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Active Filters
32 questions By Tony R. Kuphaldt
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Question 10 of 32
In this filter circuit, how will the filter’s cutoff frequency be affected by changes in the potentiometer position? Be as specific as you can in your answer.
Reveal answerThis is a “trick” question: fcutoff is unaffected by changes in the potentiometer’s position.
Follow-up question: what does change as the potentiometer wiper is moved back and forth along its adjustment range?
Notes:Ask your students what the function of the op-amp is (with potentiometer feedback), taken by itself. If there were no filter circuit in place at all, but Vin connected straight to the op-amp’s non-inverting input, what function would the potentiometer adjustment serve?
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Question 11 of 32
Determine the type (LP, HP, BP, BS) and cutoff frequency of this active filter circuit:
Reveal answerThis is a low-pass filter circuit.
f−3dB = 7.95 kHz
Follow-up question: explain what the purpose of the 9.1 kΩ feedback resistor is, since all we’re using the opamp for is a voltage buffer anyway (which theoretically does not require resistance in the feedback loop). Furthermore, explain how the Superposition theorem is used to determine the optimum value of this feedback resistance.
Notes:Ask your students to explain how they arrived at their answers: what formulae did they use, and how did they determine the type of the filter circuit that this is? Ask them if there was any information given in the diagram irrelevant to either determination.
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Question 12 of 32
Determine the type (LP, HP, BP, BS) and cutoff frequency of this active filter circuit:
Reveal answerThis is a high-pass filter circuit.
f−3dB = 482.3 Hz
Follow-up question: the feedback resistor network comprised of the 52 kΩ and 91 kΩ resistors not only provides a gain of 1.75 (4.86 dB), but these values were also intentionally chosen to compensate for the effects of DC bias current through the opamp input terminals. You will notice that the parallel combination of 52 kΩ and 91 kΩ is approximately equal to 33 kΩ. Explain why this is significant with reference to the Superposition theorem.
Notes:Ask your students to explain how they arrived at their answers: what formulae did they use, and how did they determine the type of the filter circuit that this is? Ask them if there was any information given in the diagram irrelevant to either determination.
