TETRODE 



potential (Figure 8.19). This grid forms an electrostatic screen between the 

 anode and the control grid (which enormously reduces QJ and also screens 

 the cathode region from the potential changes at the anode. The rate at 

 which electrons leave the cathode region, or the 'cathode current', is deter- 

 mined almost entirely by the screen potential, but due to the open structure 

 of the screen grid most of the electrons pass through it and strike the anode ; 



increasing 



-|HT+ 



§ 



W 



Knee 

 voltage 



Screen 

 potential 



Figure 8.19 



Figure 8.20 



a small proportion hit the screen and constitute 'screen current'. This is the 

 rationale of the tetrode and the anode characteristic for which one might 

 hope has the form of Figure 8.20. Some anode potential is necessary to get 

 anode current, for clearly if there were none at all, all the cathode current 

 would pass to the screen. However, once the anode voltage exceeds a 

 critical 'knee voltage', substantially all the cathode current passes through 

 the screen grid to the anode, giving the desirable characteristics already 

 enumerated. 



In point of fact the anode characteristic has the curious form o^ Figure 8.21. 



Screen potential 

 Figure 8.21 



The curves have 'tetrode kink' ; this may be explained as follows : When the 

 anode potential is at A the electrons pass through the screen and encounter a 

 retarding field because the anode is negative with respect to the screen. Under 

 the influence of this field they are slowed up and finally strike the anode 



145 



