- Loop gain
In
negative feedback amplifier s, loop gain refers to the product of the gain in the feedback loop and the feedback factor in that loop. In Figure 1, the loop gain is the product β"A"OL.In
telecommunication , the term loop gain has the following meanings:1. The total usable power
gain of a carrier terminal or two-wirerepeater ."Note:" The maximum usable gain is determined by, and may not exceed, the losses in the closed path.
2. The sum of the gains, expressed in dB, acting on a signal passing around a closed path, "i.e.", a loop.
Loop gain is an engineering term used to quantify the gain of a system controlled by feedback loops. Respiration is in fact such a system. The gain being influenced by controller variables (hypercapnic and hypoxic responsiveness), the ability to eliminate CO2)and circulation time, among others. Simply put, loop gain can be defined as the response of a system divided by the disturbance leading to that response:
Loop gain = response to a disturbance / the disturbance itself
It should be evident from this formula that ventilation will become quite unstable when loop gain is more than 1. If theresponse is always greater than the disturbance, then ventilation can never stabilize and will wax and wane indefinitely. Thus, the higher the loop gain (the closer it is to 1), the potentially more unstable the respiratory control system will become. In sleep medicine the term loop gain describes the respiratory insetability. Ventilatory insetability is likely to be most important in patients with apnoea with only a mild pharyngeal anatomical deficiency. If these individuals have a high loop gain, then the fluctuating upper airway resistance which occurs at sleep onset may be sufficiently destabilising that ventilation will begin to cycle leading to further loss of pharyngeal muscle activation and frank airway collapse (DP White, 2006; RB Fogel 2004).
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