Editing Positive feedback (section)

=== Basic ===
[[File:Ideal feedback model.svg|thumb|A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary [[causal system|causal]] functions.]]

A simple feedback loop is shown in the diagram.  If the loop gain AB is positive, then a condition of ''positive'' or ''regenerative'' feedback exists.

If the functions A and B are linear and AB is smaller than unity, then the overall system gain from the input to output is finite but can be very large as AB approaches unity.<ref name=smith>Electronics circuits and devices second edition. Ralph J. Smith</ref>  In that case, it can be shown that the overall or loop gain from input to output is:

:<math>G_c = A/(1-AB)</math>

When AB > 1, the system is unstable, so does not have a well-defined gain; the gain may be called infinite.

Thus depending on the feedback, state changes can be convergent, or divergent.  The result of positive feedback is to [[wikt:augment|augment]] changes, so that small perturbations may result in big changes.

A system in equilibrium in which there is positive feedback to any change from its current state may be unstable, in which case the system is said to be in an [[unstable equilibrium]]. The magnitude of the forces that act to move such a system away from its equilibrium is an [[increasing function]] of the ''distance'' of the state from the equilibrium.

Positive feedback does not necessarily imply instability of an equilibrium, for example stable ''on'' and ''off'' states may exist in positive-feedback architectures.<ref name="ReferenceA">{{cite journal|last1=Lopez-Caamal|first1=Fernando|last2=Middleton|first2=Richard H.|last3=Huber|first3=Heinrich|title=Equilibria and stability of a class of positive feedback loops|journal=Journal of Mathematical Biology|date=February 2014|pages=609–645|doi = 10.1007/s00285-013-0644-z|pmid=23358701|volume=68|issue=3|s2cid=2954380}}</ref>