Photodiode - circuit questions
Why do we need an external voltage?


Because it makes the photodiode more sensitive, it lowers its capacitance, and it makes its capacitance more predictable.


  Why do we need a resistor?


Because a photodiode acts like a current source in parallel with a diode.  So when it is reverse-biased, it just acts like a current source.







simulate this circuit &ndash; Schematic created using CircuitLab


  What is exactly happening to the resistor when light hits the photodiode?
  Why is the voltage measured between the photodiode and the resistor?


Current flows through the photodiode when light hits it.  It then flows through the resistor and generates voltage.


  Why doesn't the external voltage influence the voltage from the photodiode (assuming the voltage from the photodiode is relatively low compared to the external voltage)?


There is no "voltage from the photodiode".  There is current from the photodiode, that develops voltage across the resistor.


  For the second photo: what would be the need for a capacitor here? What would it change?


It shorts out high-frequency current from the photodiode, so the RC combination acts like a low-pass filter.