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BELL SYSTEM TECHNICAL JOURNAL 



detecting current is practically proportional to the square of the input 

 voltage, provided that this is small, thus establishing the relation, 



Jd = ae 2 , 



which corresponds in form to Equation 13 above. 



In designing this type of detector circuit, attention must be paid 

 to the value of the blocking condenser C s and its leak R s . It is clear 

 that the capacity of C s should be sufficiently small to cause the grid 

 to undergo the maximum potential change as a result of the relatively 



Grid Potential 



Plate Current 



Audio Current 



m Telephone 



Fig. 47 



small electron charge picked up, and yet it must be several times 

 larger than the tube capacity between grid and filament. Further- 

 more, the time constant of C s and R s should approximately match 

 the frequency of the detected current. With the more common de- 

 tector tubes and radio frequencies, capacities of the order of 200 Airf- 

 are satisfactory. 



Detector circuits with grid blocking condensers may be coupled to 

 amplifiers as readily as the other type of detector, and, in general, a 

 higher output resistance for the detector can be used, thus making 

 possible more efficient coupling between the detector and the first 

 stage of amplification. In increasing the output resistance of the 

 detector it should, however, be borne in mind (see Sec. 13) that a 

 secondary result is to reduce the input impedance of the detector, 

 which may entail a reduced input voltage. 



