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http://en.wikipedia.org/wiki/Positive_feedback

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License

# Positive feedback

Positive feedback is a feedback system in which the system responds to the perturbation in the same direction as the perturbation (It is sometimes referred to as cumulative causation). In contrast, a system that responds to the perturbation in the opposite direction is called a negative feedback system. The term "positive" means responding to the same direction as the perturbation whereas "negative" means responding to the opposite direction.

The end result of a positive feedback is often amplifying and "explosive." That is, a small perturbation will result in big changes. This feedback, in turn, will drive the system even further away from its own original setpoint, thus amplifying the original perturbation signal, and eventually become explosive because the amplification often grows exponentially (with the first order positive feedback), or even hyperbolically (with the second order positive feedback). It is the vicious cycle phenomenon. An intuitive example is "the rich get richer, and the poor get poorer."

Both positive and negative feedback are closed systems. They are called "closed systems" because the system is closed by a feedback loop, i.e., the response of the system depends on the feedback signal to complete its function; without such a loop, it would become an open system. In contrast, a feed-forward system is an "open system" since it does not have any feedback loop, and does not rely on feedback signal to perform its function.

Examples of positive and negative feedback, open and closed systems can be found in ecological, biological, social systems and in engineering control systems such as servo control systems.

## Explanation

When a change of variable occurs in a system, the system responds. In the case of positive feedback the response of the system is to change that variable even more in the same direction. For a simple example, imagine an ecosystem with only one species and an unlimited amount of food. The population will grow at a rate proportional to the current population, which leads to positive feedback. This has a de-stabilizing effect, so left unchecked, does not result in homeostasis. In some cases (if not controlled by negative feedback), a positive feedback loop can run out of control, and can result in the collapse of the system. This is called vicious circle, or in Latin circulus vitiosus.

Positive feedback does not necessarily imply a runaway process; it may just have an amplifying effect. An example of this is the role of water vapour in amplifying global warming; higher global temperatures lead to increased water vapour in the atmosphere, which pushes up temperatures further, and so on, but the overall effect is that of a convergent series, amplifying the original temperature rise by a relatively constant factor. But at the point of convergence, the total feedback sums up to zero or less.

Positive and negative do not mean or imply desirability. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to speed it up. Positive feedback is used in certain situations where rapid change is desirable.

One common example of positive feedback is the network effect, where more people are encouraged to join a network the larger that network becomes. The result is that the network grows more and more quickly over time.

## In electronics

Feedback is the process of sampling a part of the output signal and applying it back to the input. This technique is useful to change the parameters of an amplifier like voltage gain,input and output impedance,stability and bandwidth.

Feedback is said to be positive if any increase in the output signal results in a feedback signal which on being mixed with the input signal caused further increase in the magnitude of the output signal. Hence it is also called regenerative feedback. Positive feedback is in the same phase as the input signal,therefore the final gain of the amplifier(Af) increases.

Final gain Af=(output voltage/input voltage)=A/(1-Aß). Here A is the gain of the amplifier without feedback, and ß is the feedback factor

• Gain increases
• Bandwidth decreases

• Gain can tend to be unstable
• There is higher distortion
• Bandwidth decreases
• Stability is difficult or impossible to guarantee

### Applications

Positive feedback is used extensively in oscillators and in regenerative radio receivers and Q multipliers

Audio feedback is a common example of positive feedback. It is the familiar squeal that results when sound from loudspeakers enters a poorly placed microphone and gets amplified, and as a result the sound gets louder and louder.

## In games

In games, positive feedback is a critical and heavily exploited mechanism for controlling the resources in a game. It has a number of uses:

• To speed up a game that would otherwise be too slow. For example, if the annual income did not increase in SimCity as the city grew, it would take many years to earn enough money to fill the large map with structures.
• To create a feeling of growth and progress. For example, in a role-playing game, it's typical for players to struggle with enemies near the beginning that later become easy to destroy due to enhanced strength and weapons, purchased with the experience and gold earned by those early encounters.
• To magnify small advantages. For example, in StarCraft, a player who has more resources will be able to build more units, enabling them to seize more resource-rich territory and so gain yet more resources. This allows a player with a small resource advantage to crush their opponent in time.

However, accidental positive feedback loops in games can also be a source of degenerate strategies, destroying the game's challenge. For example, suppose a player in a first-person shooter gained 100 health points for every person they killed. Then, a careful player could quickly amass a large number of health points and become virtually indestructible. This is one reason that most FPS games place a limit on the maximum health a player can have.

## In the world system development

The hyperbolic growth of the world population observed till the 1970s has recently been correlated to a non-linear second order positive feedback between the demographic growth and technological development that can be spelled out as follows: technological growth - increase in the carrying capacity of land for people - demographic growth - more people - more potential inventors - acceleration of technological growth - accelerating growth of the carrying capacity - the faster population growth - accelerating growth of the number of potential inventors - faster technological growth - hence, the faster growth of the Earth's carrying capacity for people, and so on (see, e.g., Introduction to Social Macrodynamics by Andrey Korotayev et al.).

## Echo Chamber Effect

Metaphorically, cumulative causation may emerge on the Internet as an echo chamber effect, which refers to any situation in which information or ideas are amplified by transmission inside an enclosed space. Another emerging term used to describe this "echoing" and homogenizing effect on the Internet within social communities is "cultural tribalism".

The Internet may be seen as a complex system (e.g., emergent, dynamic, evolutionary), and as such, will at times eluminate the effects of positive feedback loops (i.e., the echo-chamber effect) to that system, where a lack of perturbation to dimensions of the network, prohibits a sense of equilibrium to the system. Complex systems that are characterized by negative feedback loops will create more stability and balance during emergent and dynamic behaviour.

For example, observers of journalism in the mass media describe an echo chamber effect in media discourse. One purveyor of information will make a claim, which many like-minded people then repeat, overhear, and repeat again (often in an exaggerated or otherwise distorted form) until most people assume that some extreme variation of the story is true.

Due to this condition arising in online communities, participants may find their own opinions constantly echoed back to them, and in doing so reinforce a certain sense of truth that resonates with individual belief systems. This can create some significant challenges to critical discourse within an online medium. The echo-chamber effect may also impact a lack of recognition to large demographic changes in language and culture on the Internet if individuals only create, experience and navigate those online spaces that reinforce their "preferred" world view.

## In biology

One example of a biological positive feedback loop is the onset of contractions in childbirth. When a contraction occurs, the hormone oxytocin is released into the body, which stimulates furthur contractions. This results in contractions increasing in amplitude and frequency.

Another example of a biological positive feedback loop is the process of blood clotting. The loop is initiated when injured tissue releases signal chemicals which activate platelets in the blood. An activated platelet releases chemicals which activate more platelets, causing a rapid cascade and the formation of a blood clot.

In most cases, once the purpose of the feedback loop is completed, counter-signals are released which suppress or break the loop.

## References

• Katie Salen and Eric Zimmerman. Rules of Play. MIT Press. 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.

• Donella Meadows' twelve leverage points to intervene in a system
• Stability criterion
• Virtuous circle and vicious circle