CATHODE FOLLOWER 



the central conductor and the outside braiding of 1,000 [XfxF. If the output 

 of /I is taken from the anode of a pentode with a load of 100,000 D, then this 

 source resistance with the cable capacitance form a low-pass filter which will 



Capacitance between core 



and braiding of 1000 p-t^F 



Figure 11.22 



turn over at co,, = 1/(10^ X 10~^) = 10^ radians/sec. This is a frequency of 

 only 10^/2-77 c/s, so the high frequency performance of the apparatus is hable 

 to be disappointing. Suppose the pentode output is passed to the cable via 

 a cathode follower of output resistance 200 Q, then the cut-off frequency will 

 be 1/(2 X 10^ X 10-^) = 5 X 10^ radians/sec. This is quite a different story, 

 and amply high for biological work. 



175 



