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# Maala

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### Maala

1. 1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite Experiment No. 3 ACTIVE LOW-PASS and HIGH-PASS FILTERSMaala, Michelle Anne C. July 14, 2011Signal Spectra and Signal Processing/BSECE 41A1 Score: Engr. Grace Ramones Instructor
2. 2. OBJECTIVES: 1. Plot the gain-frequency response and determine the cutoff frequency of a second-order (two-pole) low-pass active filter. 2. Plot the gain-frequency response and determine the cutoff frequency of a second-order (two-pole) high-pass active filter. 3. Determine the roll-off in dB per decade for a second-order (two-pole) filter. 4. Plot the phase-frequency response of a second-order (two-pole) filter.
3. 3. VOLTAGE GAIN (MEASURED VALUE): Step 3 AdB = 20 log A 4.006 = 20 log A Step 14 AdB = 20 log A 3.776 = 20 log A A = 1.54VOLTAGE GAIN(CALCULATED): Step 4PERCENTAGE DIFFERENCE: Q Step 4 Q Step 6 Q Step 15CUTOFF FREQUENCY (CALCULATED): Step 6 and 17 Step 11
4. 4. DATA SHEET:MATERIALS One function generator One dual-trace oscilloscope One LM741 op-amp Capacitors: two 0.001 µF, one 1 pF Resistors: 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,high input impedance, and low output impedance. The voltage gain reducesattenuation of the signal by the filter, the high input prevents excessive loading of thesource, and the low output impedance prevents the filter from being affected by theload. Active filters are also easy to adjust over a wide frequency range without alteringthe desired response. The weakness of active filters is the upper-frequency limit due tothe limited open-loop bandwidth (funity) of op-amps. The filter cutoff frequency cannotexceed the unity-gain frequency (funity) of the op-amp. Ideally, a high-pass filter shouldpass all frequencies above the cutoff frequency (fc). Because op-amps have a limitedopen-loop bandwidth (unity-gain frequency, funity), high-pass active filters have anupper-frequency limit on the high-pass response, making it appear as a band-pass filterwith a very wide bandwidth. Therefore, active filters must be used in applications wherethe unity-gain frequency (funity) of the op-amp is high enough so that it does not fallwithin the frequency range of the application. For this reason, active filters are mostlyused 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 frequencyresponse, it is called a Bode plot. A Bode plot is an ideal plot of filter frequencyresponse because it assumes that the voltage gain remains constant in the passbanduntil the cutoff frequency is reached, and then drops in a straight line. The filter networkvoltage gain in dB is calculated from the actual voltage gain (A) using the equation AdB = 20 log A