56 BELL SYSTEM TECHNICAL JOURNAL 



charting of the characteristics would require several such families of 

 curves taken with different screen voltages. One such additional 

 family of characteristics, in which Es is maintained at 90 volts, is shown 

 in Fig. 3. 



Since the plate in a screen-grid tube has practically no effect on the 

 total space current, so far as consideration of the latter is concerned, 

 one may regard the plate as being removed from the structure and 

 consider only the remaining elements. The cathode, control grid, and 

 screen then may be regarded as constituting an ordinary triode. By 

 maintaining the screen at a positive potential, it is enabled to perform 

 the function usually performed by the plate of a triode, viz., that of 

 supplying the positive field necessary to produce the flow of space 

 current against the opposing resistance due to space charge. This 

 function does not interfere with its screening action, so long as its 

 potential is not allowed to vary. In fact, there is an actual gain in 

 efficiency, as will be shown later, in having the electrode that provides 

 the main driving field for the space current maintained at a fixed 

 potential instead of varying over the operating cycle, as it must in the 

 triode. 



Usual design principles and equations applicable to triodes are also 

 applicable to screen-grid tetrodes. By making the spacings between 

 the electrodes small, particularly that between the cathode and control 

 grid, a high transconductance can be obtained. Using the subscript / 

 to designate total space current, the transconductance .is given by 



Si = -rr and has the same value as that for a triode of the same 

 dE„ 



dimensions. Let/ represent the fraction of the total space current that 



passes through the openings in the screen and is collected by the plate, 



which is assumed to be at a higher potential than the screen. By 



proper design, / can be made large, say from 0.7 to 0.9, in the normal 



operating range. The transconductance from the control grid to the 



plate is given by 



^"^ ~ dE,~^' dE/ 



which is 0.7 to 0.9 of the normal value of transconductance for the 

 three-electrode tube. 



The amplification factor, m, for the tetrode, is given by 



f^=^^=S„rR„. (17) 



dEr, 



