All photodiodes have an internal capacitance. In large-area diodes that capacitance can be has high as 3000 pF. The diode capacitance is in parallel and adds directly to the amplifier’s input capacitance. Added to that is the stray capacitance on the input node of the PCB (printed circuit board). Amplifiers will oscillate if this input capacitance gets large enough. This is because the capacitor creates an input pole, or lag, for the feedback signal. Since the feedback is delayed, it has a phase shift that will send the amplifier into oscillation. The cable or PCB stray capacitance on the output of the amplifier will also create a response pole. In this case, the lag is directly on the amplifier output. The amplifier is delivering current into the output, but the voltage tends not to change due to the capacitor. This delay, or lag, has the same effect, meaning that the amplifier will oscillate. The designer can compensate for the input capacitance with a small feedback capacitor CF, and for the output capacitance with a resistor in series right at the amplifier’s output pin. Although counterintuitive, sometime a faster amplifier will make for a more stable system. The faster amplifier makes a circuit with more closed-loop bandwidth, and that might allow the circuit to stabilize. In any case, if the designer increases CF it will lower the bandwidth, and a faster amplifier may be required no matter what. The value of CF may have to be tweaked for every different amplifier bandwidth tried. Remember that some amplifiers get faster at cold temperatures, so the system should be tested at the coldest temperature anticipated to ensure it does not oscillate.