Amplifying High-Impedance Sensors – Photodiode Example

Figure 1 shows the equivalent circuit of a transimpedance amplifier and a high-impedance source. IS represents the output current of the source. CS is the sum of the source’s output capacitance and the op amp’s input capacitance. RF, with the help of the op amp, converts IS to a voltage. At low frequencies, the op amp’s inverting input is forced to be at ground potential and IS must flow through RF. This combination of effects creates an output voltage of ISRF. At higher frequencies, the capacitors will affect the circuit response. The output capacitance of a current sensor has a strong effect on the stability of the op amp feedback loop. Bode plots are aids in both analyzing this effect and in properly compensating the transimpedance amplifier using the capacitor (CF). The capacitors also limit the bandwidth of the transimpedance amplifier. RN and CN reduce the output noise. The output noise is also kept low by not over-compensating the feedback loop.