Feedback

When a signal is looped back into the system that produced it, the resulting effect is called feedback.

Feedback can be positive or negative, but these terms do not imply whether the feedback is wanted or not.

Feedback causes the output to depend not only on the current values of the inputs but on what they were in the past, because one input is actually the prior output. This is why an electrical circuit with feedback is called a sequential electric circuit. One main element of sequential electric circuits is the flip flop.

Positive feedback

In general, feedback is positive when the reaction of the system goes in the same direction as the initial signal. The most likely best-known example for this is audio feedback. An example of a non-technical feedback loop is the spreading of a rumor in the mass media: One person makes a plausible claim and other sources repost it, causing yet more sources to post it.

Negative feedback

When the system reacts to a signal by going into the opposite direction, there is a negative feedback. For example, a thermostat reacts to temperature changes by countering the observed trend: When the temperature sinks under the lower border, it increases the temperature; and when the temperature is above the upper border, it reduces the temperature.