Someone argues: "A load capacitor on an amplifier's output only adds a low-pass…

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

Inside the Instrument, Ch Stability and compensation

Problem

Someone argues: "A load capacitor on an amplifier's output only adds a low-pass filter in series with the signal path; at worst it slows down the step response, it cannot make a stable amplifier ring or oscillate." A voltage follower is built from an operational amplifier with unity-gain frequency

f_u = 2.0\,\text{MHz}

, a second (output-stage) pole at

f_2 = 3.0\,\text{MHz}

(from the datasheet), and output resistance

R_o = 50\,\Omega

. It is connected to drive a

15\,\text{m}

instrumentation cable of capacitance

C_L = 3.2\,\text{nF}

, taken as a lumped load inside the feedback loop. ((a)) Find the error in the claim. ((b)) Compute the follower's phase margin with and without the cable, and state what actually happens to a step response with the cable connected.

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