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