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Op amp input resistance selection

In a circuit that amplifies a weak current signal, the op amp needs a large feedback resistor. How should the input resistor be valued at this time? Why?

  • May we know what's the specific application of the circuit? It depends on how you would be using the amplifier; for instance, if it is for the photodiode, then transimpedance amplifier configuration is mainly used. In this case, Rin or input resistance may not be needed. However, there are still factors that must be considered, including the stability, noise, dc error sources, and voltage range limitations in designing the circuit. For stability, you could add a feedback capacitor to maintain stability and compensate for the input capacitance from the photodiode and op amp at the inverting op amp input. As for noise, the dominant noise sources are the op amp voltage and current noise, and the feedback resistor, but there could still be other sources such as the feedback capacitor and internal resistance of the shunt resistor.

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