An operational amplifier with an open-loop output resistance and gain-bandwidth…

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

Electronics for Measurement, Ch The operational amplifier and negative feedback

Problem

An operational amplifier with an open-loop output resistance

R_{\rm out} = 50\,\Omega

and gain-bandwidth product

f_T = 2\,\mathrm{MHz}

is wired as a non-inverting stage with closed-loop gain

A_{\rm CL} = 20

(so

\beta = 1/A_{\rm CL} = 0.05

). Above the amplifier's open-loop corner frequency its open-loop gain falls as

A_{\rm OL}(f) \approx f_T/f

. ((a)) Estimate the open-loop gain available at

f = 50\,\mathrm{kHz}

. ((b)) Estimate the loop gain

T

the stage has left at that frequency. ((c)) Estimate the output resistance the feedback loop leaves at

50\,\mathrm{kHz}

, treating

T

as a plain number.

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