====== Active RC universal filter ======
===== Goals =====
* Understand the operation of an active RC (i.e. opamp-RC) filter
===== Lecture =====
* [[http://www.ee.iitm.ac.in/~nagendra/EC330/201401/lectures/activeRCfilter/activeRCfilter.html|Brief introductory lecture about this experiment]]
===== References =====
* Analog Circuits Course lectures on [[http://www.ee.iitm.ac.in/~nagendra/videolectures/doku.php?|VLSI group videolectures]]
* [[http://www.ee.iitm.ac.in/~nagendra/shanthi_lectures/EC201/14NovEC201/14NovEC201.html|Synthesis of a second order active-RC filter with bandpass and low-pass outputs.]]
* [[http://www.ee.iitm.ac.in/~nagendra/shanthi_lectures/EC201/15NovEC201/15NovEC201.html|Synthesis of a biquad with arbitrary numerator, the effect of finite input and output impedance of opamps in feedback amplifiers.]]
===== Preliminaries(to be done before the lab) =====
The circuit below can have inputs Vi1,R, Vi1,C, Vi2,R, Vi2,C, Vi3,R and output V1, V2, V3.
Determine the following:
* Transfer functions for all input-output combinations.
* The components on which the resonance frequency and the quality factor depend on.
* The components on which the zeroes depend on.
* Component values for a bandpass filter (with V1 as output) with a resonance frequency of 10kHz and a quality factor of 10. Determine where you will apply the input.
{{:courses:ec330:ttbiquad.png?600}}
===== Experiment =====
* Use LF347 quad opamp for this experiment
* Build a bandpass filter (with V1 as output) for a resonance frequency of 10kHz and a quality factor of 10. Where will you apply the input? (Omit all unnecessary components from the circuit) Verify its operation.
* While keeping the circuit the same, can you take the output from a different point to realize a lowpass filter? Verify it.
* What are the minimum modifications required to get a notch filter output at V1? Verify it.
* Make the minimum modifications required to obtain a maximally flat lowpass response and verify it. A maximally flat all pole lowpass response has only the highest power of ω in the denominator of |H(jω)|2.
* Modify the above circuit to get a highpass filter output at V1? Verify it.
* Restore the circuit to the bandpass filter in the first part. Replace the opamp LF347 with LM324 which has an identical pin configuration(hopefully you don't have a mess of wires running over the chip!) What do you see? Why?
===== Applications =====
* Active RC filters are the most popular topologies of RC filters. For example, they are used as intermediate frequency filters in radio receivers(=radios, mobile phones, GPS, ...). At very high frequencies, active RC filters cannot be realized because of difficulties in realizing stable feedback loops with high gains, and gm-C filters are used instead.