Positive feedback

Positive feedback (POS) is a type of feedback in which a change in the output signal of the system leads to a change in the input signal that contributes to a further deviation of the output signal from the original value, i.e., the sign of the change in the feedback signal coincides with the sign of the change in the input signal.

Positive feedback accelerates the response of the system to a change in the input signal; therefore, it is deliberately used in engineering in situations where acceleration of the reaction to a change in external parameters is required.

At the same time, positive feedback can lead to system instability. For example, we assume that there are no phase delays in the circuit. Moreover, if the gain in the positive feedback loop (in an open system, or open loop) is greater than 1, then self-oscillations occur in the system (this is used in various self-oscillators ), or the system goes into one of the stable, quasi-stationary states (for example, various triggers ). With equality of 1 gain in an open circuit, the system is on the verge of self-excitation and randomly occurring self-oscillations either slowly decay or increase to a limit. With an open loop gain of less than 1, the system is stable.

For example, an oscillator based on an amplifier with a Wien bridge in a positive feedback circuit is an example of a frequency-dependent positive feedback circuit, and in order for this generator to generate a sinusoidal signal with low distortion, the loop gain in the circuit is supported by an amplitude-dependent nonlinear negative feedback exactly equal to 1.

Another example of the use of positive feedback in a Schmitt trigger. If a digital logic element or an operational amplifier is covered by a properly selected positive feedback, a hysteresis circuit called a Schmitt trigger is formed. Schmitt trigger with an integrating RC circuit at the input is used to eliminate contact bounce , increase the noise immunity of sensor signals (or cable receivers), eliminate the state of "uncertainty" in communication channels from the influence of interference, etc.

Positive feedback is present in chain chemical reactions , autocatalytic chemical reactions, chain reactions of fission of nuclei of heavy elements in a nuclear explosion . In a controlled nuclear reaction in nuclear reactors, the effective neutron multiplication factor (loop gain, in terms of auto-regulation) is maintained at 1 using a tracking system to control the position of the neutron-absorbing rods.

Nonlinear positive feedback leads to the development of an aggravated regime in the system.

Content

Examples and Applications

In Biology

The more sheep run, the more sheep will run after them. An alarm in herds and flocks propagates through a positive feedback mechanism.

Evolutionary Biology

Speciation accelerates speciation, since the appearance of each new species creates new ecological niches, which provokes the specialization of new species. For example, the emergence of a new species of herbivores automatically creates vacant ecological niches for new predators, parasites, scavengers and insect dung beetles. In turn, the emergence of new herbivores becomes a new selection vector for plants serving as food for this species or depending on the distribution of seeds. New species of predators and parasites add positive feedback loops, becoming a selection factor for their victims ^[1] . The co-evolution of predators and their victims is known as a special case of the “ principle of the Black Queen ” (The Red Queen Effect).

In the Phanerozoic period, biodiversity shows a steady, albeit non-monotonous growth from zero to several thousand genera in a geologically short period of time.

It is assumed that the growth of human intelligence is also due to positive feedback, the arms race, due to increasing intergroup and intragroup competition ^[2] .

In Physiology

Reflex of expulsion of the fetus as an example of enhancing the body's response to a growing stimulus. During childbirth, when the fetal head is pushed through the cervix (1), it causes the activation of the afferent nerve, which excites sensitive areas in the brain (2). Irritation of afferent regions serves as a signal for the pituitary gland to release a hormone called oxytocin (3). Oxytocin through the bloodstream reaches the uterine muscles (4), enhancing contractions and involving more muscles in them, which ensures the fetus is pushed to the cervix and eventually starts delivery

Examples of positive feedback in physiology include:

. During childbirth, contraction of the walls of the uterus , through mechanosensitive receptors in them, stimulates areas in the hypothalamus to release oxytocin into the blood. An increase in oxytocin concentration enhances the amplitude and frequency of uterine wall contractions. Which further enhances the release of oxytocin. This ensures the expulsion of the fetus. In addition, during pregnancy, the background concentration of oxytocin in the mother’s blood and the sensitivity of the uterus to it (due to an increase in the number of oxytocin receptors on the surface of myocytes) change according to the POS principle ^[3] .

In immunology

Septic shock and the “ cytokine storm ” can serve as examples of positive feedback in regulatory interactions between participants in the immune response. The immune cells activated by cytokines in the focus of inflammation release new portions of cytokines, attracting and activating new doses of immune cells - as a result, the cascade of reactions can become uncontrolled and non-adaptive, causing damage to the focus of inflammation, spreading to neighboring tissues and covering the whole body.

Notes

↑ Markov A, Neymark E. Evolution. Classic ideas in the light of new discoveries. - M .: AST: Korpus, 2014 .-- 656 p. - ISBN 978-5-17-083218-7
↑ Alexander R. (1989). Evolution of the human psyche. In P. Millar & C. Stringer (Eds.), The human revolution: Behavioral and biological perspectives on the origins of modern humans (pp. 455-513). Princeton: Princeton University Press.
↑ Gaiton A.K., Hall J.E. Medical Physiology - M .: "Logosphere", 2008. - 1296 p. - ISBN 978-5-98657-013-6

[markov4-1] Markov A, Neymark E. Evolution. Classic ideas in the light of new discoveries. - M .: AST: Korpus, 2014 .-- 656 p. - ISBN 978-5-17-083218-7

[Alexander-2] Alexander R. (1989). Evolution of the human psyche. In P. Millar & C. Stringer (Eds.), The human revolution: Behavioral and biological perspectives on the origins of modern humans (pp. 455-513). Princeton: Princeton University Press.

[Gayton-3] Gaiton A.K., Hall J.E. Medical Physiology - M .: "Logosphere", 2008. - 1296 p. - ISBN 978-5-98657-013-6