A photodiode's avalanche stage produces current pulses that are rectangular in shape:…

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Feedback, operational amplifiers and analogue signal processing · Signals, systems, control and digital signal processing

Inside the Instrument, Ch Noise sources and amplifier noise

Problem

A photodiode's avalanche stage produces current pulses that are rectangular in shape: each pulse has height

A = 2.0\,\mu\mathrm{A}

and duration

\tau = 20\,\mathrm{ns}

, and the pulses arrive as a Poisson process at a mean rate

\lambda = 5.0\times10^{6}\,\mathrm{s^{-1}}

, well separated in time so they never overlap. ((a)) Find the mean current

I

. ((b)) Use Campbell's theorem to find the one-sided current-noise spectral density

S_i(0)

at frequencies well below

1/\tau

. ((c)) Compare

S_i(0)

with the value

2eI

that the elementary-charge shot-noise formula would give for the same mean current, and say why the two disagree.

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