Two claims are made about active-filter and oscillator circuits.

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

Two claims are made about active-filter and oscillator circuits. ((a)) Someone builds a passive

RC

low-pass section, adds a unity-gain op-amp follower on its output, and says: "Now it is an active filter, because there is an amplifier in the circuit." Explain what is wrong with this claim, and state what change to the wiring (not merely the presence of an amplifier) is needed to get the sharper knee and controllable damping of a genuine active section. ((b)) For a Wien-bridge oscillator with

R=10\,\mathrm{k}\Omega

and

C=10\,\mathrm{nF}

, someone trims the amplifier gain to exactly

K=3.000

with close-tolerance resistors and argues: "The Barkhausen condition

K\beta=1

is now satisfied exactly, so the circuit will settle into a fixed-amplitude sine wave at

f_0=1/(2\pi RC)

and stay there indefinitely." Explain what is wrong with this claim, given that

K

cannot be held at exactly

3.000

in practice, and state what the circuit does instead if

K

drifts even slightly above or slightly below

3

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