Page 91 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

84 Chapter Four

2 1
Current

Forward bias
Saturation
current I s
Voltage
V oc

V oc
Max
Increasing power
light I sc
level Solar cells
I sc
Reverse bias
3 4
Figure 4.4 Photodiode IV characteristic. Photodetection is pos-
sible in quadrants 1, 3, 4.

+12V
Retroreflective R 2x
L
code plate 1k0 560 1k0
+
~6V -
Instrum.
amp.
~2V
Ref.
Sensor LED LED
(source/detector) 680 220μF
Transistors:
-12V LM394 "supermatch pair"
÷
e ~ 1nV/ Hz
n
Figure 4.5 An LED can be used both as a source and simultaneously as a
detector of its own light, for example to detect a rotating reflective encoder.
For low-noise detection the transistor amplifier must be chosen for lowest
e n , lowest base spreading resistance r bb , and high collector current.

spectra is significant and useful. The LED can therefore be used simultaneously
as a light emitter and a photodetector.
The device shown in Fig. 4.5 was conceived for applications similar to the
common gapped source/detector optocouplers and as high-resolution encoder
readers embedded in the “flying” heads of magnetic storage disk systems. As
the size of these devices needs to shrink, it becomes increasingly difficult to

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