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MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August I!$30
TRANSRESISTANCE PHOTOAMPLIFIER WITH DC AND LOW-FREQUENCY NOTCHING CAPABILITY
By Tim Burke
ABSTRACT
This circuit converts modulated near-infrared light signals into electrical signals in the presence of strong ambient light such as direct sunlight or 60 Hz light. A low-noise, high gain transresistance techni- que is used.
INTRODUCTION
Nearly all infrared communications systems require a photodetector to convert modulated light signals into electrical signals for conventional processing of these signals. Normally, for near-infrared signals, a transresistance amplifier is used in conjunction with a p-i-n silicon photodetector to convert in- cident radiometric optical power into electronic power. Power gains on the order of IO8 are readily achiev- ed. The photodiode itself has an output rise time in the low nanosecond range. Unfortunately, any photodiode-amplifier combination is ultimately limited in response by either the stray capacitance across the feedback network, or, more likely, running out of gain in the linear region of the "current to voltage" connection. A typical transresistance circuit is shown in Figure 1. The transresistance gain of the circuit is dependent on the value of the load resistor, RL. The larger the value of RL, the greater the gain. The noise of the detector is inversely proportional to the value of RL Hence, it is desirable to have RL as large as possible to achieve the greatest gain and the lowest noise figure of the circuit.
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PA1 T
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id1 +
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Rb -v$,
"b eOzI.jRLo*P*RL
CONVENTlONAL TRANSAESISTMKE P"OTcmMPLlF,ER
Figure 1.
Figure2
0 Motorola, Inc. 1980 18
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MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1980
If the photodetector received only the desired signal and received no undesired signals, the circuit in Figure 1 would suffice. However, many situations require detection of low-level signals in the presence of strong ambient light, such as direct sunlight. In this situation, the circuit in Figure 2 may be used. This cir- cuit employs an adaptive-loop current sink on the photodetector to maintain a fixed bias point in the presence of low-frequency or DC light signals such as 60 Hz lights and direct sunlight, respectively.
DESCRIPTION
A circuit was designed and tested to keep the photodetector amplifier at a fixed operating point that is...
