Editing Positive feedback (section)

=== Hysteresis ===
{{main|Hysteresis}}

[[File:Hysteresis sharp curve.svg|thumb|Hysteresis causes the output value to depend on the history of the input.]]

[[File:Op-Amp Schmitt Trigger.svg|thumb|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.]]

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]]:

::"Positive feedback loops are sources of growth, explosion, erosion, and collapse in systems. A system with an unchecked positive loop ultimately will destroy itself. That's why there are so few of them. Usually, a negative loop will kick in sooner or later."<ref name=meadows>
Donella Meadows, [http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf ''Leverage Points: Places to Intervene in a System''] {{webarchive|url=https://web.archive.org/web/20131008160618/http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf |date=2013-10-08 }}, 1999</ref>

Hysteresis, in which the starting point affects where the system ends up, can be generated by positive feedback. When the gain of the feedback loop is above 1, then the output moves away from the input: if it is above the input, then it moves towards the nearest positive limit, while if it is below the input then it moves towards the nearest negative limit.

Once it reaches the limit, it will be stable. However, if the input goes past the limit,{{clarify|date=June 2012}} then the feedback will change sign{{dubious|date=June 2012}} and the output will move in the opposite direction until it hits the opposite limit. The system therefore shows [[bistability|bistable]] behaviour.