Someone argues: "This amplifier's dominant pole sits at just

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

Inside the Instrument, Ch Multistage amplifiers, the Miller effect and output stages

Problem

Someone argues: "This amplifier's dominant pole sits at just 1 kHz1\,\mathrm{kHz}, far below where the gain matters, so it must have plenty of phase to spare when used as a unity-gain follower." The amplifier has A0=2000A_0 = 2000, a first pole at fp1=1 kHzf_{p1}=1\,\mathrm{kHz} and a second pole at fp2=50 kHzf_{p2}=50\,\mathrm{kHz}. ((a)) Find the unity-gain frequency fu≈A0fp1fp2f_u\approx\sqrt{A_0f_{p1}f_{p2}}. ((b)) Find the phase remaining before −180∘-180^\circ at fuf_u, arctan⁡(fp2/fu)\arctan(f_{p2}/f_u), and say whether the argument above is correct.

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