THE CATHODALLY SCREENED CATHODE FOLLOWER 



HT + 



out 



Figure 18.3 



If the vector grid-cathode voltage is V, the valve current must be g„y. 

 The output voltage produced by this current is 



= a V- ^ 



The current through Q, must beyco V . C^, and if the grid current is negligible, 

 this current must flow through i?in, producing across it a potential difference 

 j(oV . Cg . 7?in. The total input voltage must therefore be 



therefore 



1 +JojRj^C„ 



gmR 



K 



\+jayRj^C, 



l+>i?^C + 1 +JcoC,Ri 



in 



^m^ 



1 



e: 



g,,RK + 1 - 0)2 Q Q /?in /?^^ +yco(C,/?io + Q i?A') 



Neglecting 1 in comparison with gm^K^ we now consider C'R', the geometric 

 mean of C^Rk and Q/^in, and define a so that QR^ = (!/«) ^'■^' and 

 C,/?in = a C'R'. Then, letting C'R' = l/co„ 



out 



in 



grn^K 7T 



1 





1 1/2 



This has a maximum near the frequency where (colco^Y — gm^K^ i-^- 

 where 



1/2 

 I /^ ni I 



CO 



gni \ 



.CoCsR-mJ 



260 



