NOISE 



Substituting in 1 





i+^+'-«^ 



^6V. 



Si 





1 + 



*, The gain 



^f^in ^ R + r„ 



In a typical case i? = 30 kQ, r„ = 10 kD, g„, = 5 mA/V, /^ = 50. Then 

 (i? + ra)l(raf^) = 0-08, which is <^1. Therefore gain ^ ^^i?, as for a pentode. 



HT+ 



Figure 17.4 



The cascode circuit also resembles the pentode circuit in that the input 

 capacitance is low. The change in anode potential at the lower valve for a 

 change of anode current di is not —diR but —6i{{R -\- rjj ju]. The Miller 

 input capacitance is therefore reduced by a factor {(R + rj] fxR] — 1/37| of 

 what it would be for a simple triode amphfier. 



REFERENCES 



^ Starr, A. T. Radio and Radar Technique London ; Pitman 

 2 Macfarlane, G. G. Proc.phys. Soc. Lond. 59 (1941) 366 



258 



