The Dropbox website does not work. Maybe we need to join it to see anything over there?
Instead, please attach the info about the MULTILED here with your posting.
THis is a broadband Op amplifier controlled by C ratio with DC blocking breakpoint at R19//C20 on feedback.
Since the diode is anode grounded with reverse biased PD the cathode is at V+ with null current and drops input voltage , inverts to a positive peak AC amplifier with detected light.