Negative feedback

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Negative feedback

Written by Eng.Admin

Wednesday, 05 November 2008 06:48

Negative feedback is a type of feedback in which the system responds in an opposite direction to the perturbation. It is a process of feeding back to the input a part of a system's output, so as to reverse the direction of change of the output. This tends to keep the output from changing, so it is stabilizing and attempts to maintain constant conditions. This often results in equilibrium (in physical science) or homeostasis (in biology) such that the system will return to its original setpoint.
In contrast, positive feedback 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.
(The term 'negative' does not refer to desirability, but rather to the sign of the multiplier in the mathematical feedback equation. When a change of variable occurs, a negative feedback system will attempt to re-establish equilibrium.)

Examples

Examples of the use of negative feedback to control its system are: thermostat control, phase-locked loop, hormonal regulation, and temperature regulation in animals.
A simple and practical example is a thermostat. When the 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, or a freezer.
Some biological systems exhibit negative feedback such as the baroreflex in blood pressure regulation and erythropoiesis. Many biological process (e.g., in the human anatomy) use negative feedback. Examples of this are numerous, from the regulating of body temperature, to the regulating of blood glucose 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.

Electronic amplifiers

The negative feedback amplifier was invented by Harold Stephen Black at Bell Laboratories in 1927. Fundamentally, all electronic devices (e.g. vacuum tubes, bipolar transistors, MOS transistors) are nonlinear devices. Negative feedback corrects this by trading unused gain for higher linearity (lower distortion). Though much more accurate, amplifiers with negative feedback can become unstable if not designed correctly, causing them to oscillate. Harry Nyqeduist 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). Note that negative feedback is used to "stabilize" the system, not in amplifying the signal, whereas positive feedback is used to amplify the signal itself (which may lead to instability).

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