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BELL SYSTEM TECHNICAL JOURNAL 



approaches the value rg would assume with large signal inputs than 

 does the static value. Hence, if a large signal input, e, is to be used, 

 the amplitude of the buzzer excitation voltage on the grid should 

 equal this amplitude as nearly as possible. 



When the method of drawing tangents to the static characteristic is 

 employed, a very close approximation to the value of rg to use for 

 large signal amplitudes may be obtained by drawing, not true tangents 

 but secant lines to the static characteristic, which join points on the 

 characteristic corresponding to the extreme, or peak, values of eg. 



When either method is used to obtain Tg, the value of r/ must be 

 obtained by drawing tangents to an Eg — fg curve. 



With the precautions just given, and when the assumptions made 

 in equation (43) are justifiable, an accuracy within 10% is easily 

 obtained. While this is not very exact, nevertheless, it is a real 

 advance over calculations made without taking the precautions just 

 discussed for measuring rg. 



In many vacuum tubes the value of rg is so high that the input 

 impedance of the tube, resulting from the interelectrode capacities 

 of the elements cannot justifiably be neglected. In order to include 

 this effect, the following relations are applicable. 



Consider the circuits shown in Fig. 5. This gives the equivalent 

 circuit diagram for a vacuum tube with general impedances, Si and 



F F F 



Fig. 5 — Equivalent network 



Z2, attached to the grid and plate, respectively. The plate to filament 

 capacity may conveniently be included in z-i- The impedance, ^g^, 

 is the efTective impedance of the network looking to the right from 

 the point G'F. ^g^ is the grid to filament capacity of the tube, and 

 23 is the grid to plate capacity. 

 We may write 



Zg+Zg_ 



