what is a positive feedback loop. also a negative loop.?

Question

please give simple example

Answer 1

These terms can be used when talking about lots of different applications from electrical circuits to power plants to human interactions. I'm going to describe them as they are used in control systems.

In a positive feedback loop, part of the output is fed back to the input in such a way that it increases the input.

One example of this in a sound system. You take a microphone and put it near the speaker, and then speak into the mic. The system takes your voice, amplifies it, and send it out the speaker. Since the mic is near the speaker, it picks up the sound from the speaker, and amplifies it further. The end result is the loud whining noise you get when the system maxes out.

In a negaitive feedback loop, part of the output is fed back to the input in such a way that it decreases the input.

A good example of this could be a heater with a thermostat. As temperature goes up, the controller turns down the amount of current flowing through the heater coil. This causes the temperature to go down. When the temperature goes down, the controller increases the current into the heater coil. This causes the heater output temperature to stabilize at the temperature set by the thermostat.

Answer 2

The example cited in the first answer is more appropriate to the description of a servo loop.

A simple explanation of positive feedback is that part of an output of a device or system is fed to its input such that the overall output increases.

Negative feedback, on the other hand, takes part of an output and feeds it to the input such that the overall output decreases.

A simple example of positive feedback, acoustic feedback in this case, is the awful noise produced by an audio system when a microphone is too close to a loudspeaker.

A simple example of negative feedback is more difficult to find.
Perhaps this might help. If an audio amplifier didn't have negative feedback its gain would be very high, unpredictable, and dependent upon the individual characteristics of the active devices in its circuit.
The application of negative feedback allows the designer to define the gain, which will be lower than otherwise, to a figure of his choice.
Negative feedback also has other advantages, but that's another story.

Answer 3

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.

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.

The term 'negative' do not refer to desirability, but rather to the sign of the multiplier in the mathematical feedback equation. When a change of variable occurs, a negative feedback system will attempt to re establish equilibrium.

Negative feedback is used in this way in many types of amplification systems to stabilise and improve their amplification characteristics (see e.g., operational amplifiers).

While it has many advantages, such as increased stability of the system, NFB also has disadvantages, like loss of gain.

A simple and practical example is a thermostat. When the temperature in a heated room reaches a certain upper limit the room heating is switched off so that the temperature begins to fall. When the temperature drops to a lower limit, the heating is switched on again. Provided the limits are close to each other a steady room temperature is maintained. The same applies to a cooling system, such as an air conditioner, a refrigerator, or a freezer.