CLASSICAL FILTER 



By inspection, we can write down that the transmission characteristic of 



the network is 



1 

 R 



j(oC 



Kout 



^ + 7 



1 



JojC 



in 



R 



jcoL + 



pC 



which simphfies to 



^out 



1 

 joiC 



1 



m 



1+^J^_^2^C 



R 



Now let us put in some typical values. If the apparatus consumes 100 

 milliamps at 350 V, then R = 3,500 ohms. C might well be 16 microfarads, 

 and L 20 henries. If the rectification system is half-wave (see next chapter) 

 the ripple frequency is 50 cycles and m = IttF 



47r2 X 502 X 20 X 16 

 Then oi^LC = 



and 



jojLjR = 



10« 



277 X 50 X 20 

 3,500 



^32 



=^1-8 



Clearly the o)^LC term is much the most important, and for practical purposes 

 it is sufficiently accurate to write 



f^out 



m 



1 



M^LC 



If the rectification is half-wave, this is not far from 



Kout 



Kin 



10 



Inductance in henries X Capacitance in microfarads 

 and if the rectification is full-wave 



Kin 



2-5 



Inductance in henries X Capacitance in microfarads 



CLASSICAL FILTER 



In earlier chapters we have distinguished between the potential divider (whose 

 behaviour is easy to compute if it is not required to feed a load, but which 

 becomes altogether more involved when a load is connected) and the attenua- 

 tor, which is intended for interposition between a matched generator and 

 load, and whose component values are chosen so that matching conditions 

 are preserved. We have also discussed some R-C filters (which are in effect 

 frequency-conscious potential-dividers) and their performances have been 

 worked out on the assumption that no load resistance is connected. In 

 practice, this means the load resistance must be very high and preferably 

 almost infinite — a valve. 



83 



