Positive feedback

Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a <a href="/facts/Feedback_loop/XnuWH3zp">feedback loop</a> where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system in which the results of a change act to reduce or counteract it has <a href="/facts/Negative_feedback/G9byVgpN">negative feedback</a>. Both concepts play an important role in science and engineering, including biology, chemistry, and <a href="/facts/Cybernetics/4qb58zbt">cybernetics</a>.
Mathematically, positive feedback is defined as a positive <a href="/facts/Loop_gain/h2E89W7B">loop gain</a> around a closed loop of cause and effect.
That is, positive feedback is <a href="/facts/Phase_(waves)/cpGtuVll">in phase with</a> the input, in the sense that it adds to make the input larger.
Positive feedback tends to cause <a href="/facts/Control_theory/aIuERpn6">system instability</a>. When the loop gain is positive and above 1, there will typically be <a href="/facts/Exponential_growth/m4yEqKJ7">exponential growth</a>, increasing <a href="/facts/Oscillation/6vSrm84a">oscillations</a>, <a href="/facts/Chaotic_behavior/LTC17vsF">chaotic behavior</a> or other divergences from equilibrium. System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system <a href="/facts/Latch_(electronics)/vA99xjM7">latched</a> into a new stable state. Positive feedback may be controlled by signals in the system being <a href="/facts/Filter_(signal_processing)/vydJCNle">filtered</a>, <a href="/facts/Damped/UW3RsjEd">damped</a>, or <a href="/facts/Maxima_and_minima/ADlgnyzV">limited</a>, or it can be cancelled or reduced by adding negative feedback.
Positive feedback is used in <a href="/facts/Digital_electronics/3kJcKbkw">digital electronics</a> to force voltages away from intermediate voltages into '0' and '1' states. On the other hand, <a href="/facts/Thermal_runaway/r5F7nfeY">thermal runaway</a> is a type of positive feedback that can destroy <a href="/facts/P%E2%80%93n_junction/CaKIdLPg">semiconductor junctions</a>. Positive feedback in <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reactions</a> can increase the rate of reactions, and in some cases can lead to <a href="/facts/Explosives/sUme5Pp8">explosions</a>. Positive feedback in mechanical design causes <a href="/facts/Tipping_point_(physics)/9M2pwR62">tipping-point</a>, or over-centre, mechanisms to snap into position, for example in <a href="/facts/Miniature_snap-action_switch/I0DEYXBG">switches</a> and <a href="/facts/Locking_pliers/sYQhqo3R">locking pliers</a>. Out of control, it can cause <a href="/facts/Tacoma_Narrows_Bridge_(1940)/qHJsPjTm">bridges to collapse</a>. Positive feedback in economic systems can cause <a href="/facts/Business_cycle/etH0t44s">boom-then-bust cycles</a>. A familiar example of positive feedback is the loud squealing or howling sound produced by <a href="/facts/Audio_feedback/NaMWquWb">audio feedback</a> in <a href="/facts/Public_address_system/LlLkJgNz">public address systems</a>: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.

Positive feedback open-in-new

Positive feedback