Positive feedback is a mechanism by which an output is enhanced. Here a molecular biology circuitry is used as an example is such as protein levels. however, in order to avoid any fluctuation in the protein level, the mechanism is inhibited stochastically (I), therefore when the concentration of the activated protein (A) is past the threshold ([I]), the loop mechanism is activated and the concetration of A increases exponentially if d[A]=k [A]

Positive feedback, sometimes referred to as "cumulative causation", is a feedback loop system in which the system responds to perturbation in the same direction as the perturbation. Feedback is a circular causal Process whereby some proportion of a system's output is returned (fed back to the Input. A perturbation of a biological system is an alteration of function induced by external or internal mechanisms In contrast, a system that responds to the perturbation in the opposite direction is called a negative feedback system. Negative Feedback feeds part of a System 's output inverted into the system's input generally with the result that fluctuations are attenuated These concepts were first recognized as broadly applicable by Norbert Wiener in his 1948 work on Cybernetics. Norbert Wiener ( November 26, 1894, Columbia Missouri – March 18, 1964, Stockholm, Sweden) was an American Cybernetics is the interdisciplinary study of the Structure of Complex systems especially Communication processes control mechanisms and Feedback ^[1]

Overview

A system in equilibrium in which there is positive feedback to any change in its current state is said to be in an unstable equilibrium, whereas it is possible for one with negative feedback to be in a stable equilibrium.

The end result of a positive feedback is often amplifying and "explosive", i. e. a small perturbation results in big changes. This feedback, in turn, will drive the system further away from its original setpoint, thus amplifying the original perturbation signal, and eventually become explosive because the amplification often grows exponentially (with the first order positive feedback), or even hyperbolically (with the second order positive feedback). Exponential growth (including Exponential decay) occurs when the growth rate of a mathematical function is proportional to the function's current value Indeed, chemical and nuclear fission based explosives offer an excellent physical demonstration of positive feedback. An explosive material is a material that either is chemically or otherwise Energetically unstable or produces a sudden expansion of the material usually accompanied Bombarding fissile material with neutrons causes it to emit even more neutrons, which in turn affect the material. This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. The greater the mass of fissile material, the larger the amplification, resulting in greater feedback. If the amplification is great enough, the process accelerates until the fissile material is spent or dispersed by the resulting explosion.

Both positive and negative feedback are closed systems, because the system is closed by a feedback loop, i. A closed system is a System which relies on a Feedback loop to control its output in a Control system in Systems theory. e. the response of the system depends on the feedback signal to complete its function; without such a loop, it would become an open system. An open system is a Feedforward System that does not have any Feedback loop to control its output in a Control system in Systems theory In contrast, a feed-forward system is an "open system" since it does not have any feedback loop, and does not rely on feedback signal to function. Feed-forward is a term describing a kind of System which reacts to changes in its environment usually to maintain some desired state of the system

Examples of positive and negative feedback, open and closed systems can be found in ecological, biological, social systems and in engineering control systems such as servo control systems. Ecology (from Greek grc οἶκος oikos, "house(hold" and grc -λογία -logia) is the scientific study of Foundations of modern biology There are five unifying principles In Sociology, a group can be defined as two or more Humans that interact with one another accept expectations and obligations as members of the group and share a A control system is a device or set of devices to manage command direct or regulate the behavior of other devices or systems

Basics

The effect of a positive feedback loop is not necessarily "positive" in the sense of being desirable. The name refers to the nature of change rather than the desirability of the outcome. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to speed it up.

When a change of variable occurs in a system, the system responds. A variable (ˈvɛərɪəbl is an Attribute of a physical or an abstract System which may change its Value while it is under Observation. In the case of positive feedback the response of the system is to change that variable even more in the same direction. A simple example in chemistry would be the phenomenon of autocatalysis, where a reaction is facilitated increasingly in the presence of its product. A single Chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the Catalyst for that reaction For another example, imagine an ecosystem with only one species and an unlimited amount of food. An ecosystem is a natural unit consisting of all plants animals and micro-organisms( Biotic factors in an area functioning together with all of the non-living physical ( The population will grow at a rate proportional to the current population, which leads to an accelerating increase, i. This article is about proportionality the mathematical relation e. , positive feedback. This has a de-stabilizing effect, so left unchecked, does not result in homeostasis. Homeostasis (from Greek: ὅμος hómos, "equal" and ιστημι istēmi, "to stand" lit In some cases (if not controlled by negative feedback), a positive feedback loop can run out of control, and can result in the collapse of the system. This is called vicious circle, or in Latin circulus vitiosus. People also refer to a virtuous circle, which is the same thing, but with an autocatalytic benign effect. A virtuous circle or a vicious circle is a complex of events that reinforces itself through a Feedback loop toward greater instability A single Chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the Catalyst for that reaction

Consider a linear amplifier with linear feedback. As long as the loop gain, i. e. the forward gain multiplied with the feedback gain, is lower than 1 the result is a stable (convergent) output. This is of course always true for a negative feedback but also for lower positive feedbacks. In electronic amplifiers the normal case is that the forward gain is quite high and the amplifier becomes unstable for quite small positive feedbacks.

In the real world, positive feedback loops are always controlled eventually by negative feedback of some sort; a microphone will break or a beaker will crack or a nuclear accident will result in meltdown. This outcome need not be so dramatic, however. The variety of negative feedback controls can modulate the effect. Embedded in a system of feedback loops, a positive feedback does not necessarily imply a runaway process. Combined with other processes, it may just have an amplifying effect.

One common example of positive feedback is the network effect, where more people are encouraged to join a network the larger that network becomes. In Economics and Business, a network effect (also called network externality) is the effect that one user of a good or service has The result is that the network grows more and more quickly over time. This is the basis for many social phenomena, including the infamous Ponzi Scheme. A Ponzi scheme is a Fraudulent Investment operation that involves promising or paying abnormally high returns (" Profits quot to investors out of the In this case, though, the population size is the limiting factor.

Applications

In biology

One example of a biological positive feedback loop is the onset of contractions in childbirth. In Medicine ( Obstetrics) a contraction is a forceful and painful motion of the Uterus as part of the process of Childbirth. Childbirth (also called labour, birth, partus or parturition) is the culmination of a Human Pregnancy or When a contraction occurs, the hormone oxytocin is released into the body, which stimulates further contractions. Oxytocin ( IPA: /ˌɔksɪˈtoʊsɪn/ (Greek "quick birth" is a Mammalian Hormone that also acts as a Neurotransmitter in the This results in contractions increasing in amplitude and frequency.

Another example of a biological positive feedback loop is the process of blood clotting. 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 whereby The loop is initiated when injured tissue releases signal chemicals which activate platelets in the blood. An activated platelet releases chemicals which activate more platelets, causing a rapid cascade and the formation of a blood clot.

In most cases, once the purpose of the feedback loop is completed, counter-signals are released which suppress or break the loop.

In electronics

Feedback is a process of sampling a part of the output signal and applying it back to the input. This technique is useful to change the parameters of an amplifier like voltage gain, input and output impedance, stability and bandwidth.

Feedback is said to be positive if any increase in the output signal results in a feedback signal which on being mixed with the input signal caused further increase in the magnitude of the output signal. Hence it is also called regenerative feedback. Positive feedback is in the same phase as the input signal, therefore the final gain of the amplifier (A_f) increases.

Final gain A_f = (output voltage/input voltage) = A/(1 − Aβ). Here A is the gain of the amplifier without feedback, and β is the feedback factor.

An advandage here is the Swing-up control of an inverted pendulum on a cart. Disadvantages are:

• Gain can tend to be unstable
• Higher distortion
• Bandwidth decreases
• Stability is difficult or impossible to guarantee

Positive feedback is used extensively in oscillators and in regenerative radio receivers and Q multipliers. Stability can refer to Aircraft flight Stability (aircraft In atmospheric fluid dynamics atmospheric stability, a measure of the turbulence Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states This article is about a radio receiver for other uses see Radio (disambiguation.

The schmitt trigger circuit uses positive feedback to generate hysteresis and thus provide noise immunity on digital input. In Electronics, a Schmitt trigger is a Comparator circuit that incorporates positive Feedback. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system is a one volume manga created by Tsutomu Nihei as a prequel to his ten-volume work Blame!. A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for

Audio feedback is a common example of positive feedback. Audio Feedback (also known as the Larsen effect after the Danish scientist Søren Larsen who first discovered its principles is a special kind of Feedback It is the familiar squeal that results when sound from loudspeakers enters a poorly placed microphone and gets amplified, and as a result the sound gets louder and louder. For the Marty Friedman album see Loudspeaker (album A loudspeaker, speaker, or speaker system is an electroacoustical Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies

In global economics

In the world system development

The hyperbolic growth of the world population observed till 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 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. When a quantity grows towards a Singularity under a finite variation it is said to undergo hyperbolic growth. The world population is the total number of living Humans on Earth at a given time The supportable Population of an Organism, given the food habitat, water and other necessities available within an environment is known as the environment's g. , Introduction to Social Macrodynamics by Andrey Korotayev et al. Andrey Korotayev (born in 1961 is an anthropologist, economic Historian, and sociologist. ).

Population and agriculture

Agriculture and human population can be considered in a positive feedback mode^[2], which means that one drives the other with increasing intensity. He ventures the case that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and turning to monocultures which are more susceptible to collapse.

Internet

Metaphorically, cumulative causation may emerge on the Internet as an echo chamber effect, which refers to any situation in which information or ideas are amplified by transmission inside an enclosed space. When used literally the term echo chamber refers to a hollow enclosure used to produce echoing sounds usually for recording purposes Another emerging term used to describe this "echoing" and homogenizing effect on the Internet within social communities is "cultural tribalism".

The Internet may be seen as a complex system (e. g. , emergent, dynamic, evolutionary), and as such, will at times illuminate the effects of positive feedback loops (i. e. , the echo-chamber effect) to that system, where a lack of perturbation to dimensions of the network, prohibits a sense of equilibrium to the system. Complex systems that are characterized by negative feedback loops will create more stability and balance during emergent and dynamic behaviour. Negative Feedback feeds part of a System 's output inverted into the system's input generally with the result that fluctuations are attenuated

For example, observers of journalism in the mass media describe an echo chamber effect in media discourse. Journalism is the profession of writing or communicating formally employed by publications and broadcasters for the benefit of a particular Community of people "Popular press" redirects here note that the University of Wisconsin Press publishes under the imprint "The Popular Press" One purveyor of information will make a claim, which many like-minded people then repeat, overhear, and repeat again (often in an exaggerated or otherwise distorted form) until most people assume that some extreme variation of the story is true.

Due to this condition arising in online communities, participants may find their own opinions constantly echoed back to them, and in doing so reinforce a certain sense of truth that resonates with individual belief systems. This can create some significant challenges to critical discourse within an online medium. The echo-chamber effect may also impact a lack of recognition to large demographic changes in language and culture on the Internet if individuals only create, experience and navigate those online spaces that reinforce their "preferred" world view.

See also

• Bus bunching
• Donella Meadows' twelve leverage points to intervene in a system
• Hyperbolic growth
• Matthew effect
• Reflexivity (social theory)
• Strategic complementarity
• Stability criterion
• Virtuous circle and vicious circle
• System dynamics

References

Further reading

• Norbert Wiener (1948), Cybernetics or Control and Communication in the Animal and the Machine, Paris, Hermann et Cie - MIT Press, Cambridge, MA. Norbert Wiener ( November 26, 1894, Columbia Missouri – March 18, 1964, Stockholm, Sweden) was an American
• Katie Salen and Eric Zimmerman. Rules of Play. MIT Press. The MIT Press is a University press affiliated with the Massachusetts Institute of Technology (MIT in Cambridge Massachusetts ( USA) 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.