THE CATHODALLY SCREENED CATHODE FOLLOWER 



and 



out 



Fi 



in 



a+ \(a 



a -\- Xfa has a rather blunt minimum value of 2 when a = 1. Thus, if the 

 two time constants CqRk and Q/?in are approximately equal, the gain rises 

 to (gmRKy'^12 . gm^K is typically 100, so the gain will rise to 5. 



Some curves for this circuit for g,„RK = 100 and various a are plotted in 

 Graph 33. Evidently to avoid the hump in the characteristic a must be 

 > 1 or < 1 , but it does not much matter which ; that is, C^R^ and C^Rij^ must 

 be dissimilar. The turn-over frequency of the device, for a given valve and 

 electrode, is inversely proportional to the product of €„ and C^. 



Transient response 



For the analysis which follows I am indebted to Professor A. L. Hodgkin. 

 In Figure 18.4 



HT+ 



Close at 



i = Rin 



,\ 







y 



HT- 



Figure 18.4 



If u and V are departures from the initial voltages, 



du 

 E=v + u + RinC, -^ 



^ dw , ^ df , y 



dt 



dt ' R, 



....(1) 



....(2) 



Combining 



dh 



(RinCsRKCo) ^ + (RinCs + RrCo + i?A'Q ;^ + {g^nRR + 0^ 



dv 

 dt 



= gm^KE + RrCs 



dE 



'dt 



The roots are 



-(/?i,Q + Rj^C, + R^C) ± 



2 RiuC^Rk^o 

 Att = 0, V ^0 and dv/dt = £'/(7?inQ. As ordinarily used, Rin ^ R^ so 



261 



