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Large area photodiodes come with a large capacitance, typically on the order of 3000pF or larger. This capacitance to ground at the input of a transimpedance amplifier has a significant effect on the output noise of the circuit as the noise gain increases dramatically at higher frequencies. The addition of a low noise JFET clamps (or bootstraps) the diode capacitance voltage to the Vgs of the FET and allows the photodiode current to pass to the feedback resistance. The result is much lower output noise over the conventional implementation.