Regeneration Theory and Experiment

TN a comparatively recent paper on "Regeneration Theory," 1 Dr. Nyquist presented a mathematical investigation of the conditions under which instability 2 exists in a system made up of a linear amplifier and a transmission path connected between its input and output circuits. The results of the investigation are of interest because of their obvious application to amplifiers provided with feed-back paths, 3 as well as to the starting conditions in oscillators. As a result of his general analysis, Dr. Nyquist arrived at a criterion for stability, expressed in particularly simple and convenient form, which is not restricted in its range of application to particular amplifier and circuit configurations. The great value attached to a criterion as precise and as general as Nyquist's makes it desirable to submit the criterion to an experimental test. One particularly striking conclusion drawn from this criterion is that under certain conditions a feed-back amplifier may sing within certain limits of gain, but either reduction or increase of gain beyond these limits may stop singing. A feed-back amplifier satisfying these conditions was set up, and the experimental results were found to be in agreement with this conclusion. * Published in Proc. I. R. E., October, 1934. 1 Bell. Sys. Tech. Jour., vol. X I , p. 126. 2 Instability is used in the sense that a small impressed force, which dies out in course of time, gives rise to a response which does not die out. 3 Electrical Engineering, July, 1933; Bell Sys.