Positive feedback
When the loop gain is positive and above 1, there will typically be exponential growth, increasing oscillations, chaotic behavior or other divergences from equilibrium.For example, when part of an electronic output signal returns to the input, and is in phase with it, the system gain is increased.[3] Such systems can give rich qualitative behaviors, but whether the feedback is instantaneously positive or negative in sign has an extremely important influence on the results.Positive and negative in this sense refer to loop gains greater than or less than zero, and do not imply any value judgements as to the desirability of the outcomes or effects.Positive feedback does not necessarily imply instability of an equilibrium, for example stable on and off states may exist in positive-feedback architectures.[9] In the real world, positive feedback loops typically do not cause ever-increasing growth but are modified by limiting effects of some sort.According to Donella Meadows: Hysteresis, in which the starting point affects where the system ends up, can be generated by positive feedback.Modern radio receivers use the superheterodyne design, with many more amplification stages, but much more stable operation and no positive feedback.By the use of tuned circuits or a piezoelectric crystal (commonly quartz), the signal that is amplified by the positive feedback remains linear and sinusoidal.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.When an input voltage is expected to vary in an analogue 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.This effect is called hysteresis: the input voltage has to drop past a different, lower threshold to 'un-latch' the output and reset it to its original digital value.[19] An electronic flip-flop, or "latch", or "bistable multivibrator", is a circuit that due to high positive feedback is not stable in a balanced or intermediate state.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.If a microphone picks up the amplified sound output of loudspeakers in the same circuit, then howling and screeching sounds of audio feedback (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and filtered by the characteristics of the audio system and the room."I Feel Fine" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music.Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl.He helped develop the controlled and musical use of audio feedback in electric guitar playing,[24] and later Brian May was a famous proponent of the technique.The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.Genetic engineers have constructed and tested simple positive feedback networks in bacteria to demonstrate the concept of bistability.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 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.When isolated from the rest of apoptotic pathway, this positive feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.[44] Programs such as Facebook and Twitter depend on positive feedback to create interest in topics and drive the take-up of the media.[45][46] In the age of smartphones and social media, the feedback loop has created a craze for virtual validation in the form of likes, shares, and FOMO (fear of missing out).[citation needed] Agriculture and human population can be considered to be in a positive feedback mode,[52] which means that one drives the other with increasing intensity.[54]: 146 [55] Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as circular cumulative causation.Less water vapour means both low dew point temperatures and more efficient daytime heating, decreasing the chances of humidity in the atmosphere leading to cloud formation.If the heat produced is not removed from the reactants fast enough, thermal runaway can occur and very quickly lead to a chemical explosion.
Causal loop diagramstampedenetwork effectbank runfeedback loopnegative feedbackcyberneticsloop gainin phase withexponential growthoscillationschaotic behaviorlatchedfiltereddampedlimiteddigital electronicsthermal runawaysemiconductor junctionschemical reactionsexplosionstipping-pointswitcheslocking pliersbridges to collapseboom-then-bust cyclesaudio feedbackpublic address systemsPlateletloop gainsvalue judgementscausalunstable equilibriumincreasing functionHysteresisSchmitt triggerDonella MeadowsbistableWorld War IIregenerative circuitHarold Stephen Blackvacuum tubeRegenerative circuitstransistorsuperheterodyneelectronic oscillatorstuned circuitspiezoelectriccrystalquartzsinusoidalharmonic oscillatorsArmstrong oscillatorHartley oscillatorColpitts oscillatorWien bridge oscillatorinput impedanceoutput impedancebandwidthphase shiftlow-pass filteringBarkhausen stability criterionparasitic oscillationsoscilloscopedigital electroniclogic gatesBooleananaloguelatchingflip-flopmultivibratormemoryrandom access memorymicrophoneloudspeakersguitar pickuploudspeakeramplifiedfrequencyPA systemssound reinforcement systemPA systemAudio engineerselectric guitarrock musicguitar amplifiersdistortionI Feel FinefeedbackSøren Absalon LarsenRecord deckpickup cartridgesJimi Hendrixguitar solosBrian MayVideo feedbackvideo cameramonitorDoctor Whoelectrical switchesbimetallic stripoxytocinuterusphysiologycontractionschildbirthFerguson reflexhypothalamusamplitudeblood clottingLactationpituitaryprolactinestrogenfollicular phaseovulationnerve signalsHodgkin cycleaction potentialexcitation–contraction couplingbistabilitylac operonmolecular dynamicscell signalingevolutionAlfred J. Lotka