All Topics  
Negative feedback

 

   Email Print
   Bookmark   Link






 

Negative feedback



 
 
Negative feedback
Feedback

Feedback describes the situation when output from an event or phenomenon in the past will influence the same event/phenomenon in the present or future....
 feeds part of a system
System

System is a set of interacting or interdependent entities, real or abstract, forming an integrated whole.The concept of an "integrated whole" can also be stated in terms of a system embodying a set of relationships which are differentiated from relationships of the set to other elements, and from relationships between an element of the se...
's output, inverted, into the system's input; generally with the result that fluctuations are attenuated. Many real-world systems have one or several points around which the system gravitates. In response to a perturbation
Perturbation (biology)

A perturbation of a biological system is an alteration of function, induced by external or internal mechanisms. Biological systems can be perturbed through a number of means....
, a negative feedback system with such point(s) will tend to re-establish equilibrium
Mechanical equilibrium

A standard definition of is:This is a strict definition, and often the term "static equilibrium" is used in a more relaxed manner interchangeably with "mechanical equilibrium", as defined next....
.

any physical and biological systems, qualitatively different influences can oppose each other.






Discussion
Ask a question about 'Negative feedback'
Start a new discussion about 'Negative feedback'
Answer questions from other users
Full Discussion Forum



Recent Posts









Encyclopedia


Negative feedback
Feedback

Feedback describes the situation when output from an event or phenomenon in the past will influence the same event/phenomenon in the present or future....
 feeds part of a system
System

System is a set of interacting or interdependent entities, real or abstract, forming an integrated whole.The concept of an "integrated whole" can also be stated in terms of a system embodying a set of relationships which are differentiated from relationships of the set to other elements, and from relationships between an element of the se...
's output, inverted, into the system's input; generally with the result that fluctuations are attenuated. Many real-world systems have one or several points around which the system gravitates. In response to a perturbation
Perturbation (biology)

A perturbation of a biological system is an alteration of function, induced by external or internal mechanisms. Biological systems can be perturbed through a number of means....
, a negative feedback system with such point(s) will tend to re-establish equilibrium
Mechanical equilibrium

A standard definition of is:This is a strict definition, and often the term "static equilibrium" is used in a more relaxed manner interchangeably with "mechanical equilibrium", as defined next....
.

Overview

In many physical and biological systems, qualitatively different influences can oppose each other. For example, in biochemistry, one set of chemicals drives the system in a given direction, whereas another set of chemicals drives it in an opposing direction. If one, or both of these opposing influences are non-linear, equilibrium point(s) result.

In biology
Biology

Biology is a branch of the natural sciences concerned with the study of living organisms and their interaction with each other and their environment ....
, this process (generally biochemical
Biochemistry

Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
) is often referred to as homeostasis
Homeostasis

Homeostasis is the property of a system, either open system or closed system, that regulates its internal environment and tends to maintain a stable, constant condition....
; whereas in mechanics
Mechanics

Mechanics is the branch of physics concerned with the behaviour of physical body when subjected to forces or Displacement , and the subsequent effect of the bodies on their environment....
, the more common term is equilibrium
Mechanical equilibrium

A standard definition of is:This is a strict definition, and often the term "static equilibrium" is used in a more relaxed manner interchangeably with "mechanical equilibrium", as defined next....
.

In engineering
Engineering

Engineering is the discipline and profession of applying Technology and science knowledge and utilizing natural laws and physical resources in order to design and implement materials, structures, machines, devices, systems, and process that safely realize a desired objective and meet specified criteria....
, mathematics
Mathematics

Mathematics is the study of quantity, structure, space, change, and related topics of pattern and form. Mathematicians seek out patterns whether found in numbers, space, natural science, computers, imaginary abstractions, or elsewhere....
 and the physical and biological sciences, common terms for the points around which the system gravitates include: attractors, stable
Stability theory

In mathematics, stability theory deals with the stability of solutions for differential equations and dynamical systems....
 states, eigenstates/eigenfunctions, equilibrium points, and setpoint
Setpoint

Setpoint is the target value that an automatic control system, for example PID controller, will aim to reach. For example, a boiler control system might have a temperature setpoint, that is a temperature the control system aims to attain....
s.

Negative refers to the sign of the multiplier in mathematical models for feedback. In delta notation, -output is added to or mixed into the input. In multivariate systems, vectors help to illustrate how several influences can both partially complement and partially oppose each other.

In contrast, positive feedback
Positive feedback

Positive feedback, sometimes referred to as "cumulative causation", is a feedback loop system in which the system responds to Perturbation of biological system in the same direction as the perturbation....
 is a feedback in which the system responds in the same direction as the perturbation, resulting in amplification of the original signal instead of stabilizing the signal. Both positive and negative feedback require a feedback loop to operate, as opposed to feedforward, which does not rely on a feedback loop for its control of the system.

Examples

Examples of the use of negative feedback to control its system are: thermostat
Thermostat

A thermostat is a Measuring instrument for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature....
 control, phase-locked loop
Phase-locked loop

A phase-locked loop or phase lock loop is a control system that generates a Signal that has a fixed relation to the phase of a "reference" signal....
, hormonal regulation (see diagram above), and temperature regulation in animals.

A simple and practical example is a thermostat
Thermostat

A thermostat is a Measuring instrument for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature....
. When the temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
 in a heated room reaches a certain upper limit the room heating is switched off so that the temperature begins to fall. When the temperature drops to a lower limit, the heating is switched on again. Provided the limits are close to each other, a steady room temperature is maintained. The same applies to a cooling system, such as an air conditioner, a refrigerator
Refrigerator

A refrigerator is a cooling appliance comprising a thermal insulation compartment and a heat pump - a mechanism to transfer heat from it to the external environment, cooling the contents to a temperature below ambient....
, or a freezer.

Some biological systems exhibit negative feedback such as the baroreflex
Baroreflex

In cardiovascular physiology, the baroreflex or baroreceptor reflex is one of the body's homeostasis mechanisms for maintaining blood pressure....
 in blood pressure
Blood pressure

Blood pressure is the pressure exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs. The pressure of the circulating blood decreases as it moves away from the heart through artery and capillary, and toward the heart through veins....
 regulation and erythropoiesis
Erythropoiesis

Erythropoiesis is the process by which red blood cells are produced. In human adults, this usually occurs within the bone marrow. In the early fetus, erythropoiesis takes place in the mesodermal cells of the yolk sac....
. Many biological process (e.g., in the human anatomy
Human anatomy

Human anatomy, which, with physiology and biochemistry, is a complementary basic medical science is primarily the scientific study of the morphology of the adult human body....
) use negative feedback. Examples of this are numerous, from the regulating of body temperature, to the regulating of blood glucose
Glucose

Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
 levels. The disruption of negative feedback can lead to undesirable results: in the case of blood glucose levels, if negative feedback fails, the glucose levels in the blood may begin to rise dramatically, thus resulting in diabetes
Diabetes mellitus

Diabetes mellitus , often referred to simply as diabetes , is a syndrome of disordered metabolism, usually due to a combination of genetic disorder and environmental causes, resulting in abnormally high blood sugar levels ....
.

Electronic amplifiers

The negative feedback amplifier was invented by Harold Stephen Black
Harold Stephen Black

Harold Stephen Black was an American electrical engineer, who revolutionized the field of applied electronics by inventing the negative feedback amplifier in 1927....
 at Bell Laboratories in 1927, and patented by him in 1934. Fundamentally, all electronic devices (e.g. vacuum tube
Vacuum tube

In electronics, a vacuum tube, electron tube , thermionic valve, or just valve is a device used to amplifier, switch, otherwise modify, or create an Electricity signal by controlling the movement of electrons in a low-pressure space....
s, bipolar transistors, MOS transistors
MOSFET

The metal?oxide?semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925....
) exhibit some nonlinear behavior. Negative feedback corrects this by trading unused gain for higher linearity (lower distortion). An amplifier with too large of an open-loop gain, possibly in a specific frequency range, will additionally produce too large of a feedback signal in that same range. This feedback signal, when subtracted from the original input, will act to reduce the original input, also by "too large" an amount. This "too small" input will be amplified again by the "too large" open-loop gain, creating a signal that is "just right". The net result is a flattening of the amplifier's gain over all frequencies (desensitising). Thus, though much more accurate, amplifiers with negative feedback can become unstable if not designed correctly, causing them to oscillate. Harry Nyquist
Harry Nyquist

Harry Nyquist , was an important contributor to information theory....
 of Bell Laboratories managed to work out a theory regarding how to make it stable.

Negative feedback is used in this way in many types of amplification systems to stabilize and improve their operating characteristics (see e.g., operational amplifiers).

See also

  • Asymptotic gain model
    Asymptotic gain model

    The asymptotic gain model is a representation of the gain of negative feedback amplifiers given by the asymptotic gain relation:where is the return ratio with the input source disabled , G8 is the asymptotic gain and is the direct transmission term....
  • Control theory
    Control theory

    Control theory is an interdisciplinary branch of engineering and mathematics, that deals with the behavior of dynamical systems. The desired output of a system is called the reference....
  • Cybernetics
    Cybernetics

    Cybernetics is the interdisciplinary study of the structure of regulatory systems. Cybernetics is closely related to control theory and systems theory....
  • Harold Stephen Black
    Harold Stephen Black

    Harold Stephen Black was an American electrical engineer, who revolutionized the field of applied electronics by inventing the negative feedback amplifier in 1927....
  • Nyquist stability criterion
    Nyquist stability criterion

    The Nyquist stability criterion, named after Harry Nyquist, provides a simple test for BIBO stability of a closed-loop control system by examining the open-loop system's Nyquist plot....
  • Positive feedback
    Positive feedback

    Positive feedback, sometimes referred to as "cumulative causation", is a feedback loop system in which the system responds to Perturbation of biological system in the same direction as the perturbation....
  • Stability criterion
  • Step response
    Step response

    The step response of a system in a given initial state consists of the time evolution of its outputs when its input are Heaviside step functions....


External links