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# 3 (1)

## on Jan 21, 2012

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## 3 (1)Document Transcript

• NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite Experiment No. 3 ACTIVE LOW-PASS and HIGH-PASS FILTERSAgdon, Berverlyn B. July 14, 2011Signal Spectra and Signal Processing/BSECE 41A1 Score: Engr. Grace Ramones Instructor
• Objectives Plot the gain-frequency response and determine the cutoff frequency of a second- order (two-pole) low-pass active filter. Plot the gain-frequency response and determine the cutoff frequency of a second- order (two-pole) high-pass active filter. Determine the roll-off in dB per decade for a second-order (two-pole) filter. Plot the phase-frequency response of a second-order (two-pole) filter.
• Sample ComputationsStep 3 AdB = 20 log A 4.006 = 20 log AStep 4Question Step 4Step 6Question Step 6Q uestion Step 7 -36.146 dB – 0.968 dB = -37.106 dBStep 14 A = 1.54
• Data SheetMATERIALSOne function generatorOne dual-trace oscilloscopeOne LM741 op-ampCapacitors: two 0.001 µF, one 1 pFResistors: one 1kΩ, one 5.86 kΩ, two 10kΩ, two 30 kΩTHEORY In electronic communications systems, it is often necessary to separate a specificrange of frequencies from the total frequency spectrum. This is normally accomplishedwith filters. A filter is a circuit that passes a specific range of frequencies while rejectingother frequencies. Active filters use active devices such as op-amps combined withpassive elements. Active filters have several advantages over passive filters. The passiveelements provide frequency selectivity and the active devices provide voltage gain, highinput impedance, and low output impedance. The voltage gain reduces attenuation ofthe signal by the filter, the high input prevents excessive loading of the source, and thelow output impedance prevents the filter from being affected by the load. Active filtersare also easy to adjust over a wide frequency range without altering the desiredresponse. The weakness of active filters is the upper-frequency limit due to the limitedopen-loop bandwidth (funity) of op-amps. The filter cutoff frequency cannot exceed theunity-gain frequency (funity) of the op-amp. Ideally, a high-pass filter should pass allfrequencies above the cutoff frequency (fc). Because op-amps have a limited open-loopbandwidth (unity-gain frequency, funity), high-pass active filters have an upper-frequencylimit on the high-pass response, making it appear as a band-pass filter with a very widebandwidth. Therefore, active filters must be used in applications where the unity-gainfrequency (funity) of the op-amp is high enough so that it does not fall within thefrequency range of the application. For this reason, active filters are mostly used in low-frequency applications. The most common way to describe the frequency response characteristics of afilter is to plot the filter voltage gain (Vo/Vin) in dB as a function of frequency (f). Thefrequency at which the output power gain drops to 50% of the maximum value is calledthe cutoff frequency (fc). When the output power gain drops to 50%, the voltage gaindrops 3 dB (0.707 of the maximum value). When the filter dB voltage gain is plotted as afunction of frequency using straight lines to approximate the actual frequency response,it is called a Bode plot. A Bode plot is an ideal plot of filter frequency response because itassumes that the voltage gain remains constant in the passband until the cutofffrequency is reached, and then drops in a straight line. The filter network voltage gain indB is calculated from the actual voltage gain (A) using the equation AdB = 20 log A