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2017-08-18 08:54  
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A common method of transforming the output current of a photodiode into a voltage signal, paralleling the photodiode with a high-value load resistor, produces a nonlinear response. Also the combination of the load`s transresistance, Rr. and the photodiode`s junction capacitance, C` slows the circuit`s response time. Figure 67-11B shows virtually the same components as Fig. 67-llA rearranged to maximize the inherent speed and linearity of the photodiode.

The SP4010 (available from Hybrid Systems, Billerica, MA) is a unity voltage-gain buffer with aJFET input, 60-MHz 3-dB bandwidth, and 18-bit, 0.0004%, linearity over a ?±10 V input range. In the circuit of Fig. 67-11B, the photodiode sees a constant voltage across its terminals, which is essential for linear photodiode outputs. The optional zener diode, Dz, sets a reverse bias at the photodiode for lower junction capacitance and higher speed. If you don`t use Dz, be sure to connect the feedback loop. An optional diode, DcLAMP. limits the output in case of unexpected light bursts, but results in increased dark-current leakage and lower speed. The buffered output of the circuit equals the photodiode current times the transresistance, RT?· Figure 67-llC shows the circuit`s response to a fast light pulse.

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