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Encyclopedia > Positive feedback
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. In contrast, a system that responds to the perturbation in the opposite direction is called a negative feedback system. These concepts were first recognized as broadly applicable by Norbert Wiener in his 1948 work on Cybernetics.[1] For other uses, see Feedback (disambiguation). ... A perturbation of a biological system is an alteration of function, induced by external or endogenous mechanisms. ... This article does not cite any references or sources. ... Norbert Wiener Norbert Wiener (November 26, 1894, Columbia, Missouri – March 18, 1964, Stockholm Sweden) was an American theoretical and applied mathematician. ... For other uses, see Cybernetics (disambiguation). ...

Contents

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 one with negative feedback is said 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). Indeed, chemical and nuclear fission based explosives offer an excellent physical demonstration of positive feedback. Bombarding fissile material with neutrons causes it to emit even more neutrons, which in turn affect the material. 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. Generally, amplification is a basic process sometimes seen in nature, and often used in processes which involve a signal which must be made stronger. ... Setpoint is the target value that an automatic control system, for example PID controller, will aim to reach. ... In mathematics, exponential growth (or geometric growth) occurs when the growth rate of a function is always proportional to the functions current size. ... Look up fission in Wiktionary, the free dictionary. ... This article is concerned solely with chemical explosives. ... This article or section does not adequately cite its references or sources. ...


Both positive and negative feedback are closed systems, because the system is closed by a feedback loop, i.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. 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. A closed system is a system which relies on a feedback loop to control its output in a control system in systems theory. ... 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. ... 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. For the journal, see Ecology (journal). ... For the song by Girls Aloud see Biology (song) Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle Biology (from Greek: Βιολογία - βίος, bio, life; and λόγος, logos, speech lit. ... In sociology, a group is usually defined as a collection consisting of a number of people who share certain aspects, interact with one another, accept rights and obligations as members of the group and share a common identity. ... A control system is a device or set of devices to manage, command, direct or regulate the behaviour of other devices or systems. ... Look up servo in Wiktionary, the free dictionary. ...


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. 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. For another example, imagine an ecosystem with only one species and an unlimited amount of food. The population will grow at a rate proportional to the current population, which leads to an accelerating increase, i.e., positive feedback. This has a de-stabilizing effect, so left unchecked, does not result in homeostasis. 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. In computer science and mathematics, a variable (pronounced ) (sometimes called an object or identifier in computer science) is a symbolic representation used to denote a quantity or expression. ... A single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the catalyst for that reaction. ... A coral reef near the Hawaiian islands is an example of a complex marine ecosystem. ... This article is about proportionality, the mathematical relation. ... Homeostasis (from Greek: ὅμος, homos, equal; and ιστημι, histemi, to stand lit. ... Vicious Circle is an album released in 1995 by L.A. Guns. ... In many parts of economics there is an assumption that a complex system of determinants will tend to lead to a state of equilibrium. ... 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. Generally, amplification is a basic process sometimes seen in nature, and often used in processes which involve a signal which must be made stronger. ...


One common example of positive feedback is the network effect, where more people are encouraged to join a network the larger that network becomes. 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. In this case, though, the population size is the limiting factor. A network effect is a characteristic that causes a good or service to have a value to a potential customer which depends on the number of other customers who own the good or are users of the service. ... A Ponzi scheme is a fraudulent investment operation that involves paying abnormally high returns (profits) to investors out of the money paid in by subsequent investors, rather than from net revenues generated by any real business. ...


Applications

In biology

One example of a biological positive feedback loop is the onset of contractions in childbirth. When a contraction occurs, the hormone oxytocin is released into the body, which stimulates further contractions. This results in contractions increasing in amplitude and frequency. In medicine (obstetrics), a contraction is a forceful motion of the uterus, generated by the release of oxytocin (quick labor) by the pituitary gland, culminating in childbirth. ... Parturition redirects here. ... Oxytocin (Greek: quick birth) is a mammalian hormone that also acts as a neurotransmitter in the brain. ...


Another example of a biological positive feedback loop is the process of blood clotting. 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. This article is about the clotting of blood. ...


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 (Af) increases.


Final gain Af = (output voltage/input voltage) = A/(1 −). 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. Look up stability in Wiktionary, the free dictionary. ... Oscillation is the periodic variation, typically in time, of some measure as seen, for example, in a swinging pendulum. ... In radio terminology, a receiver is an electronic circuit that receives a radio signal from an antenna and decodes the signal for use as sound, pictures, navigational-position information, etc. ...


The schmitt trigger circuit uses positive feedback to generate hysteresis and thus provide noise immunity on digital input. In electronics, a Schmitt (or Schmidt) 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. ... This article is about noise as in sound. ... For other uses, see Digital (disambiguation). ...


Audio feedback is a common example of positive 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. 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 which occurs when a sound loop exists between an audio input (for example, a microphone or guitar pickup) and an audio output (for example... An inexpensive low fidelity 3. ... Microphones redirects here. ... This article is about audible acoustic waves. ...


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.g., Introduction to Social Macrodynamics by Andrey Korotayev et al.). When a quantity grows towards a singularity under a finite variation it is said to undergo hyperbolic growth. ... Map of countries by population — China and India, the only two countries to have a population greater than one billion, together possess more than a third of the worlds population. ... The equilibrium maximum of the population of an organism is known as the ecosystems carrying capacity for that organism. ... 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. Another emerging term used to describe this "echoing" and homogenizing effect on the Internet within social communities is "cultural tribalism". This article is about the technological device. ...


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. This article does not cite any references or sources. ...


For example, observers of journalism in the mass media describe an echo chamber effect in media discourse. 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. // Journalism is the discipline of gathering, writing and reporting news, and broadly it includes the process of editing and presenting the news articles. ... Popular press redirects here; note that the University of Wisconsin Press publishes under the imprint The Popular Press. Mass media is a term used to denote a section of the media specifically envisioned and designed to reach a very large audience such as the population of a nation state. ...


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

An example of bus bunching seen on the Kings Road, London. ... The twelve leverage points to intervene in a system were proposed by Donella Meadows. ... When a quantity grows towards a singularity under a finite variation it is said to undergo hyperbolic growth. ... The term Matthew effect may refer to a number of ideas all centrally related to a parable in the Gospel of Matthew, depending on context: // Matthew effect derives its name from a line spoken by the Master in Jesuss parable of the talents in the Christian Bibles book... // The concept of reflexivity In general, reflexivity is an act of self-reference where examination or action bends back on, refers to, and affects the entity instigating the action or examination. ... The Nyquist Stability Criterion is a unique and powerfull method for determining the stability of a closed-loop control system. ... In many parts of economics there is an assumption that a complex system of determinants will tend to lead to a state of equilibrium. ... System dynamics is an approach to understanding the behaviour of complex systems over time. ...

References

  1. ^ Norbert Wiener (1948), Cybernetics or Control and Communication in the Animal and the Machine, Paris, Hermann et Cie - MIT Press, Cambridge, MA.
  2. ^ Brown, A. Duncan. (2003) [1] Feed or Feedback. Publisher: International Books.

Norbert Wiener Norbert Wiener (November 26, 1894, Columbia, Missouri – March 18, 1964, Stockholm Sweden) was an American theoretical and applied mathematician. ...

Further readings

  • Norbert Wiener (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. 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.
Norbert Wiener Norbert Wiener (November 26, 1894, Columbia, Missouri – March 18, 1964, Stockholm Sweden) was an American theoretical and applied mathematician. ... MIT Press Books The MIT Press is a university publisher affiliated with the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. ...

  Results from FactBites:
 
Feedback (420 words)
In every feedback loop, as the name suggests, information about the result of a transformation or an action is sent back to the input of the system in the form of input data.
Positive feedback leads to divergent behavior: indefinite expansion or explosion (a running away toward infinity) or total blocking of activities (a running away toward zero).
In some cases the goal is self-determined and is preserved in the face of evolution: the system has produced its own purpose (to maintain, for example, the composition of the air or the oceans in the ecosystem or the concentration of glucose in the blood).
  More results at FactBites »

 
 

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