CATHODE FOLLOWER 



from a special battery which is returned to the cathode. This method has the 

 advantage that the screen current is kept out of the load (Figure 11.16). 



Advantage of high input impedance 



If our source of input voltage has a high internal resistance r, and the valve 

 into which it works has input resistance R and input capacitance C (Figure 

 11.17), then there occurs: 



^R 



B] 



Figure 11.17 



(1) A loss of voltage deUvered to the input terminals AB, due to the poten- 

 tial divider action of R and r\ the fraction of the e.m.f. transferred is 



Rl{r + R). 



(2) An additional loss at high frequencies due to the shunting effect of C. 

 We have a simple low-pass filter turning over at 



1 



ft>. = 



U+'"/ 



R 



To get fo^ high we need r, R and C low. We have no power to alter r 

 must be ^r for reason (1), so C must be small. 



With the cathode follower C is small and R is large, so it forms the ideal 

 stage to follow generators of very high internal resistance. A conventional 

 amplifier may then follow the cathode follower. 



Output impedance 



The output impedance of a device is its internal resistance regarded as a 

 generator, and may be inferred by seeing what happens to the output voltage 

 when some kind of external load is connected across it. 



In the case of the cathode follower, suppose as the result of the connection 

 of a load R^ (Figure 11.18) the output voltage falls dV^. The grid potential is 

 unaffected, so dVgj,^dVj^ and an extra current flows in the valve 

 f^dVj(R + O where R = R„ in parallel with Rj^. R^ is invariably much 

 smaller than Rj,, so the extra current is approximately /nd VJ(Rl + ^a) and 

 nearly all flows in the external load. 



The output impedance is 



