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1949
LEAD-SULFIDE PHOTOCONDUCTIVE CELLS
695
because as the signal voltage is developed it reduces the voltage across the sensitive element (i.e., the cell is not ideally a constant-current device). This is borne out by the appearance in (9) of distortion terms of the second and higher order. Equation (9) can be expanded and rearranged to give the harmonic components :
¥(«
«* r» i i **•
cos 2ut + -^ cos
(10)
To give a rough practical example, if the polarizing voltage E were 100 volts, and the light modulation sufficient to give an alternatingcurrent fundamental component of 1 volt peak amplitude, then the
I 2
LOAD RESISTANCE
Fig. 3 — Signal output (in decibels relative to an arbitrary zero) versus direct-current load resistance for a number of lead-sulfide cells; the same curve is plotted for an SI and S4 vacuum phototube to serve as a reference.
second-harmonic term would be 1 per cent. If the peak amplitude of the fundamental is n per cent of the polarizing voltage, one may expect second-harmonic distortion of n per cent. In all practical cases, distortion terms higher than the second order are negligible.
SENSITIVITY
A number of tests have been made to determine the sensitivity of the cells under practical test conditions. Fig. 3 is a plot of the signal output from a large number of photoconductive cells and from two photoemissive tubes, plotted against the value of the load resistance. The conditions of the test were the same for all cells, but at a load resistance of 1 megohm, there are variations in output of nearly 30