Actually, the circuit has a positive output voltage. For a vivid explanation, I suggest two different ways:
- assume zero current flow at the beginning. Then the photo current will build up a photoelement voltage, negative at the
cathode. The OP gain causes a positive voltage at the output.
- consider, that the photocurrent is a reverse current. Only a positive output voltage can cause a reverse current through
R1 and D1.
To anticipate a possible question. Yes, the photocrrent has the same direction in all photo diode operation modes.
But in photoelement mode, the photo current is cancelled by a forward current, if no external load exists.
I notice that this basic circuit often causes confusion. It has been discussed repeatedly at edaboard, see e.g.: