590 BELL SYSTEM TECHNICAL JOURNAL 



The arrangement of the several photoelectric cells in their cabinet, 

 as shown in Fig. 3, is such that one amplifier can be connected directly 

 to two of the cells leaving the third to operate a second amplifier. 

 The outputs of these two amplifiers are then connected in parallel to 

 the common battery supply equipment shown at the bottom of the 

 two vertical cells. 



By the use of two stages of amplification in the photoelectric cell 

 amplifier, the signal is brought to such a level that it may be carried 

 by suitably shielded leads to other amplifiers outside the photoelectric 

 cell cabinet. This permits of using the convenient relay rack form of 

 mounting. The signal level is, however, still low and may be ade- 

 quately handled in amplifier units which differ but little from those 

 used with the photoelectric cell. 



The remaining requirements placed on the amplifiers at the trans- 

 mitting terminal are those set by the telephone line. One of primary 

 importance is that which determines the amount of energy needed. 

 In order that the signal wave shall be of such magnitude that any 

 interference present in the line may be negligible in comparison, it is 

 desired that the alternating current delivered by the final amplifier 

 stage shall be at least 4 milliamperes into an impedance of 600 ohms. 

 The energy to be supplied is, therefore, approximately 0.01 watt, 

 which determines the choice of the last amplifier stage. To build up 

 the signal to a value sufficient to operate this output tube it has been 

 found that eight stages of the small-sized tubes and one stage of greater 

 load-carrying capacity must be used. The total amplification given 

 by these ten stages is approximately 130 T U. It is through this 

 known gain of the amplifiers that we get our only accurate quantitative 

 data as to the magnitude of the initial signal wave. This comes out 

 to be about 10~^^ watts or, with a 100,000-ohm resistance in series 

 with the photoelectric cell, the potential available at the first tube is 

 roughly 10 microvolts. 



The characteristics of the line also determine the means by which 

 it shall be coupled to the final amplifier stage. In order to secure the 

 proper impedance matching and to prevent the line from being un- 

 balanced with respect to ground, it was felt desirable to use trans- 

 formers if possible rather than to attempt the design of a tube circuit 

 capable of meeting the requirements directly. The problem included 

 both output and input transformers, and specified an amplitude- 

 frequency characteristic constant to within ± 0.5 T U from 10 cycles 

 to 25,000 cycles. The input coils intended for use at the receiving 

 terminal had the additional requirement that a minimum of inter- 

 ference current should be induced in the secondary due to potentials 



