Positive feedback

Positive feedback is a type of feedback. Open systems (ecological, biological, social) contain many types of regulatory systems, among which are systems that involve positive feedback and its relative negative feedback.

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 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

Advantages: Gain increases

Disadvantages:
 * 1) Gain with feedback is unstable as it is affected by changes in temperature.
 * 2) There is higher rate of distortion

Application: Positive feedback is used extensively in oscillators.

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.

Source

 * Katie Salen and Eric Zimmerman. Rules of Play. MIT Press. 2004. ISBN 0262240459. Chapter 18: Games as Cybernetic Systems.