Transconductance as a Criterion of Electron Tube 

 Performance 



By T. SLONCZEWSKI 



eUANTITATIVE evaluation of electron tube performance has assumed 

 added importance with the increasing extension of electronics into the 

 fields of measurement and control. Simplification of the process of selection 

 of suitable tube types and operating conditions from the general data avail- 

 able is of considerable value to all engineers concerned with electronics 

 circuit design. The conventional procedure involves analysis of the plate 

 current-grid voltage characteristics. The simpler method presented herem 

 supplies the same information from an analysis of the transconductance-grid 

 voltage characteristics. These are usually supplied by the manufacturer 

 or can be obtained readily by measurement^ 



The method presented herein has been employed successfully for a number 

 of years in the development of electronic measuring apparatus by a group 

 of engineers who attended lectures on the subject given by the author. It 

 applies chiefly to pentodes, where the internal plate impedance is high with 

 respect to the load impedance. Its merit resides in the comparative brevity 

 of the formulae, the ease of computation and the facility in obtaming the 

 data from which the computations are made. It allows one to form a pre- 

 liminary judgment of the performance of a tube from a brief glance at the 

 characteristics furnished by the manufacturer better and faster than any 

 other method known to the author. It should prove of value to the instruc- 

 tor teaching electron tube theory. 



In the interest of simplicity some of the subscripts m, p, c and g appended 

 usually to symbols for transconductance, plate current and grid voltages are 

 deleted below. The scope of the discussion is so limited that no confusion 

 may arise from this omission. The formulae are expressed in terms of 

 amplitudes of voltage and current, capital letters being used for their 

 symbols. All values are in peak volts. Levels are in decibels. 



The g-e characteristic is introduced into the problem by starting with the 

 general expression for the plate current 



where v is the voltage measured from the bias point Ec , where the deriva- 

 tives are taken, and utilizing the definition of the transconductance 



> Radio Engineers' Handbook, F. E. Terman, McGraw-Hill, 1943, p. 961. 



315 



