Positive feedback

• 1. Defining The Loop

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  A small disturbance in a system can grow into a massive change. This process is called positive feedback. It occurs when the outcome of a process reinforces the inciting process to build momentum. A produces more of B which in turn produces more of A. Such forces exacerbate the effects of a small disturbance. That means the magnitude of the perturbation increases over time. In contrast, negative feedback reduces or counteracts a change. Both concepts play an important role in science and engineering. They appear in biology, chemistry, and cybernetics. Mathematically, positive feedback is defined as a positive loop gain around a closed loop of cause and effect. When the loop gain is positive and above 1, there will typically be exponential growth. This leads to increasing oscillations or chaotic behavior. System parameters accelerate towards extreme values. These extremes may damage or destroy the system. Or they may end with the system latched into a new stable state. Positive feedback tends to cause system instability. It does not necessarily imply instability if stable on and off states exist.

• 2. Electronics And Engineering

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  Regenerative circuits were invented and patented in 1914 for amplifying weak radio signals. Carefully controlled positive feedback around a single transistor amplifier can multiply its gain by 1,000 or more. A signal can be amplified 20,000 times in one stage. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable. They start to oscillate. The radio operator has to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the superheterodyne design instead. They have many more amplification stages but much more stable operation. The oscillation that breaks out in a regenerative radio circuit is used in electronic oscillators. By using tuned circuits or a piezoelectric crystal like quartz, the signal remains linear and sinusoidal. There are several designs for such harmonic oscillators including the Armstrong oscillator and the Hartley oscillator. Thermal runaway occurs in electronic systems because some aspect of a circuit passes more current when it gets hotter. The hotter it gets, the more current it passes. This heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. Positive feedback is also used in digital electronics to force voltages away from intermediate values into '0' and '1' states.

• 3. Biology And Physiology

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  When a contraction occurs during childbirth, the hormone oxytocin causes a nerve stimulus. This stimulates the hypothalamus to produce more oxytocin which increases uterine contractions. Contractions increase in amplitude and frequency until the baby is born. Another example is the process of blood clotting. Injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets. This causes a rapid cascade and the formation of a blood clot. Lactation also involves positive feedback as the baby suckles on the nipple. A nerve response goes into the spinal cord and up into the hypothalamus of the brain. This stimulates the pituitary gland to produce more prolactin to produce more milk. A spike in estrogen during the follicular phase of the menstrual cycle causes ovulation. The generation of nerve signals is another example where slight initial leakage results in an explosion of sodium leakage. This creates the nerve action potential known as the Hodgkin cycle. Most such feedback loops culminate in counter-signals being released that suppress or break the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot.

• 4. Economics And Social Dynamics

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  Markets with social influence show how product recommendations influence consumers' choices significantly. Popular products tend to become even more popular through this mechanism. George Soros advanced the theory of reflexivity where price changes are driven by a positive feedback process. Investors' expectations are influenced by price movements so their behavior acts to reinforce movement in that direction. Until it becomes unsustainable, the feedback drives prices in the opposite direction. A classic sociological example is a bank run. When more people are encouraged to join a network this increases the reach of the network. The network expands ever more quickly. Programs such as Facebook and Twitter depend on positive feedback to create interest in topics. In the age of smartphones and social media, the feedback loop has created a craze for virtual validation. Outrageous statements and negative comments often produce much more feedback than positive comments. Systemic risk is the risk that an amplification or leverage presents to a system. It can amplify exponentially and rapidly lead to destructive or chaotic behavior. 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 attract more new investors. This causes rapid growth toward collapse. W. Brian Arthur studied positive feedback in the economy.

• 5. Environment And Climate

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  Drought intensifies through positive feedback when lack of rain decreases soil moisture. Plants die or release less water through transpiration. Both factors limit evapotranspiration and add dry dust to the atmosphere. Less water vapour means both low dew point temperatures and more efficient daytime heating. Without clouds there cannot be rain and the loop is complete. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapour in the atmosphere. This leads to further warming. Other examples include a warmer atmosphere melting ice which changes the albedo surface reflectivity. This further warms the atmosphere. Methane hydrates can be unstable so that a warming ocean could release more methane. 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 cloud distribution where clouds at higher altitudes enhance greenhouse effects. The Intergovernmental Panel on Climate Change Fourth Assessment Report states that anthropogenic warming could lead to some effects that are abrupt or irreversible depending upon the rate and magnitude of climate change.