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692 LEE December
REVIEW
A brief review of the salient features of the cell is probably in order. The photoelectric action is internal (as opposed to the external photoeffect or photoemission). An excess of free electrons in the thin layer of active material results in an increased electrical conductivity. Cells of this type may be used as receptors in circuits much the same as those used with photoemissive tubes; the signal voltage is developed across a resistance in series with the cell, and polarizing voltages of 45 to -90 volts are commonly used. The voltage sensitivity is high; in some cases, up to 30 decibels increase in output signal may be realized over a vacuum photoemissive tube. The frequency response seems to be adequate; Cashman mentions a loss of the order of 7 decibels at" 10 kilocycles per second. The spectral response is concentrated in the infrared1 with a peak at 2 to 2.5 microns, and cutoff at approximately 3.5 microns. The response, however, can be varied considerably,2 by varying the amount of oxide in the active material. The noise level is very low, and attainable signal-to-noise ratios are very high. Furthermore, the absolute noise output decreases when the cell is illuminated, in contrast to the behavior of photoemissive tubes, in which the noise voltage is proportional to the square root of the mean photocurrent. Finally, the effect is apparently quite linear with light intensity, at least within certain limits. This point will be discussed in more detail later.
THEORY OF A SIGNAL CIRCUIT USING A PHOTOCONDUCTIVE CELL
Additional understanding of the use of a photoconductive cell may be gained by a rather elementary mathematical approach. Consider a light-sensitive area like that shown cross-hatched in Fig. 1. The following symbols will be used in the treatment:
L = distance between electrodes
X = width of electrodes
T = thickness of photoconductive coating
po = specific resistance of the coating
gr0 = l/po = specific conductance of the coating
: G = conductance of the tube
R = I/Or = resistance of the tube
/ = light intensity (flux per unit area on sensitive surface)
. •** ; =?.. angular frequency of alternating-current light excitation
RO = load resistance in signal circuit, Fig. 2
E = supply voltage to signal circuit
EQ = signal voltage
eo «= alternating-current component of signal voltage