Negative feedback
A classic example of negative feedback is a heating system thermostat — when the temperature gets high enough, the heater is turned OFF.
Negative feedback loops in which just the right amount of correction is applied with optimum timing, can be very stable, accurate, and responsive.
Negative feedback is widely used in mechanical and electronic engineering, and also within living organisms,[1][2] and can be seen in many other fields from chemistry and economics to physical systems such as the climate.
Negative feedback loops also play an integral role in maintaining the atmospheric balance in various systems on Earth.
Even before the point where the phase shift becomes 180 degrees, stability of the negative feedback loop will become compromised, leading to increasing under- and overshoot following a disturbance.
This problem is often dealt with by attenuating or changing the phase of the problematic frequencies in a design step called compensation.
This quantity, then, is converted by the thermostat (a 'comparator') into an electrical error in status compared to the 'set point' S, and subsequently used by the regulator (containing a 'controller' that commands gas control valves and an ignitor) ultimately to change the heat provided by a furnace (an 'effector') to counter the initial weather-related disturbance in heat input to the house.
The operational amplifier was originally developed as a building block for the construction of analog computers, but is now used almost universally in all kinds of applications including audio equipment and control systems.
Operational amplifier circuits typically employ negative feedback to get a predictable transfer function.
Since the open-loop gain of an op-amp is extremely large, a small differential input signal would drive the output of the amplifier to one rail or the other in the absence of negative feedback.
[28] Consequently, the voltage gain of the circuit in the diagram, assuming an ideal op amp, is the reciprocal of feedback voltage division ratio β: A real op-amp has a high but finite gain A at low frequencies, decreasing gradually at higher frequencies.
As discussed in the previous section, the feedback circuit stabilizes the closed-loop gain and desensitizes the output to fluctuations generated inside the amplifier itself.
Similar control mechanisms are used in heating and cooling systems, such as those involving air conditioners, refrigerators, or freezers.
Some biological systems exhibit negative feedback such as the baroreflex in blood pressure regulation and erythropoiesis.
As shown in the figure, most endocrine hormones are controlled by a physiologic negative feedback inhibition loop, such as the glucocorticoids secreted by the adrenal cortex.
[32] In economics, automatic stabilisers are government programs that are intended to work as negative feedback to dampen fluctuations in real GDP.
However Norbert Wiener wrote in 1948: The notion of economic equilibrium being maintained in this fashion by market forces has also been questioned by numerous heterodox economists such as financier George Soros[34] and leading ecological economist and steady-state theorist Herman Daly, who was with the World Bank in 1988–1994.
[35] A basic and common example of a negative feedback system in the environment is the interaction among cloud cover, plant growth, solar radiation, and planet temperature.
[42] Negative feedback as a control technique may be seen in the refinements of the water clock introduced by Ktesibios of Alexandria in the 3rd century BCE.
Self-regulating mechanisms have existed since antiquity, and were used to maintain a constant level in the reservoirs of water clocks as early as 200 BCE.
Cornelius Drebbel had built thermostatically controlled incubators and ovens in the early 1600s,[44] and centrifugal governors were used to regulate the distance and pressure between millstones in windmills.
[47] Harold Stephen Black came up with the idea of using negative feedback in electronic amplifiers in 1927, submitted a patent application in 1928,[15] and detailed its use in his paper of 1934, where he defined negative feedback as a type of coupling that reduced the gain of the amplifier, in the process greatly increasing its stability and bandwidth.
[49] Early researchers in the area of cybernetics subsequently generalized the idea of negative feedback to cover any goal-seeking or purposeful behavior.
: 54 To reduce confusion, later authors have suggested alternative terms such as degenerative,[53] self-correcting,[54] balancing,[55] or discrepancy-reducing[56] in place of "negative".
