A technician designs an equal-component Sallen-Key low-pass stage for and

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 Active filters, oscillators and timers

Problem

A technician designs an equal-component Sallen-Key low-pass stage for f0=20 kHzf_0 = 20\,\mathrm{kHz} and Q=5Q = 5, computing the required gain from Q=1/(3−K)Q = 1/(3-K) as K=2.8K = 2.8. The chosen operational amplifier's datasheet quotes a gain-bandwidth product fT=1 MHzf_T = 1\,\mathrm{MHz}. The technician writes: "Since fT=1 MHzf_T = 1\,\mathrm{MHz} is fifty times the corner frequency of 20 kHz20\,\mathrm{kHz}, the amplifier has ample margin, and the built circuit will show the designed sharp peak at 20 kHz20\,\mathrm{kHz}." ((a)) Find the error in this reasoning. ((b)) Compute the gain-bandwidth product actually required by the rule of thumb fT≥100KQf0f_T \ge 100 K Q f_0, and state what the built circuit will show instead.

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