Positive feedback

positive feedback looppositiveexacerbatedpositive feedback cyclereinforcing looppositive-feedback loopsreinforcing feedbackrunawayself-reinforcement positive feedback loop
Positive feedback (or exacerbating feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance.wikipedia
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Negative feedback

negative feedback loopnegative-feedbacknegative
In contrast, a system in which the results of a change act to reduce or counteract it has negative feedback.
Whereas positive feedback tends to lead to instability via exponential growth, oscillation or chaotic behavior, negative feedback generally promotes stability.

Thermal runaway

runawayrunaway reactionthermal explosion
On the other hand, thermal runaway is a type of positive feedback that can destroy semiconductor junctions.
It is a kind of uncontrolled positive feedback.

Feedback

feedback loopfeedback loopsfeedback control
Positive feedback (or exacerbating feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance.
There are two types of feedback: positive feedback and negative feedback.

Audio feedback

feedbackguitar feedbackacoustic feedback
A familiar example of positive feedback is the loud squealing or howling sound produced by audio feedback in public address systems: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.
Audio feedback (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive loop gain 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 power amplified loudspeaker).

Regenerative circuit

regenerative receiverregenerativeregeneration
The idea of positive feedback was already current in the 1920s with the introduction of the regenerative circuit.
A regenerative circuit is an amplifier circuit that employs positive feedback (also known as regeneration or reaction).

Exponential growth

exponentiallyexponentialgrow exponentially
When the loop gain is positive and above 1, there will typically be exponential growth, increasing oscillations, chaotic behavior or other divergences from equilibrium. It has been shown that changes in biodiversity through the Phanerozoic correlate much better with hyperbolic model (widely used in demography and macrosociology) than with exponential and logistic models (traditionally used in population biology and extensively applied to fossil biodiversity as well). Technological innovation and human population can be similarly considered, and this has been offered as an explanation for the apparent hyperbolic growth of the human population in the past, instead of a simpler exponential growth.

Electronic oscillator

oscillatoroscillatorsaudio oscillator
The oscillation that can break out in a regenerative radio circuit is used in electronic oscillators.
The most common form of linear oscillator is an electronic amplifier such as a transistor or operational amplifier connected in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide positive feedback.

Schmitt trigger

schmitt-triggerSchmidt triggerSchmitt-triggering
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.
In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier.

Wien bridge oscillator

There are several designs for such harmonic oscillators, including the Armstrong oscillator, Hartley oscillator, Colpitts oscillator, and the Wien bridge oscillator.
The oscillator can also be viewed as a positive gain amplifier combined with a bandpass filter that provides positive feedback.

Amplifier

amplifiersamplificationelectronic amplifier
In this example, a signal received by the microphone is amplified and passed out of the loudspeaker.
Early tube amplifiers often had positive feedback (regeneration), which could increase gain but also make the amplifier unstable and prone to oscillation.

Action potential

action potentialsnerve impulsenerve impulses
Voltage-gated ion channels are capable of producing action potentials because they can give rise to positive feedback loops: The membrane potential controls the state of the ion channels, but the state of the ion channels controls the membrane potential.

Ferguson reflex

It is an example of positive feedback in biology.

Evolutionary arms race

evolutionary conflictarms racecoevolutionary arms race
The analogy of Evolutionary arms races provide further examples of positive feedback in biological systems.
These are often described as examples of positive feedback.

Hodgkin cycle

The Hodgkin cycle represents a positive feedback loop in which an initial membrane depolarization leads to uncontrolled deflection of the membrane potential to near V Na.

Reflexivity (social theory)

reflexivityreflexivefalse as imposed
According to the theory of reflexivity advanced by George Soros, price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction.
This is an instance of a positive feedback loop.

Biodiversity

diversitybiological diversitybiodiverse
It has been shown that changes in biodiversity through the Phanerozoic correlate much better with hyperbolic model (widely used in demography and macrosociology) than with exponential and logistic models (traditionally used in population biology and extensively applied to fossil biodiversity 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 arising from resource limitation.

Parasitic oscillation

parasite signalself-oscillation
Such oscillations are sometimes called parasitic oscillations.
Using Bode plots, a design engineer checks whether there is a frequency where both conditions for oscillations are met: the phase is zero (positive feedback) and the loop gain is 1 or greater.

Hyperbolic growth

hyperbolic
Technological innovation and human population can be similarly considered, and this has been offered as an explanation for the apparent hyperbolic growth of the human population in the past, instead of a simpler exponential growth.
In the real world hyperbolic growth is created by certain non-linear positive feedback mechanisms.

Chain reaction

chain reactionschemical chain reactionchain
In a chain reaction, positive feedback leads to a self-amplifying chain of events.

Flip-flop (electronics)

flip-flopflip-flopslatch
System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system latched into a new stable state.

Network effect

network effectsnetwork externalitiesnetwork externality
Another sociological example of positive feedback is the network effect.
The network effect can create a bandwagon effect as the network becomes more valuable and more people join, resulting in a positive feedback loop.

Muscle contraction

contractionmuscular contractionexcitation-contraction coupling
The increase in is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum, which releases in a positive feedback physiological response.

Virtuous circle and vicious circle

vicious circlevicious cyclevirtuous circle
Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".
Both systems of events have feedback loops in which each iteration of the cycle reinforces the previous one (positive feedback).

W. Brian Arthur

Brian ArthurWilliam Brian Arthur
W. Brian Arthur has also studied and written on positive feedback in the economy (e.g. W. Brian Arthur, 1990).
Arthur is noted for his seminal works "studying the impacts of positive feedback or increasing returns in economies, and how these increasing returns magnify small, random occurrences in the market place."