In one scheme the cathode of the first photodiode is common with the anode of the second photodiode and an inverting terminal of {HEAD ~ an operational amplifier} with {TAIL ~ negative feedback} 
with
In one scheme the cathode of the first photodiode is common with the anode of the second photodiode and an inverting terminal of { HEAD ~ an operational amplifier } with { TAIL ~ negative feedback }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL