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