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 — courses:ec330_2013:activercfilter [2013/01/17 02:47] (current) Line 1: Line 1: + ====== Active RC universal filter ====== + * **Goals:** + * Understand the operation of an active RC (i.e. opamp-RC) filter + * Use LF347 quad opamp for this experiment + ===== To be done before the lab session ===== + The circuit below can have inputs V<​sub>​i1,​R,​ V<​sub>​i1,​C,​ V<​sub>​i2,​R,​ V<​sub>​i2,​C,​ V<​sub>​i3,​R​ and output V<​sub>​1,​ V<​sub>​2,​ V<​sub>​3​. ​ + + 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 V<​sub>​1​ 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}} + + ===== To be done in the lab session ===== + + * Build a bandpass filter (with V<​sub>​1​ 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 V<​sub>​1?​ 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)|<​sup>​2​. + * Modify the above circuit to get a highpass filter output at V<​sub>​1?​ 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 g<​sub>​m​-C filters are used instead.