A laser-pulse energy meter uses a photodiode of responsivity and capacitance

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Feedback, operational amplifiers and analogue signal processing · Sensors, detectors and experimental instrumentation

Electronics for Measurement, Ch The operational amplifier and negative feedback

Problem

A laser-pulse energy meter uses a photodiode of responsivity

0.55\,\mathrm{A\,W^{-1}}

and capacitance

100\,\mathrm{pF}

, wired directly into the inverting input of an operational amplifier whose own input capacitance is

5\,\mathrm{pF}

and whose gain-bandwidth product is

f_T = 1\,\mathrm{MHz}

. The design target is a transimpedance sensitivity of

0.6\,\mathrm{V}

per microwatt of optical power, averaged over a pulse. ((a)) Choose the feedback resistor

R_f

for the design sensitivity, round it to the nearest E12 preferred value, and state the sensitivity it actually gives. Then use the rule

C_f \approx \sqrt{C_{\rm in}/(2\pi R_f f_T)}

, with

C_{\rm in}

the total capacitance at the input, to choose the feedback capacitor (again the nearest E12 value) and compute the

-3\,\mathrm{dB}

bandwidth this leaves the stage. ((b)) The transimpedance output feeds an operational-amplifier integrator (input resistor

R_1

, capacitor

C

, no resistor across

C

) that integrates over a laser pulse of width

50\,\mu\mathrm{s}

to give a reading proportional to pulse energy. Choose

R_1 = 100\,\mathrm{k}\Omega

(fixing the integrator's input resistance at that value) and choose

C

, to the nearest E12 preferred value, so that the time constant

R_1C

equals the pulse width; state the actual time constant achieved and its percentage deviation from

50\,\mu\mathrm{s}

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