THEORY OF MULTI-ELECTRODE VACUUM TUBES 47 



the output anode or plate of a multi-electrode tube is defined, as it is in 

 the triode, by the rate of change of plate current with variation of the 

 control-grid voltage; that is, it is the slope of the plate current-grid 

 voltage characteristic at the given operating point, the potentials of all 

 electrodes other than the control grid remaining constant. 



In conventional screen-grid tetrodes and pentodes, the grid next to 

 the cathode is the control grid. Consequently, for such structures, the 

 transconductance is defined from equations 2 and 3 by 



r) T 



Transconductance = -—— — Spi. (6) 



In space-charge-grid tetrodes and pentodes, the grid next to the 

 cathode is maintained at a fixed positive potential and the second grid 

 acts as the control grid. Consequently, in these and similar structures 



r) T 



Transconductance = -^r— = 5^?. (7) 



Similarly, considering the control grid (assumed to be the first grid) 

 and the output electrode of a multi-electrode tube, the amplification 

 factor is defined, as it is in the triode, by the ratio of the transcon- 

 ductance to the plate conductance. It is expressed by 



Amplification factor = ^ipg^ — -—— ■ (8) 



dEr. 



Or, assuming that Eg^ and Ep are varied in such a manner that /;, 

 remains constant, the amplification factor is expressed in the usual 

 form by 



_ dE-p 1 



"•-'-'Ol J Ip = constant 



Combining equations 5 and 6 with equation 8 gives 



(9) 



Transconductance Spi — -^ (10) 



just as in the case of the triode. Exactly similar equations apply if 

 g2 is used as the control grid. 



Obviously, the currents to the other electrodes in a multi-electrode 

 tube may be expressed by functions of the electrode voltages, similar to 

 equations 1 and 2. The various difTerential coefficients of these equa- 



