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Active Filters
32 questions By Tony R. Kuphaldt
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Question 7 of 32
Identify what factor(s) determine the cutoff frequency of this passive filter circuit:
Give an exact equation predicting this filter circuit’s cutoff frequency, and also identify what type of filter it is.
Reveal answerThis is a high-pass filter circuit, with a cutoff frequency of:
f−3 dB = 1 2 πR CNotes:Nothing fancy here, just a review of passive RC filter circuitry.
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Question 8 of 32
In this passive filter circuit, how will the filter’s cutoff frequency be affected by changes in the load resistance? Be as specific as you can in your answer.
Reveal answerfcutoff will increase as the load resistance decreases.
Notes:Ask your students to define what “cutoff frequency” means. There is more than one definition: one based on output voltage, and one based on output power. When defined in terms of power, the cutoff frequency is sometimes described as f−3 dB.
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Question 9 of 32
In this active filter circuit, how will the filter’s cutoff frequency be affected by changes in the load resistance? Be as specific as you can in your answer.
Reveal answerfcutoff is unaffected by changes in load resistance.
Follow-up question: explain the opamp’s role in providing immunity to the filter circuit from load resistance changes. How does it accomplish this feat?
Notes:Ask your students what the function of the opamp is, taken by itself. What do we call an opamp that has its output directly connected to its inverting input? How does this function and name relate to the granting of load-impedance immunity in the filter circuit shown in the question?
