Simplified schematic diagram of a negative-feedback current preamplifier. The photodiode current is turned into a voltage through feedback resistor RF. Stray capacitance CS sets the dominant pole of the preamplifier, which dictates the circuit's bandwidth.
Prior art. The dc gain is lower than the ac gain, thanks to the feedback path through R1, C, and R2.
New solution. The main change is the insertion of a proportional-integrative (P-I) block along the feedback loop, which yields complete removal of the output voltage offset.
Four-channel prototype of the circuit. Four 1 mm2 silicon PIN diodes are mounted on the front panel. The preamplifiers are realized in hybrid technology on alumina substrates.
Response of the circuit to a step current pulse with (upper curve) or without (lower curve) dc cancellation device.
Response of the circuit to an input step current as observed in the ms time scale. It is an exponential decay with a decay time constant of about 100 μs.
Noise waveform observed at the preamplifier output when the photodiode is lighted in such a way to deliver a current of ∼60 μA. The rms noise is lower than 1 mV.
Response of the four circuit channels to the optical signals caused by a ∼0.7 μm diameter particle traversing the laser beam at a speed of ∼140 m/s. The electronic noise as represented in particle size units is ∼10 nm rms.
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