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694 Then
December
G = (A +kB)^ + ^% cos co/ LJ y>
which can be simplified to
G = n(\ + m cos co/) if a = (A + kB)(rX/L) and m =
kC
A +
(5)
EO-
The voltage developed across the load resistance £0 is just
aRo (1 -f m cos co/)
#0 =
(6)
+ afto(l 4 m cos o>0'
When this expression is expanded in a series of powers of cos at, and coefficients collected, we find the fundamental component of the alternating-current signal.
(fundamental) =
(7)
Fig. 2 -Simple signal ri rcui t usi ng photoconductive
cell as detector; signal voltage is developed across the resistor 7?ft.
This expression has a maximum value for a RQ = l,or
(max) = TT i-^ov v. (8)
(A + kB)rX'
This points up the fact that maximum alternating-current signal output is obtained when the load resistance is matched to that value of the cell resistance corresponding to the mean light level, and not necessarily to the dark resistance of the cell. Subject to the matching condition expressed by (8), the signal voltage can be expressed as
m cos
ni . m3 ,
cos2 co/ + — COS3 co/ 4 o
(9)
The literature is rather vague on the matter of linearity. Cashman states that the effect is linear up to intensities of 30 to 40 foot-candles. Anderson and Pakswer say that linearity is obtained "up to light levels of about 0.01 lumen" but say nothing of the area, and the limit presumably is determined by intensity and not by total flux. Neither paper makes it clear whether' the "effect" referred to is fundamental (i.e., change of conductivity) or simply the signal voltage developed across a load resistor. Even though the fundamental effect were ideally linear, one would expect a nonlinearity in the voltage output