182 



BELL SYSTEM TECHNICAL JOURNAL 



treated as an ordinary a.c. circuit, consisting of the external impedance 

 in series with a resistance Rp, and in which tlie imjiresscd enif. is 

 /ifj, Rp being the internal plate impedance of the tube, y. the amplifica- 

 tion constant of the tube and Cg the voltage applied to the grid. 



In Fig. 1 (b) Cp is the plate to filament capacity of tube I, and the 

 input impedance of tube II is represented by a resistance Rg in parallel 

 with a condenser Cg. 



The maximum amplifiralion wliich can l)e olHained by this amplifier 

 is gi\eii b\- the well-known exi)ression 



A' 



^^\Rp 



but this maximum am|)lil'i 



and 



at ion can onh' be obtained when 

 coLi> >Rp, 

 0)7.2 > >Rg, 

 wL\ (11L2 



(1) 



(2) 



Rp 



R. 



Large reactances o> L\ and w L« can only be obtained at low fre- 

 quencies because at higher fre(|uencies the effects of internal tube 

 capacities and the distributed capacity of the coil become large. This 

 may best be illustrated b\- means of the tabic gi\en below: 



The tube capacity plus the distributed capacity of each of the three 

 coils for which these data are gi\en is assumed to be 10 nnj. Since 

 transformers in order to gi\e a fiat band must work below their 

 natural frequency a much higher impedance than given by it f L in the 

 Table can therefore not be ol)tained. It is thus seen that only at 

 audio frequencies is it possible to build a transformer with an im- 

 pedance which is high compared with the tube resistances, the plate 

 resistance being of the order of 0,(X)0 .50,000 ohms for ordinar>- re- 

 ceiving tubes and the grid resistance Rg being as high as 4X 10" ohms 

 but often limited to .500,000 ohms by an added resistance. 



