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BELL SYSTEM TECHNICAL JOURNAL 



that the output plotted against input will be too steep as shown in the 

 curve 1, Fig. 8. If the bias is too great, the output input curve will be 

 inclined too much as shown in curve 2 of the same figure. When the 



o 30 



3 20 



10 



20 



30 40 50 



INPUT IN DECIBELS 



60 



70 



80 



Fig. 8 — The effect of the biasing current on the slope of the output-input level 



curves. 



proper value of bias is used the curve 3 is obtained which is inclined 

 at 45 degrees. Measurements of output versus input may therefore 

 be used to determine the proper bias current. 



Another method of determining the proper amount of bias is to plot 

 the output obtained from records of very weak signals as a function of 



12 3 4 



BIAS CURRENT IN MILLIAMPERES 



Fig. 9 — Effect of the biasing current on the intensity of weak signals. 



the bias current used during recording. Figure 9 shows such a curve 

 for a 1000-cycle record. In selecting the proper bias current from the 

 curve, the point at the crest gives the most efficient value but it is 



