Positive feedback

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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 gets richer, and the poor gets 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 feedforward 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.

1 Explanation
2 In electronics
3 In games
4 In the world system development
5 References
6 See also
7 External Links

[edit] 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 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.

[edit] 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(A_f) increases.

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

[edit] Advantages

Gain increases
Bandwidth decreases

[edit] Disadvantages

Gain can tend to be unstable
There is higher distortion
Bandwidth decreases

[edit] 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.

[edit] 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.

[edit] 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.).