signals is another example, in which the membrane of a nerve fibre causes slight leakage of sodium ions through sodium channels, resulting in a change in the membrane potential, which in turn causes more opening of channels, and so on. So a slight initial leakage results in an explosion of sodium leakage which creates the nerve action potential
    Action potential
    In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

    .


In most cases, such feedback loops culminate in counter-signals being released that suppress or breaks the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot. Lactation stops when the baby no longer nurses.

The analogy of Evolutionary arms race
Evolutionary arms race
In evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback...

s provide further examples of positive feedback in biological systems. While analogies used to describe, theorise, or explicate evolutionary positive feedback are considered by some as an adaptive process, the essential feature of positive feedback is that of the process itself, namely cumulative causation and amplification, as outlined further above. This is unrelated to what people want to believe about it (for example that it must be progressive), or whether they like the outcome which can be favourable or unfavourable. Thus it is "a means of conceptualising the adaptive or maladaptive consequences of given processes or actions".

Positive feedback loops have been utilised in several adaptive theories and explanations pertaining to human evolution and performance. For example, beginning at the macro level, Alfred J. Lotka
Alfred J. Lotka
Alfred James Lotka was a US mathematician, physical chemist, and statistician, famous for his work in population dynamics and energetics. An American biophysicist best known for his proposal of the predator-prey model, developed simultaneously but independently of Vito Volterra...

 (1945) argued that the evolution of the species was most essentially a matter of selection that fed back energy flows to capture more and more energy for use by living systems. At the human level, Richard Alexander
Richard Alexander
Richard Thain Alexander was a British politician. He was Conservative Party Member of Parliament for Newark until losing his seat in the landslide of the 1997 general election....

 (1989) proposed that social competition between and within human groups fed back to the selection of intelligence thus constantly producing more and more refined human intelligence. Since humans have collectively evolved to be capable of capturing and using more energy that any other species, Lotka’s rigorous energy model of positive feedback and Alexander’s social model of positive feedback appear to be in mutually supportive agreement. Crespi (2004) discussed several other examples of positive feedback loops in evolution. In psychology, Winner (1996) described gifted children, perhaps representative of the highest class of human intelligence, as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on. Winner termed this positive feedback loop as a “rage to master.” Vandervert (2009a, 2009b) proposed that the child prodigy
Child prodigy
A child prodigy is someone who, at an early age, masters one or more skills far beyond his or her level of maturity. One criterion for classifying prodigies is: a prodigy is a child, typically younger than 18 years old, who is performing at the level of a highly trained adult in a very demanding...

 (a gifted child who reaches adult status in a particular domain of learning, for example, mathematics, music or art by age 10) can be explained in terms of a positive feedback loop between the output of thinking/performing in working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...

, which then is fed to the cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

 where it is streamlined, and then fed back to working memory thus steadily increasing the quantitative and qualitative output of working memory. Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for language
Language
Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

 evolution in working memory.

It have been shown that changes in biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

 through the Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...

 correlate much better with hyperbolic model (widely used in demography
Demography
Demography is the statistical study of human population. It can be a very general science that can be applied to any kind of dynamic human population, that is, one that changes over time or space...

 and macrosociology
Macrosociology
Macrosociology is an approach to the discipline which emphasizes the analysis of social systems and populations on a large scale, at the level of social structure, and often at a necessarily high level of theoretical abstraction. Microsociology, by contrast, focuses on the individual social agency...

) than with exponential
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...

 and logistic
Logistic function
A logistic function or logistic curve is a common sigmoid curve, given its name in 1844 or 1845 by Pierre François Verhulst who studied it in relation to population growth. It can model the "S-shaped" curve of growth of some population P...

 models (traditionally used in population biology
Population biology
Population biology is a study of populations of organisms, especially the regulation of population size, life history traits such as clutch size, and extinction...

 and extensively applied to fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

 as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

 arising from resource limitation. Hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the world population growth
Population growth
Population growth is the change in a population over time, and can be quantified as the change in the number of individuals of any species in a population using "per unit time" for measurement....

 has been demonstrated (see below) to arise from a second-order positive feedback between the population size and the rate of technological growth. The hyperbolic character of biodiversity growth can be similarly accounted for by a positive feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

 and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend (produced by the positive feedback) with cyclical and stochastic dynamics.

In electronics


Electronic amplification
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...

 devices may have positive feedback signal paths intentionally added, or such paths may come into being inadvertently. In addition, thermal positive feedback may take place in electronic circuits. This is called thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

 and can be destructive.

In networking, a form of positive feedback known as a broadcast storm can result when multiple switches are connected in such a way that they form a loop. Say for example, you have two switches, each with 4 ports. By accident, ports 1 and 2 are connected to the other switches ports 1 and 2. A single multi-cast packet is sent, switch one receives it, and sends it out through every port besides the one it came in on. Switch 2 receives 2 multi-casts, and sends each of them out on every port besides the ones they came in on. Switch 1 then receives 2 again, and the process repeats. This begins flooding the network with packets being rapidly bounced back and forth until the entire network is crippled.


Regenerative circuit
Regenerative circuit
The regenerative circuit or "autodyne" allows an electronic signal to be amplified many times by the same vacuum tube or other active component such as a field effect transistor. It consists of an amplifying vacuum tube or transistor with its output connected to its input through a feedback...

s were invented and patented in 1914 for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 amplifier can multiply its gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

 by 1,000 or more. Therefore a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the superheterodyne design, with many more amplification stages, but much more stable operation and no positive feedback.

The oscillation that can break out in a regenerative radio circuit can be put to good use in the design of electronic oscillator
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...

s. By the use of tuned circuits or a piezoelectric
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...

 crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 (commonly quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

), the signal that is amplified by the positive feedback remains linear and sinusoidal. There are several designs for such harmonic oscillator
Harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: \vec F = -k \vec x \, where k is a positive constant....

s, including the Armstrong oscillator
Armstrong oscillator
The Armstrong oscillator is named after its inventor, the electrical engineer Edwin Armstrong. It is sometimes called a tickler oscillator because the feedback needed to produce oscillations is provided using a tickler coil via magnetic coupling between coil L and coil T...

, hartley oscillator
Hartley oscillator
The Hartley oscillator is an electronic oscillator circuit that uses an inductor and a capacitor in parallel to determine the frequency. Invented in 1915 by American engineer Ralph Hartley, the distinguishing feature of the Hartley circuit is that the feedback needed for oscillation is taken from...

, colpitts oscillator
Colpitts oscillator
A Colpitts oscillator, invented in 1920 by American engineer 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...

, and the 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 oscillator is based on a bridge circuit originally developed by Max Wien in 1891....

. They all use positive feedback to maintain the oscillations.

Many electronic circuits, especially amplifiers, incorporate negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

. This reduces their gain, but improves their input impedance
Input impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...

, output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...

, and bandwidth, and stabilises all of these parameters, including the closed-loop gain. These parameters also become less dependent on the details of the amplifying device itself, and more dependent on the feedback components, which are less likely to vary with manufacturing tolerance, age and temperature. The difference between positive and negative feedback for AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 signals is one of phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

: if the signal is fed back out of phase, the feedback is negative and if it is in-phase the feedback is positive. One problem for amplifier designers who use negative feedback is that some of the components of the circuit will introduce phase shift in the feedback path. If there is a frequency (usually a high frequency) where the phase shift reaches 180°, then the designer must ensure that the amplifier gain at that frequency is very low (usually by low-pass filter
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...

ing). If the loop gain
Loop gain
Loop gain is an engineering term used to quantify the gain of a system controlled by feedback loops. As such, the concept of loop gain is useful in a variety of disciplines. Traditionally, most of those have been in the field of electronics, telecommunications, or control systems...

 (the product of the amplifier gain and the extent of the positive feedback) at any frequency is greater than one, then the amplifier will oscillate at that frequency (Barkhausen stability criterion
Barkhausen stability criterion
The Barkhausen stability criterion is a mathematical condition to determine when a linear electronic circuit will oscillate. It was put forth in 1921 by German physicist Heinrich Georg Barkhausen...

). Such oscillations are sometimes called parasitic oscillation
Parasitic oscillation
Parasitic oscillation is an undesirable oscillation caused by feedback. The problem occurs notably in RF , audio, and other electronic amplifiers as well as in digital signal processing. It is one of the fundamental issues addressed by control theory....

s. An amplifier that is stable in one set of conditions can break into parasitic oscillation in another. This may be due to changes in temperature, supply voltage, adjustment of front-panel controls, or even the proximity of a person or other conductive item. Amplifiers may oscillate gently in ways that are hard to detect without an oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...

, or the oscillations may be so extensive that only a very distorted or no required signal at all gets through, or that damage occurs. Low frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note.


Digital electronic circuits are sometimes designed to benefit from positive feedback. Normal logic gate
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...

s usually rely simply on gain to push digital signal voltages away from intermediate values to the values that are meant to represent boolean
Boolean logic
Boolean algebra is a logical calculus of truth values, developed by George Boole in the 1840s. It resembles the algebra of real numbers, but with the numeric operations of multiplication xy, addition x + y, and negation −x replaced by the respective logical operations of...

 '0' and '1'. When an input voltage is expected to vary in an analogue
Analogue electronics
Analogue electronics are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two different levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that represents the signal...

 way, but sharp thresholds are required for later digital processing, the Schmitt trigger circuit uses positive feedback to ensure that if the input voltage creeps gently above the threshold, the output is forced smartly and rapidly from one logic state to the other. One of the corollaries of the Schmitt trigger's use of positive feedback is that, should the input voltage move gently down again past the same threshold, the positive feedback will hold the output in the same state with no change. This effect is called hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...

: the input voltage has to drop past a different, lower threshold to 'un-latch' the output and reset it to its original digital value. By reducing the extent of the positive feedback, the hysteresis-width can be reduced, but it can not entirely be eradicated. The Schmitt trigger is, to some extent, a latching circuit.
An electronic latch is a circuit that has the effect of memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...

. For example, a Schmitt trigger circuit in a stable state, with an input voltage between its two threshold levels, in the middle of its hysteresis band, can tell the observer whether the last input swing it saw was higher or lower than the outer edges of its hysteresis band. This is the basis of one bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

 of electronic memory. The actual memory circuits used in most digital electronics are based on the 'flip-flop
Flip-flop (electronics)
In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic...

' or 'bistable multivibrator
Multivibrator
A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices cross-coupled by resistors or capacitors...

'. A bistable multivibrator uses logic gates connected to each other so that positive feedback maintains the state of the circuit after the input signal has been removed, until a suitable alternative signal is applied to change the state. This is usually a form of volatile memory
Volatile memory
Volatile memory, also known as volatile storage, is computer memory that requires power to maintain the stored information, unlike non-volatile memory which does not require a maintained power supply...

, in the sense that removing power from the flip-flop circuit will usually cause it to lose state. Computer random access memory (RAM) can be made in this way, with one latching circuit for each bit of memory, eight to the byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

.

Thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

 occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which, directly or indirectly, makes it pass yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.
Audio
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...

 and video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 systems can easily be made to demonstrate positive feedback. If a microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

 picks up the amplified sound loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

s in the same circuit, then howling and screeching sounds of audio feedback
Audio feedback
Audio feedback is a special kind of positive feedback which occurs when a sound loop exists between an audio input and an audio output...

 (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and filtered
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...

 by the characteristics of the audio system and the room. Microphones are not the only transducers subject to this effect. Phonograph turntables are also prone to picking up acoustic feedback, usually in the low frequency range < 100Hz, manifesting as a low rumble. Usually moving the placement or orientation (due to the directional nature of all transducers) of the mic, turntable, or speakers is the cure for this, although using EQ can help if the system tends to be resonant mostly in certain bands. This resonance is due to acoustical characteristics of the room and nature of electrical circuits.

Similarly, if a video camera
Video camera
A video camera is a camera used for electronic motion picture acquisition, initially developed by the television industry but now common in other applications as well. The earliest video cameras were those of John Logie Baird, based on the electromechanical Nipkow disk and used by the BBC in...

 is pointed at a monitor
Video monitor
A video monitor also called a broadcast monitor, broadcast reference monitor or just reference monitor, is a display device similar to a television set, used to monitor the output of a video-generating device, such as playout from a video server, IRD, video camera, VCR, or DVD player. It may or...

 screen that is displaying the camera's own signal, then weird repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to early series of the television programme Dr Who. Jimi Hendrix
Jimi Hendrix
James Marshall "Jimi" Hendrix was an American guitarist and singer-songwriter...

 helped to develop the controlled and musical use of audio feedback in electric guitar playing, and later Brian May
Brian May
Brian Harold May, CBE is an English musician and astrophysicist most widely known as the guitarist and a songwriter of the rock band Queen...

 was a famous proponent of the technique.

In the World System development


The exponential growth
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...

 of the world population
World population
The world population is the total number of living humans on the planet Earth. As of today, it is estimated to be  billion by the United States Census Bureau...

 observed until the 1970s has recently been correlated to a non-linear second order positive feedback between the demographic growth and technological development that can be spelled out as follows: technological growth - increase in the carrying capacity
Carrying capacity
The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment...

 of land for people - demographic growth - more people - more potential inventors - acceleration of technological growth - accelerating growth of the carrying capacity - the faster population growth - accelerating growth of the number of potential inventors - faster technological growth - hence, the faster growth of the Earth's carrying capacity for people, and so on (see, e.g., Introduction to Social Macrodynamics by Andrey Korotayev
Andrey Korotayev
Andrey Korotayev is an anthropologist, economic historian, and sociologist, with major contributions to world-systems theory, cross-cultural studies, Near Eastern history, and mathematical modeling of social and economic macrodynamics.Education and career=Born in Moscow, Andrey Korotayev attended...

 et al.).

Systemic risk


Systemic risk
Systemic risk
In finance, systemic risk is the risk of collapse of an entire financial system or entire market, as opposed to risk associated with any one individual entity, group or component of a system. It can be defined as "financial system instability, potentially catastrophic, caused or exacerbated by...

 is the risk that an amplification or leverage or positive feedback process is built into a system, this is usually unknown, and under certain conditions this process can amplify exponentially and rapidly lead to destructive or chaotic behavior. A Ponzi scheme
Ponzi scheme
A Ponzi scheme is a fraudulent investment operation that pays returns to its investors from their own money or the money paid by subsequent investors, rather than from any actual profit earned by the individual or organization running the operation...

 is a good example of a positive-feedback system, because its output (profit) is fed back to the input (new investors), causing rapid growth toward collapse. W. Brian Arthur
W. Brian Arthur
William Brian Arthur is an economist credited with influencing and describing the modern theory of increasing returns. He has lived and worked in Northern California for many years. He is an authority on economics in relation to complexity theory, technology and financial markets...

 has also studied and written on positive feedback in the economy (e.g. W. Brian Arthur, 1990)

Simple systems that clearly separate the inputs from the outputs are not prone to systemic risk
Systemic risk
In finance, systemic risk is the risk of collapse of an entire financial system or entire market, as opposed to risk associated with any one individual entity, group or component of a system. It can be defined as "financial system instability, potentially catastrophic, caused or exacerbated by...

. This risk is more likely as the complexity of the system increases, because it becomes more difficult to see or analyze all the possible combinations of variables in the system even under careful stress testing conditions. The more efficient a complex system is, the more likely it is to be prone to systemic risks, because it takes only a small amount of deviation to disrupt the system. Therefore well-designed complex systems generally have built-in features to avoid this condition, such as a small amount of friction, or resistance, or inertia, or time delay to decouple the outputs from the inputs within the system. These factors amount to an inefficiency, but they are necessary to avoid instabilities.

Population and agriculture


Agriculture and human population can be considered to be in a positive feedback mode, which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly-efficient monocultures which are more susceptible to systemic risk
Systemic risk
In finance, systemic risk is the risk of collapse of an entire financial system or entire market, as opposed to risk associated with any one individual entity, group or component of a system. It can be defined as "financial system instability, potentially catastrophic, caused or exacerbated by...

.

Prejudice, social institutions and poverty


Gunnar Myrdal
Gunnar Myrdal
Karl Gunnar Myrdal was a Swedish Nobel Laureate economist, sociologist, and politician. In 1974, he received the Nobel Memorial Prize in Economic Sciences with Friedrich Hayek for "their pioneering work in the theory of money and economic fluctuations and for their penetrating analysis of the...

 described a vicious circle
Vicious Circle
Vicious Circle is an album released in 1994 by L.A. Guns. Most of the songs have Phil Lewis on lead vocals, but the track "Nothing Better to Do" features Kelly Nickels on lead vocals, and "Tarantula" is instrumental. MC Bones drums on several songs. Lewis and Bones also played together in the band...

 of increasing inequalities, and poverty, which is known as "Circular Cumulative Causation (CCC)".

In climatology


Within climate a positive feedback subsystem never acts in isolation, but is always embedded within the overall climate system, which itself is always subject to a very powerful negative feedback, the Stefan–Boltzmann law: that emitted radiation rises with the fourth power of temperature. Hence, on earth the gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

 of the overall system is always less than one, stopping the system from suffering runaway effects. While there may have been periods of time such as the exit from an ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 where the gain was greater than one, this has not lasted long enough for extreme effects such as the evaporation of the oceans as is believed to have happened on Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

.

Examples of positive feedback subsystems in climatology include:
  • A warmer atmosphere will melt ice and this changes the albedo
    Albedo
    Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

     which further warms the atmosphere.
  • Methane hydrates can be unstable so that a warming ocean could release more methane
    Methane
    Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

    , which is also a greenhouse gas.

In sociology


A self-fulfilling prophecy
Self-fulfilling prophecy
A self-fulfilling prophecy is a prediction that directly or indirectly causes itself to become true, by the very terms of the prophecy itself, due to positive feedback between belief and behavior. Although examples of such prophecies can be found in literature as far back as ancient Greece and...

 is a social positive feedback loop between beliefs and behavior: if enough people believe that something is true, their behavior can make it true, and observations of their behavior may turn increase belief. A classic example is a bank run
Bank run
A bank run occurs when a large number of bank customers withdraw their deposits because they believe the bank is, or might become, insolvent...

.

Another sociological example of positive feedback is the network effect
Network effect
In economics and business, a network effect is the effect that one user of a good or service has on the value of that product to other people. When network effect is present, the value of a product or service is dependent on the number of others using it.The classic example is the telephone...

. When more people are encouraged to join a network this increases the reach of the network therefore the network expands ever more quickly. A viral video
Viral video
A viral video is one that becomes popular through the process of Internet sharing, typically through video sharing websites, social media and email...

 is an example of the network effect in which links
Hyperlink
In computing, a hyperlink is a reference to data that the reader can directly follow, or that is followed automatically. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks...

 to a popular video are shared and redistributed, ensuring that more people see the video and then re-publish the links. This is the basis for many social phenomena, including Ponzi scheme
Ponzi scheme
A Ponzi scheme is a fraudulent investment operation that pays returns to its investors from their own money or the money paid by subsequent investors, rather than from any actual profit earned by the individual or organization running the operation...

s and chain letter
Chain letter
A typical chain letter consists of a message that attempts to the recipient to make a number of copies of the letter and then pass them on to as many recipients as possible...

s. In many cases population size is the limiting factor to the feedback effect.

See also

  • Chain reaction
    Chain reaction
    A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....

  • Donella Meadows' twelve leverage points to intervene in a system
  • Hyperbolic growth
    Hyperbolic growth
    When a quantity grows towards a singularity under a finite variation it is said to undergo hyperbolic growth.More precisely, the reciprocal function 1/x has a hyperbola as a graph, and has a singularity at 0, meaning that the limit as x \to 0 is infinity: any similar graph is said to exhibit...

  • Reflexivity (social theory)
    Reflexivity (social theory)
    Reflexivity refers to circular relationships between cause and effect. A reflexive relationship is bidirectional with both the cause and the effect affecting one another in a situation that does not render both functions causes and effects...

  • Strategic complementarity
  • Stability criterion
  • System dynamics
    System dynamics
    System dynamics is an approach to understanding the behaviour of complex systems over time. It deals with internal feedback loops and time delays that affect the behaviour of the entire system. What makes using system dynamics different from other approaches to studying complex systems is the use...

  • Technological Singularity
    Technological singularity
    Technological singularity refers to the hypothetical future emergence of greater-than-human intelligence through technological means. Since the capabilities of such an intelligence would be difficult for an unaided human mind to comprehend, the occurrence of a technological singularity is seen as...

  • Thermal runaway
    Thermal runaway
    Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

  • Negative feedback
    Negative feedback
    Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...


Analogous concepts

  • Self-fulfilling prophecy
    Self-fulfilling prophecy
    A self-fulfilling prophecy is a prediction that directly or indirectly causes itself to become true, by the very terms of the prophecy itself, due to positive feedback between belief and behavior. Although examples of such prophecies can be found in literature as far back as ancient Greece and...

  • Virtuous circle and vicious circle
    Virtuous circle and vicious circle
    A virtuous circle and a vicious circle are economic terms. They refer to a complex of events that reinforces itself through a feedback loop. A virtuous circle has favorable results, while a vicious circle has detrimental results...

  • Matthew effect (sociology)
    Matthew effect (sociology)
    In sociology, the Matthew effect is the phenomenon where "the rich get richer and the poor get poorer". Those who possess power and economic or social capital can leverage those resources to gain more power or capital. The term was first coined by sociologist Robert K...

  • Matthew effect (education)
    Matthew effect (education)
    The Matthew effect in education was described by Keith Stanovich based on the Matthew Effect in sociology. It derives its name from a passage in the New Testament: "For unto every one that hath shall be given, and he shall have abundance: but from him that hath not shall be taken away even that...

  • Law of Attraction
  • Karma
    Karma
    Karma in Indian religions is the concept of "action" or "deed", understood as that which causes the entire cycle of cause and effect originating in ancient India and treated in Hindu, Jain, Buddhist and Sikh philosophies....


Examples

  • Bus bunching
    Bus bunching
    In public transport bus bunching, clumping, or platooning refers to a group of two or more transit vehicles along the same route, such as buses or trains, which are scheduled to be evenly spaced, running in the same location at the same time...

  • Cytokine storm
    Cytokine storm
    A cytokine storm, or hypercytokinemia is a potentially fatal immune reaction consisting of a positive feedback loop between cytokines and immune cells, with highly elevated levels of various cytokines.-Symptoms:...

  • Matthew effect
    Matthew effect
    The Matthew effect may refer to:* Matthew effect , the phenomenon in sociology where "the rich get richer and the poor get poorer"* Matthew effect , the phenomenon in education that has been observed in research on how new readers acquire the skills to read...

  • Autocatalysis
    Autocatalysis
    A single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product itself is the catalyst for that reaction....

  • Meander
    Meander
    A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...

  • Runaway greenhouse effect
    Runaway greenhouse effect
    A runaway greenhouse effect is not a clearly defined term, but is understood to mean an event analogous to that which is believed to have happened in the early history of Venus, where positive feedback increased the strength of its greenhouse effect until its oceans boiled away...

  • Larsen effect

Further reading

  • Norbert Wiener
    Norbert Wiener
    Norbert Wiener was an American mathematician.A famous child prodigy, Wiener later became an early researcher in stochastic and noise processes, contributing work relevant to electronic engineering, electronic communication, and control systems.Wiener is regarded as the originator of cybernetics, a...

     (1948), Cybernetics or Control and Communication in the Animal and the Machine, Paris, Hermann et Cie - MIT Press, Cambridge, MA.
  • Katie Salen and Eric Zimmerman. Rules of Play. MIT Press
    MIT Press
    The MIT Press is a university press affiliated with the Massachusetts Institute of Technology in Cambridge, Massachusetts .-History:...

    . 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.