ABSTRACT
Photodiodes are also used in active circuits, using op-amps as in Fig. 7-36, to achieve better speed and linearity, and to reduce noise levels. Note that the diode shunt resistance and junction capacitance have no effect on circuit performance because they are "short-circuited" at the op-amp input. That is, the voltage across the op-amp input is essentially zero (our standard op-amp assumption), so no voltage exists across Rsh and Cj, so no current flows through them. Thus all the photocurrent goes into the op-amp feedback path:
(7-93)
We again see the generic first-order response, so all our previous results apply also to this circuit. Note that performance now depends not on the parasitic Rsh and Cj of the photodiode, but on Rfb and Cfb, "wired in" components which we can select from a wide range to get improved performance. (When we said that the diode "parasitics" had no effect on performance this was not totally correct. They don't affect the above analysis, but they do show up in studies of the random noise effects in the system. These noise analyses can be found in the Melles Griot reference but are beyond our scope here.)
voltage • 0 \
