Photodiodes and op amps

The photodiode is basically the opposite of the LED (in fact you can just use the LED! though it will be a little week), instead of emitting a photon for every electron that bases with the current, it will create current for every photon absorbed.  The current is produced going the opposite direction from the diode wired normally.  Here's a schematic from the lab write up:
Unfortunately the current produced will be quite small... good thing we know how to use the Op Amp!!

Lab 8-6: Current to Voltage Converter
a) The Photodiode

Here is the diagram for wiring up the photo-diode, there were some high frequency oscillations (fuzz) which I was able to remove by adding a capacitor (see smaller pic).  After that, you could see the signal from the fluorescent lights much better.  The oscillations are pretty extreme... The output voltage was -10mV with a 100% modulation from the lights (dipped to 0V).  If you covered up the photodiode, the signal dropped to 0.  Looking at the summing junction (x on the diagram), the voltage was always 0, (makes sense right? remember Golden Rule #1?)

b) The PhotoTransistor

The idea is essentially the same, same parts and everything, but this time we wire up the photodiode as a transistor by attaching it to a current source.  This time the output voltage was 2V!  A much stronger signal. With 50% modulation.  Summing junction is still 0V.