Positive feedback

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Positive feedback

Written by Eng.Admin

Wednesday, 05 November 2008 06:46

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.
A system in which there is positive feedback to any change in its current state is said to be in an unstable equilibrium, whereas one with negative feedback is said to be in a stable equilibrium.
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). Indeed, chemical and nuclear fission based explosives offer an excellent physical demonstration of positive feedback. Bombarding fissile material with neutrons causes it to emit even more neutrons, which in turn affect the material. The greater the mass of fissile material, the larger the amplification, resulting in greater feedback. If the amplification is great enough, the process accelerates until the fissile material is spent or dispersed by the resulting explosion.
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.
Consider a linear amplifier with linear feedback. As long as the loop gain, i.e. the forward gain multiplied with the feedback gain, is lower than 1 the result is a stable (convergent) output. This is of course always true for a negative feedback but also for lower positive feedbacks. In electronic amplifiers the normal case is that the forward gain is quite high and the amplifier gets instable for quite small positive feedbacks.
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

Disadvantages

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 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 further 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.

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