EFFECT OF FEEDBACK ON IMPEDANCE 271 



the normal condition of the network these terminals may be connected 

 through an external impedance branch. This is the case, for example, when 

 terminals 1,1' are the input terminals of a feedback amplifier whose input 

 impedance is under investigation. However, this external impedance may 

 also be zero or infinite according as terminals 1, 1' are "mesh-terminals" 

 obtained by breaking open a mesh of the network, or "junction-terminals" 

 obtained by bringing out two junctions of the network. 



It is assumed that the network, including all of the vacuum tubes, is a 

 linear system in which, therefore, the Superposition Principle holds. Hence, 

 if an e.m.f. £i is applied in series with terminals 1, 1' and a second e.m.f. 

 £2 is applied between the grid and the cathode of the tube, the potential 

 difference Vi developed across the input terminals 1,1' and the potential 

 difference V2 developed between the terminal 2 and the cathode of the tube 

 will be linearly related to Ei and £2. If the source of £1 has internal im- 

 pedance the coefficients in these relations will depend upon this impedance. 

 However, if the input current /i is used as an independent variable in place 

 of the e.m.f. £1 the coefficients will not depend upon the impedance of the 

 source of the current 7i. It is also convenient to consider the potential 

 difference £2 — V2 developed across the terminals 2, 2' as one of the de- 

 pendent variables in place of V2. Therefore, 



Vi = Ah + BE2 

 E2- Vi = CI I + DE2 



where the coefficients are independent of Z. 

 From these equations we obtain 



(1) 



- BC 



/FA _ AD- 



V/iA2=r, D 



(r) -^ 



AD - BC 



Hence 



/ £2 - V2 \ 



\ £2 /fi=0 



\ £2 //i=0 



(h) 1 _ (}^) 



\/l/^2-0 \E2/,, 



(2) 



