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.

[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.

Low frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note.

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.

A lack of rain decreases soil moisture, which kills plants or causes them to release less water through transpiration.

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 diagram that depicts the causes of a stampede as a positive feedback loop. Alarm or panic can sometimes be spread by positive feedback among a herd of animals to cause a stampede .
In sociology a network effect can quickly create the positive feedback of a bank run . The above photo is of the UK Northern Rock 2007 bank run .
Platelet clotting demonstrates positive feedback. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot through platelet congregation. As more platelets gather, more chemicals are released that speed up the process. The process gets faster and faster until the blood vessel wall is completely sealed and the positive feedback loop has ended. The exponential form of the graph illustrates the positive feedback mechanism.
A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary causal functions.
Hysteresis causes the output value to depend on the history of the input.
In a Schmitt trigger circuit, feedback to the non-inverting input of an amplifier pushes the output directly away from the applied voltage towards the maximum or minimum voltage the amplifier can generate.
A vintage style regenerative radio receiver. Due to the controlled use of positive feedback, sufficient amplification can be derived from a single vacuum tube or valve (centre).
The effect of using a Schmitt trigger (B) instead of a comparator (A)
Positive feedback is a mechanism by which an output is enhanced, 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 concentration of A increases exponentially if d[A]=k [A].
Illustration of an R-S ('reset-set') flip-flop made from two digital nor gates with positive feedback. Red and black mean logical '1' and '0', respectively.
A phonograph turntable is prone to acoustic feedback.
Video feedback
Positive feedback is the amplification of a body's response to a stimulus. For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called oxytocin (3). Oxytocin is then carried via the bloodstream to the uterus (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth.
During the Phanerozoic the biodiversity shows a steady but not monotonic increase from near zero to several thousands of genera.