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Test! OpAmp Active Filters Q. What is v0? Filter • What is Active filter? • What is Passive filter? • What is analog filter? • What is digital filter? An analog filter processes continuous-time signals A digital filter processes discrete-time signals • • • • • • What is filter? Purpose of filter? Types of filters? What is cut-off frequency? Draw frequency responses of 4 filters? What is pass-band? • Pros and cons of active filter? • Pros and cons of passive filter? Passive Filters • made up of passive components - resistors, capacitors and inductors • no amplifying elements (- transistors, op-amps, etc) • no signal gain • 1st order - design is simple (just use standard equations to find resonant frequency of the circuit) • 2nd order - complex equations • require no power supplies • not restricted by the bandwidth limitations of the op-amps • can be used at very high frequencies • can handle larger current or voltage levels than active devices • They are particularly non-ideal (lossy) • They are bulky and expensive Ref:080222HKN EE3110 Active Filter (Part 1) 6 Passive elements : Inductor • high accuracy (1% or 2%), small physical size, or large inductance values are required?? • standard values of inductors are not very closely spaced • difficult to find an off-the-shelf inductor within 10 percent of any arbitrary value • adjustable inductors are used • tuning such inductors to the required values is timeconsuming and expensive for larger quantities of filters • inductors are often prohibitively expensive EE3110 Active Filter (Part 1) 7 Active Filter • • • • no inductors made up of op-amps, resistors and capacitors provides virtually any arbitrary gain generally easier to design • high input impedance prevents excessive loading of the driving source • low output impedance prevents the filter from being affected by the load • at high frequencies is limited by the gain-bandwidth of the opamps • easy to adjust over a wide frequency range without altering the desired response Ref:080222HKN EE3110 Active Filter (Part 1) 8 • Draw ideal filter responses of 4 types of filters. • Draw the same for practical filters. • Draw the Bessel characteristics of a LPF. • Draw the same for a HPF. Bessel Characteristic • Flat response in the passband. • Phase response is linear. • Used for filtering pulse waveforms without distorting the shape of the waveform. Ref:080222HKN Av EE3110 Active Filter (Part 1) f 11 • Draw Butterworth characteristic for HPF. Butterworth Characteristic • Very flat amplitude, Av(dB) , response in the passband. • • • Av Phase response is not linear. Used when all frequencies in the passband must have the same gain. Often referred to as a maximally flat response. Ref:080222HKN EE3110 Active Filter (Part 1) f 13 • Draw Chebyshev characteristic for LPF • Draw the same for BPF Chebyshev Characteristic • Overshoot or ripples in the passband. • • • Av Role-off rate greater than 20dB/decade/pole. Phase response is not linear worse than Butterworth. Used when a rapid roll-off is required. Ref:080222HKN EE3110 Active Filter (Part 1) f 15 • Draw a passive LPF filter circuit. • Draw an active LPF filter circuit. Passive LPF R Vo XC Vi XC R Vi 1 1 jC Vo Vi Vi 1 1 jCR R jC C Vo Active LPF using OpAmp • What is cut-off frequency for LPF? • What is ,,, for HPF? • What is ,,, for BPF? • What is ,,, for BRF? • What is dB/decade? dB/… • In many situations, the magnitude attenuation of a signal is given in units of dB/decade or dB/octave. • The decibel is defined in terms of a power (P) ratio, or the magnitude ratio (M). • A decade is defined as a 10-fold increase in frequency. • An octave is defined as a 2-fold increase (doubling) in frequency. • How can we improve the performance of a filter? Better response? 2nd order? • Draw a circuit for active HPF. HPF • Draw a circuit for 2nd order HPF. 2nd order HPF • Draw a circuit for BPF. Two-Stage Band-Pass Filter C2 R4 +V R2 R1 +V + C4 C3 + vin C1 - R3 - Rf1 -V vout -V Rf3 Rf2 Rf4 Stage 1 Two-pole low-pass Stage 2 Two-pole high-pass Av Stage 2 response BW f1 Ref:080222HKN Stage 1 response fo BW = f2 – f1 Q = f0 / BW f f2 EE3110 Active Filter (Part 1) 30 • Draw a model for BRF, BSF, BEF, notch filter. Band-Stop (Notch) Filter The notch filter is designed to block all frequencies that fall within its bandwidth. The circuit is made up of a high pass filter, a low-pass filter and a summing amplifier. The summing amplifier will have an output that is equal to the sum of the filter output voltages. Av(dB) Low pass filter f1 vin High pass filter low-pass high-pass { Summing amplifier -3dB vout f2 f f1 Block diagram Ref:080222HKN f2 Frequency response EE3110 Active Filter (Part 1) 32 Notch filter Ref:080222HKN EE3110 Active Filter (Part 1) 33