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