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BELL SYSTEM TECHNICAL JOURNAL 



well as by the removal of any impedance elements or circuits which are 

 normally connected to the amplifier. 



These dii3ficulties are avoided by the method of derivation adopted in this 

 paper. Illustrative examples are then given of some of the uses to which the 

 general relationship between feedback and impedance may be put. 



Derivation 



The derivation of the general relationship between feedback and im- 

 pedance will be made here with reference to the diagram shown in Fig. 1. 



One of the vacuum tubes in the network, namely that one to which the 

 feedback is to be referred, is shown explicitly at the top of the box in the 

 diagram. The grid lead to this tube is broken at terminals 2, 2'. In prac- 

 tice, the break in the grid lead would leave the grid still coupled to some 



Fig. 1 — Relation between feedback and impedance. 



degree to the other electrodes of the tube through parasitic interelectrode 

 admittance. For analytical purposes, however, it may be assumed that the 

 parasitic admittances between the grid and the other electrodes of the tubes 

 are connected not directly to the grid within the tube but to some point 

 farther out along the grid lead. Under this assumption the break in the 

 grid lead not only removes the feedback to the tube completely, but also 

 leaves the parasitic admittances connected in the network in such a way 

 that their contribution to the feedback is implicitly taken into account. 

 Furthermore, the impedance looking into the grid of the tube is now infinite 

 so that if a voltage is applied to the grid no current will be drawn from the 

 source of the voltage. 



At the left-hand side of the box in the diagram, terminals 1,1' are brought 

 out. These are the terminals to which the impedance is to be referred. In 



