A signal-conditioning group must choose between a bipolar-input and a MOS-input…

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

Inside the Instrument, Ch The differential pair and the current mirror

Problem

A signal-conditioning group must choose between a bipolar-input and a MOS-input operational amplifier for two different transducers, using the typical input bias currents

I_{B,\rm bip} = 80\,\mathrm{nA}

(as for the 741) and

I_{B,\rm MOS} = 2\,\mathrm{pA}

(typical of a CMOS input pair). ((a)) A glass pH electrode presents a source resistance

R_{s1} = 1.0\times10^{9}\,\Omega

and its output changes by

59\,\mathrm{mV}

per pH unit. The design requires the voltage error caused by input bias current to correspond to no more than

0.1

pH unit. Compute the bias-current-induced voltage error for each amplifier type at this source resistance and state whether each meets the specification. ((b)) A strain-gauge bridge with

350\,\Omega

arms presents a source resistance of at most

R_{s2}=350\,\Omega

to each input and delivers a signal of order

1\,\mathrm{mV}

. Compute the bias-current-induced voltage error for each amplifier type at this source resistance, and compare each to the

1\,\mathrm{mV}

signal. ((c)) State, with a reason tied to a number from (a) or (b), which amplifier type should be used for each transducer, and explain in one sentence why bias current alone does not decide the choice for the bridge.

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