Positive feedback, sometimes referred to as "cumulative causation", refers to situations where some effect causes more of itself. Under strong positive feedback, most systems quickly move to a limit state, where the limit is provided by external factors, or into some other new stable state where the positive feedback is somehow negated. Positive feedback can also lead to oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes is used to be synonymous with "oscillation.&.
A system exhibiting positive feedback Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same (i.e. same defined) event / phenomenon (or the continuation / development of the original phenomenon) in the present or future. When an event is part of a chain of cause-and-effect that forms a circuit or, in response to perturbation, acts to increase the magnitude of the perturbation. That is, "A produces more of B which in turn produces more of A".[1] In contrast, a system that responds to the perturbation in the opposite direction is said to exhibit 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. These concepts were first recognized as broadly applicable by Norbert Wiener Norbert Wiener was an American mathematician in his 1948 work on cybernetics Cybernetics is the interdisciplinary study of the structure of regulatory systems. Cybernetics is closely related to control theory and systems theory. Both in its origins and in its evolution in the second-half of the 20th century, cybernetics is equally applicable to physical and social systems.[2]
The effect of a positive feedback loop may not be "positive" in the sense of being desirable. Positive refers to the direction of change rather than the desirability of the outcome. A negative feedback loop tends to reduce or inhibit or stabilise a process, while a positive feedback loop tends to expand or promote it and will often ultimately destabilise it.
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Overview
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Why is positive feedback important? 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 a positive feedback is often amplifying and "explosive," i.e. a small perturbation results in big changes. Indeed, chemical- and nuclear fission-based explosives An explosive material, also called an explosive, is a substance that contains a great amount of stored energy that can produce an explosion, a sudden expansion of the material after initiation, usually accompanied by the production of light, heat, sound, and pressure. An explosive charge is a measured quantity of explosive material offer an excellent physical demonstration of positive feedback. Bombarding a single atom of Uranium 235 Uranium-235 has a half-life of 700 million years and was discovered in 1935 by Arthur Jeffrey Dempster. Its nuclear cross section for slow thermal neutrons is about 1000 barns. For fast neutrons it is on the order of 1 barn. Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236. The with a neutron causes it to emit on average slightly over two neutrons, which (if losses do not overcome this) can hit two or more atoms, which in turn can hit four or more atoms, etc. The number of atoms involved increases exponentially and very soon the entire mass of uranium is involved.
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 are an increasing function In mathematics, a monotonic function is a function which preserves the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order theory of the "distance" from the set point.
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. A variety of negative feedback loops in the same system can modulate the effect. Embedded in a system of positive and negative feedback loops, positive feedback does not necessarily imply a runaway process. Combined with other processes, it may just have an amplifying effect.
Examples and Applications
In biology
In biology, a number of examples of positive feedback systems may be found in physiology, some of which are well described in works such as Arthur Guyton's 'Textbook of Medical Physiology'.[3]
- One example is the onset of contractions In medicine , a contraction is a motion of the uterus as part of the process of childbirth. Contractions, and labour in general, is one condition that releases the hormone oxytocin into the body. Contractions become longer as labour intensifies in 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. The process of normal human childbirth is categorized in three stages of labour: the shortening and dilation of the cervix, descent and birth of the infant, and birth of the placenta. In many cases, with. When a contraction occurs, the hormone oxytocin Oxytocin is a mammalian hormone that acts primarily as a neurotransmitter in the brain. Also known as alpha-hypophamine (α–hypophamine), oxytocin has the distinction of being the very first polypeptide hormone to be sequenced and synthesized biochemically by Vincent du Vigneaud et al. in 1953 is released into the body, which stimulates further contractions. This results in contractions increasing in amplitude and frequency.[4]
- Another example is the process of blood clotting Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis , 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. Disorders of coagulation can lead to an increased risk of bleeding (hemorrhage) or obstructive clotting. 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.[5]
- Lactation Lactation describes the secretion of milk from the mammary glands, the process of providing that milk to the young, and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, and in humans it is commonly referred to as breastfeeding or nursing. In most species milk comes out of the mother's nipples; also involves positive feedback in that the more the baby suckles, the more milk is produced, via a surge in prolactin secretion.[6]
- Estrogen Estrogens , oestrogens (BE), or œstrogens, are a group of steroid compounds, named for their importance in the estrous cycle, and functioning as the primary female sex hormone, their name comes from estrus/oistros (period of fertility for female mammals) + gen/gonos = to generate that functions during the follicular phase of menstruation is also an example of positive feedback.[7]
- 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 An action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a stereotyped trajectory. Action potentials occur in several types of excitable cells, including neurons, muscle cells, and endocrine cells. In neurons, they play a central role in cell-to-cell communication. In.[8]
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.[3]
The analogy of Evolutionary arms races 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. The co-evolving gene sets may be in different species, as in an evolutionary arms race provide further examples of positive feedback in biological systems.[9] 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".[10]
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 (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.[11] At the human level, Richard Alexander (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.[12] 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.[13] 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.[14] Winner termed this positive feedback loop as a “rage to master.” Vandervert (2009a, 2009b) proposed that the child prodigy A child prodigy is someone who at an early age masters one or more skills at an adult level. One heuristic for classifying prodigies is: a prodigy is a child, typically younger than 15 years old, who is performing at the level of a highly trained adult in a very demanding field of endeavor (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 is the executive and attentional aspect of short-term memory involved in the interim integration, processing, disposal, and retrieval of information. Working memory tasks include the active monitoring or manipulation of information or behaviors. It is a theoretical construct within cognitive psychology and neuroscience. Theories, which then is fed to the cerebellum The cerebellum is a region of the brain that plays an important role in motor control. It is also involved in some cognitive functions such as attention and language, and probably in some emotional functions such as regulating fear and pleasure responses, but it is its function in movement that is most clearly understood. The cerebellum does not where it is streamlined, and then fed back to working memory thus steadily increasing the quantitative and qualitative output of working memory.[15][16] Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for language Language is a term most commonly used to refer to so-called "natural languages" — the spoken forms of communication ubiquitous among humankind. By extension the term also refers to the type of thought process which creates and uses language. Essential to both meanings is the systematic creation, maintenance and use of systems of evolution in working memory.
In electronics
Feedback is a process of sampling a part of the output signal, compounding it with some derived part of the source signal, and applying the compound to the input of the active feed-forward Feed-forward is a term describing an element or pathway within a control system which passes a controlling signal from a source in the control system's external environment, often a command signal from an external operator, to a load elsewhere in its external environment. A control system which has only feed-forward behavior responds to its element of the feedback loop. The input to the system as a whole comes from outside the system; it is energy derived from an external signal source, which is subject to leakage and noise on its way to and within the system, and within the system can be compounded with a sample from the output by way of the feedback element. The notion of feedback relies on the presence of a well defined loop around which signal power propagates, with a well-defined feed-forward Feed-forward is a term describing an element or pathway within a control system which passes a controlling signal from a source in the control system's external environment, often a command signal from an external operator, to a load elsewhere in its external environment. A control system which has only feed-forward behavior responds to its pathway inside the feedback loop, and in electronics this is achieved by use of active devices such as transistors or thermionic valves, which have access to a reservoir of power that they can tap to provide power gain for amplification. Feedback implies also the occurrence of a loop delay because that signal power propagation is causal. Negative feedback (patented by H.S. Black in 1934) is useful to set the parameters of an amplifier like voltage gain, input and output impedance, stability and bandwidth. On the other hand, positive feedback is rarely useful in amplifiers; it is useful only in very exceptional circumstances, one of which is to control the input impedance of the amplifier, and even then the amplifier is at serious risk of likely destructive instability.
Feedback is said to be positive if any increase in the output signal results in a feedback signal which on being compounded with a derivative of the source signal causes 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 'internal gain' of the amplifier (Ai) increases.
If the circuit elements are practically linear, the 'internal gain', Ai , of the feedback loop is given by Ai = (output voltage/input voltage) = A/ (1 − Aβ). Here A is the gain of the feed-forward Feed-forward is a term describing an element or pathway within a control system which passes a controlling signal from a source in the control system's external environment, often a command signal from an external operator, to a load elsewhere in its external environment. A control system which has only feed-forward behavior responds to its active part of the amplifier without feedback, and β is the gain of the feedback element. The 'loop gain' is Aβ. Final or amplifier gain refers to the relation between source signal and load quantity; as well as depending on the 'internal gain' of the feedback loop, the final amplifier gain depends also on the presence of leakage or parasitic pathways, at the input, at the output, and as feed-forward Feed-forward is a term describing an element or pathway within a control system which passes a controlling signal from a source in the control system's external environment, often a command signal from an external operator, to a load elsewhere in its external environment. A control system which has only feed-forward behavior responds to its in parallel with the feedback loop, and it depends also on the load, which may be reactive.
An advantage 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 Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes is used to be synonymous with "oscillation.& and in regenerative radio receivers A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, and Q multipliers.
The schmitt trigger In electronics, a Schmitt trigger is a comparator circuit that incorporates positive feedback circuit uses positive feedback to generate hysteresis Hysteresis refers to systems that have memory, where the effects of the current input to the system are experienced with a certain delay in time. Such a system may exhibit path dependence, or "rate-independent memory" . Hysteresis phenomena occur in magnetic materials, ferromagnetic materials and ferroelectric materials, as well as in and thus provide noise In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is an unwanted perturbation to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission. Signal noise is heard as acoustic noise if played through a loudspeaker; it manifests as immunity on digital A digital system is a data technology that uses discrete values. By contrast, non-digital (or analog) systems use a continuous range of values to represent information. Although digital representations are discrete, the information represented can be either discrete, such as numbers, letters or icons, or continuous, such as sounds, images, and input.
Audio feedback Audio feedback 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, a loudspeaker). In this example, a signal received by the microphone is amplified and passed out of the loudspeaker. The sound from the loudspeaker can then be or acoustic feedback is a common example of positive feedback. It is the familiar squeal that results when sound from loudspeakers A loudspeaker is an electroacoustic transducer that converts an electrical signal into sound. The speaker moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water enters a closely-placed microphone A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1876, Emile Berliner invented the first microphone used as a telephone voice transmitter. Microphones are used in many applications such as telephones, tape recorders, karaoke systems, hearing aids, motion picture production, live and and gets amplified, and as a result the sound Sound is a travelling wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations gets louder and louder. To avoid this condition, the microphone A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1876, Emile Berliner invented the first microphone used as a telephone voice transmitter. Microphones are used in many applications such as telephones, tape recorders, karaoke systems, hearing aids, motion picture production, live and must be prevented from "hearing" its own loudspeaker A loudspeaker is an electroacoustic transducer that converts an electrical signal into sound. The speaker moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water.
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Franchising.com (press release)
"Already we are receiving positive feedback from retailers about the new gifts, as it allows them to diversify and carry additional stock at no risk. ...
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niet dat deze trouwens veel goedkoper zijn maar het zijn wel 1 van de mooiste stands die ik ooit heb gezien
Fahad Khan
Fri, 09 Jul 2010 14:46:04 GM
It can be frustrating, disheartening and discouraging when you put in such an effort only to have no recognition, no . positive feedback. to help keep the momentum going. Now you have doubts, fears, and feel despondent. ...
