. The Bell System technical journal . g up and adjusting the apparatus. Test Amplifier and Experimental ResultsTest Amplifier The stability criterion indicates three distinct conditions of in-terest, one of which is unstable, the other two being stable. The un-stable condition (1) is that in which the transfer factor curve enclosesthe point (1, 0). Two stable conditions are those in which (1, 0) is notenclosed by the curve, but in which (2) the curve crosses the zero phaseshift axis at points greater than unity, and (3) the curve does not crossthe zero phase shift axis at points greater than u
. The Bell System technical journal . g up and adjusting the apparatus. Test Amplifier and Experimental ResultsTest Amplifier The stability criterion indicates three distinct conditions of in-terest, one of which is unstable, the other two being stable. The un-stable condition (1) is that in which the transfer factor curve enclosesthe point (1, 0). Two stable conditions are those in which (1, 0) is notenclosed by the curve, but in which (2) the curve crosses the zero phaseshift axis at points greater than unity, and (3) the curve does not crossthe zero phase shift axis at points greater than unity. Condition (2)is of particular interest because while it is judged stable on the basisof Nyquists criterion it would appear to be unstable on the basis of theolder transfer criterion discussed in the first and second sections. For test purposes an amplifier was designed which, upon variationof an attenuator in the feed-back path, would satisfy each of the threeabove conditions in turn. The amplifier schematic is shown in Fig. Fig. 9—Circuit diagram of the feed-back amplifier used in testing the stabilitycriterion. The dashed line indicates the point at which the loop was broken formeasurement of the transfer factor. At the left of this line is shown the resistanceattenuator provided to vary the gain around the feed-back loop. 694 BELL SYSTEM TECHNICAL JOURNAL It has three stages, the first two tubes being space charge grid pentodes,and the last one a triode. The interstage coupHng circuits were madeup of simple inductances and resistances as shown. The amplifier wasdesigned by E. L. Norton and E. E. Aldrich to provide a transfer factorcharacteristic having the desired shape, , a loop crossing the zerophase axis in the neighborhood of 10 kilocycles. It will be observedthat the feed-back circuit is connected between bridge networks inboth input and output circuits, which were provided to eliminate reac-tion of the input and output circuits upon the feed-back networ
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