The photodiode circuit is very popular and is one of the most often used applications for an op-amp. This circuit changes a current into a voltage. It is often used in instrumentation, sensors, or as an encoder for motor applications. There are many uses, and therefore, analog engineers will end up using this or seeing it, or dealing with this circuit in their jobs. It helps to be able to recognize the potential issues and how to fix them. This presentation concentrates on using an op-amp in this type of circuit and the related amplifier issues. As an overview, when light hits the photodiode, it generates a small current flowing in the negative diode direction. Think of it like a diode leakage current. When a voltage is formed across the diode,  it is operating it like a solar cell. That allows power to be taken out of the diode, but the response is very non-linear and unusable for measurement and control applications. Letting a voltage form across the diode also slows its response significantly. That is why the classic photodiode circuit above either keeps no voltage across the diode, when its anode is grounded, or is operated with a negative bias across the diode to reduce its capacitance and make the circuit faster. Since the op-amp input current is ideally zero, the diode current is forced through the feedback resistor. That gives a voltage difference equal to diode current times the resistor value, which appears at the output of the amplifier.