Photodiodes can also be used as current generators for fast response. You can identify this circuit when the photodiode is directly connected to the inverting input of an opamp that also has a feedback resistor.
Usually, photodiodes are reversed-biased with a resistor, and light (or IR energy) causes them to leak some current. Since the leakage current is very small, the resistor has a high value and the voltage across it is sensed with a non-inverting (high impedance) opamp.