PRINCIPLE OF FEEDBACK
Conception, which is called feedback in the engineering sphere, is used in increasing frequency in non-engineering fields of knowledge, including biology. Initially the term was used in the conception of the design of automatic control systems. Such kind of systems is often called also feedback-control systems, or closed contour (closed-loop) systems.
Closed contour system is a system, where the true state of a controlled variable (called "output") is continually compared with the desirable state (called "input"), and a signal, which is proportional to the difference between these two states, influences the control element for the purpose of decreasing of the error.
The definition says nothing about the energy, which is necessary to change the output; or about the energy existing at the input. As a rule, small energy is delivered to the input of the control system, and energy for output is obtained from an external power supply. Thus, a peculiarity of the most automatic control systems is considerable increasing of power.