In electronics or valve (centre).
Regenerative circuits were invented and patented in 1914 for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single
transistor amplifier can multiply its
gain by 1,000 or more. Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, which would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception.
Superregenerative receivers use even more gain. Modern radio receivers use the
superheterodyne design, with many more amplification stages, but much more stable operation and no positive feedback. The oscillation that can break out in a regenerative radio circuit is used in
electronic oscillators. By the use of
tuned circuits or a
piezoelectric crystal (commonly
quartz), the signal that is amplified by the positive feedback remains linear and
sinusoidal. There are several designs for such
harmonic oscillators, including the
Armstrong oscillator,
Hartley oscillator,
Colpitts oscillator, and the
Wien bridge oscillator. They all use positive feedback to create oscillations. Many electronic circuits, especially amplifiers, incorporate
negative feedback. This reduces their gain, but improves their linearity,
input impedance,
output impedance, and
bandwidth, and stabilises all of these parameters, including the loop gain. These parameters also become less dependent on the details of the amplifying device itself, and more dependent on the feedback components, which are less likely to vary with manufacturing tolerance, age and temperature. The difference between positive and negative feedback for
AC signals is one of
phase: if the signal is fed back out of phase, the feedback is negative, and if it is in phase, the feedback is positive. One problem for amplifier designers who use negative feedback is that some of the components of the circuit will introduce
phase shift in the feedback path. If there is a frequency (usually a high frequency) where the phase shift reaches 180°, then the designer must ensure that the amplifier gain at that frequency is very low (usually by
low-pass filtering). If the loop gain (the product of the amplifier gain and the extent of the positive feedback) at any frequency is greater than one, then the amplifier will oscillate at that frequency (
Barkhausen stability criterion). Such oscillations are sometimes called
parasitic oscillations. An amplifier that is stable in one set of conditions can break into parasitic oscillation in another. This may be due to changes in temperature, supply voltage, adjustment of front-panel controls, or even the proximity of a person or other conductive item. Amplifiers may oscillate gently in ways that are hard to detect without an
oscilloscope, or the oscillations may be so extensive that only a very distorted or no required signal at all gets through, or that damage occurs. Low-frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note. Many common
digital electronic circuits employ positive feedback. While normal, simple Boolean
logic gates usually rely simply on gain to push digital signal voltages away from intermediate values to the values that are meant to represent
Boolean '0' and '1', but many more complex gates use feedback. When an input voltage is expected to vary in an
analogue way, but sharp thresholds are required for later digital processing, the
Schmitt trigger circuit uses positive feedback to ensure that if the input voltage creeps gently above the threshold, the output is forced smartly and rapidly from one logic state to the other. One of the corollaries of the Schmitt trigger's use of positive feedback is that, should the input voltage move gently down again past the same threshold, the positive feedback will hold the output in the same state with no change. This effect is called
hysteresis: the input voltage has to drop past a different, lower threshold to 'un-latch' the output and reset it to its original digital value. By reducing the extent of the positive feedback, the hysteresis width can be reduced, but it can not entirely be eradicated. The Schmitt trigger is, to some extent, a
latching circuit. File:Positive feedback bistable switch.svg|thumb|Positive feedback is a mechanism by which an output is enhanced, such as protein levels. However, in order to avoid any fluctuation in the protein level, the mechanism is inhibited stochastically (I), therefore when the concentration of the activated protein (A) is past the threshold ([I]), the loop mechanism is activated and the concentration of A increases exponentially if d[A]=k [A]. gates with positive feedback. Red and black mean logical '1' and '0', respectively. An electronic
flip-flop, or "latch", or "bistable
multivibrator", is a circuit that, due to high positive feedback is not stable in a balanced or intermediate state. Such a bistable circuit is the basis of one
bit of electronic
memory. The flip-flop uses a pair of amplifiers, transistors, or logic gates connected to each other so that positive feedback maintains the state of the circuit in one of two unbalanced stable states after the input signal has been removed until a suitable alternative signal is applied to change the state. Computer
random access memory (RAM) can be made in this way, with one latching circuit for each bit of memory.
Thermal runaway occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.
Audio and
video systems can demonstrate positive feedback. If a
microphone picks up the amplified sound output of
loudspeakers in the same circuit, then howling and screeching sounds of
audio feedback (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and
filtered by the characteristics of the audio system and the room.
Audio and live music Audio feedback (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a
microphone or
guitar pickup) and an audio output (for example, a loudly-amplified
loudspeaker). In this example, a signal received by the microphone is
amplified and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The
frequency of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small
PA systems the sound is readily recognized as a loud squeal or screech. Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a
sound reinforcement system or
PA system.
Audio engineers use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s,
electric guitar players in
rock music bands using loud
guitar amplifiers and
distortion effects have intentionally created guitar feedback to create a desirable musical effect. "
I Feel Fine" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive
feedback note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way." The principles of audio feedback were first discovered by Danish scientist
Søren Absalon Larsen. Microphones are not the only transducers subject to this effect.
Phone cartridges can do the same, usually in the low-frequency range below about 100 Hz, manifesting as a low rumble.
Jimi Hendrix was an innovator in the intentional use of guitar feedback in his
guitar solos to create unique sound effects. He helped develop the controlled and musical use of audio feedback in
electric guitar playing, and later
Brian May was a famous proponent of the technique.
Video Similarly, if a
video camera is pointed at a
monitor screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the first ten seasons of the television program
Doctor Who.
Switches In
electrical switches, including
bimetallic strip based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.
In biology (3). Oxytocin is then carried via the bloodstream to the
uterus (4), causing contractions, pushing the fetus towards the cervix, eventually inducing childbirth.
In physiology A number of examples of positive feedback systems may be found in
physiology. • One example is the onset of
contractions in
childbirth, known as the
Ferguson reflex. When a contraction occurs, the hormone
oxytocin causes a nerve stimulus, which stimulates the
hypothalamus to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in
amplitude and
frequency. • Another example is the process of
blood clotting. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.
Immune system A
cytokine storm, or
hypercytokinemia is a potentially fatal immune reaction consisting of a positive feedback loop between
cytokines and
immune cells, with highly elevated levels of various cytokines. In normal immune function, positive feedback loops can be utilized to enhance the action of B lymphocytes. When a B cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B cell's antibodies can bind to a pathogen, and when a B cell has its antibodies bind to a pathogen with C3, it speeds up that B cell's secretion of more antibodies and more C3, thus creating a positive feedback loop.
Cell death Apoptosis is a
caspase-mediated process of cellular death, whose aim is the removal of long-lived or damaged cells. A failure of this process has been implicated in prominent conditions such as
cancer or
Parkinson's disease. The very core of the apoptotic process is the auto-activation of caspases, which may be modelled via a positive-feedback loop. This positive feedback exerts an auto-activation of the
effector caspase by means of intermediate caspases. When isolated from the rest of the apoptotic pathway, this positive feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.
In psychology Winner (1996) described gifted children as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on.
Market dynamics According to the theory of
reflexivity advanced by
George Soros, price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction.
In social media Programs such as
Facebook and
Twitter depend on positive feedback to create interest in topics and drive the take-up of the media. In the age of smartphones and social media, the feedback loop has created a craze for virtual validation in the form of likes, shares, and FOMO (fear of missing out). This is intensified by the use of bots which are designed to respond to particular words or themes and transmit posts more widely. What is called negative feedback in social media should often be regarded as positive feedback in this context. Outrageous statements and negative comments often produce much more feedback than positive comments.
Systemic risk Systemic risk is the risk that an amplification or leverage or positive feedback process presents to a system. This is usually unknown, and under certain conditions, this process can amplify exponentially and rapidly lead to destructive or
chaotic behaviour. A
Ponzi scheme is a good example of a positive-feedback system: funds from new investors are used to pay out unusually high returns, which in turn attract more new investors, causing rapid growth toward collapse.
W. Brian Arthur has also studied and written on positive feedback in the economy (e.g. W. Brian Arthur, 1990).
Hyman Minsky proposed a theory that certain credit expansion practices could make a market economy into "a deviation amplifying system" that could suddenly collapse, sometimes called a
Minsky moment. Simple systems that clearly separate the inputs from the outputs are not prone to
systemic risk. This risk is more likely as the complexity of the system increases because it becomes more difficult to see or analyze all the possible combinations of variables in the system even under careful stress testing conditions. The more efficient a complex system is, the more likely it is to be prone to systemic risks because it takes only a small amount of deviation to disrupt the system. Therefore, well-designed complex systems generally have built-in features to avoid this condition, such as a small amount of friction, or resistance, or inertia, or time delay to decouple the outputs from the inputs within the system. These factors amount to an inefficiency, but they are necessary to avoid instabilities. The
2010 Flash Crash incident was blamed on the practice of
high-frequency trading (HFT), although whether HFT really increases systemic risk remains controversial.
Human population growth Agriculture and human population can be considered to be in a positive feedback mode, which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly efficient monocultures, which are more susceptible to
systemic risk. Technological innovation and human population can be similarly considered, and this has been offered as an explanation for the apparent
hyperbolic growth of the human population in the past, instead of a simpler
exponential growth. It is proposed that the growth rate is accelerating because of second-order positive feedback between population and technology. Technological growth increases the
carrying capacity of land for people, which leads to a growing population, and this in turn drives further technological growth.
Prejudice, social institutions and poverty Gunnar Myrdal described a
vicious circle of increasing inequalities and poverty, which is known as
circular cumulative causation. James Moody, Assistant Professor at
Ohio State University, states that students who
self-segregate or grow up in segregated environments have "little meaningful exposure to other races because they never form relationships with students of another race...[; as a result,...] they are viewing other racial groups at a social distance, which can bolster stereotypes," which ultimately causes a positive feedback loop in which segregated groups become more prejudiced, polarized, and segregated against each other, similar to that of
political polarization.
In meteorology Drought intensifies through positive feedback. A lack of rain decreases soil moisture, which kills plants or causes them to release less water through
transpiration. Both factors limit
evapotranspiration, the process by which water vapour is added to the atmosphere from the surface, and add dry dust to the atmosphere, which absorbs water. Less water vapour means both low
dew point temperatures and more efficient daytime heating, decreasing the chances of humidity in the atmosphere leading to cloud formation. Lastly, without clouds, there cannot be rain, and the loop is complete.
In climatology Climate
forcings may push a climate system in the direction of warming or cooling, for example, increased atmospheric concentrations of
greenhouse gases cause warming at the surface. Forcings are external to the climate system and feedbacks are internal processes of the system. Some feedback mechanisms act in relative isolation to the rest of the climate system while others are tightly coupled. Forcings, feedbacks and the dynamics of the climate system determine how much and how fast the climate changes. The main positive feedback in
global warming is the tendency of warming to increase the amount of water vapour in the atmosphere, which in turn leads to further warming. The main negative feedback comes from the
Stefan–Boltzmann law, the amount of heat radiated from the Earth into space is proportional to the fourth power of the temperature of Earth's surface and atmosphere. Other examples of positive feedback subsystems in climatology include: • A warmer atmosphere melts ice, changing the
albedo (surface reflectivity), which further warms the atmosphere. • Methane hydrates can be unstable so that a warming ocean could release more
methane, which is also a greenhouse gas. •
Peat, occurring naturally in
peat bogs, contains carbon. When peat dries it
decomposes, and may additionally burn. Peat also releases
nitrous oxide. • Global warming affects the cloud distribution. Clouds at higher altitudes enhance the greenhouse effect, while low clouds mainly reflect back sunlight, having opposite effects on temperature. The
Intergovernmental Panel on Climate Change (IPCC)
Fourth Assessment Report states that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."
In sociology A
self-fulfilling prophecy is a social positive feedback loop between beliefs and behaviour: if enough people believe that something is true, their behaviour can make it true, and observations of their behaviour may in turn increase belief. A classic example is a
bank run. Another sociological example of positive feedback is the
network effect. When more people are encouraged to join a network, this increases the reach of the network, therefore the network expands ever more quickly. A
viral video is an example of the network effect in which
links to a popular video are shared and redistributed, ensuring that more people see the video and then re-publish the links. This is the basis for many social phenomena, including
Ponzi schemes and
chain letters. In many cases, population size is the limiting factor to the feedback effect.
In political science In politics, institutions can reinforce norms, which can subsequently be a source of positive feedback. This rationale is frequently utilized to comprehend public policy processes, which may be dissected into a sequence of events. Self-reinforcing processes are understood to be affected by positive feedback mechanisms (e.g., supportive policy constituencies). Conversely, unsuccessful policy processes encounter negative feedback mechanisms (e.g., veto points with veto power). A comparative illustration of policy feedback can be observed in the economic foreign policies of Brazil and China, particularly in their execution of state capitalism tactics during the 1990s and 2000s. Although both nations initially embraced similar state capitalist ideas, their paths in executing economic policies diverged over time due to distinct incentives. In China, a positive feedback mechanism reinforced previous policies, whereas in Brazil, negative feedback mechanisms compelled the country to abandon state capitalism policies and dynamics.
In chemistry If a chemical reaction causes
the release of heat, and the reaction itself
happens faster at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough,
thermal runaway can occur and very quickly lead to a chemical
explosion.
In conservation Many wildlife are hunted for their parts, which can be quite valuable. The closer to extinction that targeted species become, the higher the price there is on their parts. == See also ==