THERMIONIC VACUUM TUBES 



77 



for the high frequency and not the low frequency. When using an 

 amplifier on the output of a detector (see Fig. 45), it is important to 

 choose r to give the maximum detecting voltage. As in amplifier cir- 



Fig. 45 



cuits it is essential in this type of detector that the grid remain always 

 negative. See also the sections on amplifiers. 



39. Detection by Rectification in Grid Circuit. This type of circuit 

 (see Fig. 46) is now in very general use for radio purposes, and is 

 characterized by the grid blocking condenser C s . Contrary to the 

 preceding type of detector, the present requires the flow of electrons 

 to the grid and works best when the grid is held permanently at a 



Fig. 46 



small positive potential. The action of the high frequency input 

 causes the periodic accumulation of a negative charge upon the grid 

 and the blocking condenser, thus lowering E s and diminishing I p . 

 This action is clearly illustrated in Fig. 47. This circuit is most 

 effective when the carrier frequency is much greater than the signal 

 frequency, and not as efficient as the circuit described in Sec. 38 

 when the carrier is say only four or five times as great as the signal 

 frequency. 



Attempts to deduce a quantitative relation for the detecting current 

 in this type of detector have as yet met with little success, one of the 

 principal reasons being that very little is known about the "dynamic" 

 grid current characteristic. 33 Experiments show, however, that the 



33 For a discussion of this topic see Hulbert & Breit, Phys. Rev., Nov., 1920, pp. 

 408-419; Oct., 1920, pp. 274-281. 



