I am building a pulse oximeter, and there is a photo diode that is reversed biased by 4.5 volts. When there is a light shining on the diode the voltage changes between .1-.3V. I need to cubtract off the 4.5 volt bias so the change can be multiplied and analyzed. I tried using a basic differential op amp. but it doesn't do a good job of detecting differences in voltage, in fact I don't think it notices at all. I am using an lm324. Do you have any better ideas on how I can achieve this or insight into what may be wrong with it? I have isolated just the differential part and even when i input the two voltages with a .2 v difference it doesn't out put .2 V.