VACUUM TUBE ELECTRONICS 87 



as the more usual one, the difference being mainly one of mechanical 

 convenience in securing low-loss leads between the tube and the 

 external circuit. 



The modified circuit, then, will be employed for analysis, and the 

 assumption will be made that the necessary direct-current connections 

 are made through chokes which are sufficiently good so that it may be 

 considered that no external high-frequency impedance is connected 

 between either the grid and the plate, or between the cathode and the 

 grid. 



It is easy to see that under these conditions there can be no high- 

 frequency current carried away by the grid. It follows that for plane- 

 parallel structures, the alternating-current density, /], will be the same 

 both in the cathode-grid region and in the grid-plate region. The ar- 

 rangement thus reduces the problem to the consideration of the single 

 current, /i, and the resulting potential difference between cathode and 

 plate. 



There are several possible combinations of direct-current biasing 

 potentials. For the first of these, the plate will be supposed to be 

 biased at a potential sufficiently positive to collect all electrons which 

 are not captured by the grid on their first transit. Complete space 

 charge will be assumed both in the cathode region and in the plate 

 region. 



Under these conditions, we have the grid-cathode potential differ- 

 ence given by (26) and the grid-plate potential difference given by (35), 

 where the velocity, M + iN, is given by (25). We can write, 



V,- v. = (v,-v„) + {v,~ ig 



(55) 

 = - [Eq. 35] + [Eq. 26]. 



It will be remembered that the current was assumed to be positive in 

 (26) when directed away from the origin, and positive in (35) when 

 directed toward the origin. Therefore, since the same current exists 

 in both regions, and they are joined together at the grid, the sign of 

 the current Jy remains the same in both (35) and (26), its direction 

 being from cathode to plate. The impedance looking into the cathode- 

 plate terminals may be obtained from (55) by dividing by the ampli- 

 tude A of Ji and reversing the sign of the result to correspond to a 

 current from plate to cathode. Letting 



Zo = Ro + iXo (56) 



represent the impedance looking into the cathode-plate terminals, we 

 can write the result as follows 



