CASCODE CIRCUIT 



negatively fed back {Figure 17.3). Let the output of the device be instan- 

 taneously 1 . Then the fedback amplifier input is B and the amplifier output 

 is —AB. The instantaneous generator potential must therefore be 1 + AB. 



h-fc> 



Figure 17.2 



No input 



Figure 17.3 



n 



n 



But this is by definition n, so the output in terms of « is t— — j^ «y — ^ ,thatis, 



1 -\- AB AB 



the effect of the feedback is to make the noise in the output AB times less. 



Cathode follower noise 



An important application of the above is to the cathode followers which 

 frequently precede the first amplifying valve in electrophysiology. Here B 

 equals 1 and A is the gain, without feedback, gm^^, between 30 and 100. 

 Thus the noise generated by the cathode follower is only between 1/30 and 

 1/100 of the noise it would produce if it were an amplifying stage. If the 

 cathode follower is preceded by a glass microelectrode, as is usually the case, 

 its noise is quite unimportant. 



CASCODE CIRCUIT 



There is an interesting circuit which has triode-like noise but pentode-like 

 gain. It is called the cascode and is shown in Figure 17.4. Two triodes are 

 used, usually similar, the grid of the upper one being returned to a fixed 

 positive potential in a similar manner to a pentode screen grid. Both valves 

 run under space charge limited conditions and so there is reduced shot noise 

 from each but no partition noise. We have for the lower valve 



di^- 



and for the upper valve 



MR + O 



....(1) 



.(2) 



From 2 



dV. 



/* 



257 



