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The Marvelous Disappearing Capacitor
The performance of photodiode amplifiers can be improved in a clever way.
My goal is to improve the device’s sensitivity and reduce signal settling time, so that we can make more precise measurements of faint and extremely short-lived phosphorescence — and hopefully, publish some interesting results. In effect, it works to keep both terminals of the photodiode at the same potential: if the cathode (right) ever moves up or down, the anode (left) is coerced to the same voltage by sinking or sourcing current through the chip’s low-impedance output path. The capacitor in tandem with the resistor form a highpass filter; as long as Rf ≫ Rp, the cutoff frequency for the follower’s meddling in the diode’s charge state is given by the standard RC formula:
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