Lab 7

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University Of Connecticut *

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Course

2001

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Electrical Engineering

Date

Feb 20, 2024

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6

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Experiment Number Seven: Active Filter Circuits Jack Taylor ECE 2001, 11/16/2023 Abstract In this experiment we are testing second order active filter circuits. We tested the amplitude response to see how these circuits react to different frequencies. We calculated the center frequency, quality factor, and the resonant gain. We found that our graphs matched the same shapes as the calculated results, but the values were off. Introduction In this lab, we will be creating three different second order active filter circuits using op amps. We will explore the characteristics of the circuits both experimentally and theoretically using PSpice. Theory For this experiment, we will be using active filter circuits. There are three types of filter that we will be using. There are low pass filters, which only allow low frequencies to pass through. High pass filters only allow for higher frequencies to pass. Lastly, band pass filters only allow a certain range of frequencies to pass. For the third circuit, we will have to design our own bandpass filter. To do this we need to use equations for the center frequency (f 0 ), quality factor (Q), and resonant gain (A r ). 𝑓 0 = 1 ? 1 ? 2 (1) ? = 1 2 ? 2 ? 1 (2) 𝐴 𝑟 =− ? 2 2? 1 (3)
Experimental Procedures For our first circuit, we will be building the low pass filter in figure 7-1. We will use two 1kΩ resistors and two 10nF capacitors. We will measure the amplitude response over ten points. In PSpice, we will measure the same amplitude response and compare our results. For the second circuit, we will build the high pass filter circuit in figure 7-2. We will use the same resistor and capacitor values and measure the amplitude response. We will
also use PSpice to calculate the amplitude response. For our last circuit, we will be using the band pass filter circuit in figure 7-3 to design our own circuit. The center frequency will be 12 kHz ± 10%, the quality factor, q, will be 0.4 ± 5%. Results and Discussion
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For circuit one, we measured and plotted our results in figure 7-4. We can see that the experimental circuit has a lower cutoff frequency than our PSpice circuit does. The theoretical amplitude response also has a lower peak than the experimental data does. frequency (Hz) Amplitude (dB) 1000 0 2000 0 3000 -1.462865821 5000 -1.012199867 10000 -4.2934033 100000 -26.37517525 250000 -20.17547849 500000 -16.47817482 750000 -14.8945499 1000000 -14.15487857 5000000 -11.70053304 10000000 -18.06179974 For the second circuit, we measured the amplitude response is figure 7-5. We can see that the two cutoff frequencies for the experimental data are at about 80 kHz and 150 kHz. Our calculated cutoff frequencies were at 25 kHz and 1.2 MHz. These values are drastically different. frequency (Hz) Amplitude (dB) 1000 -40 2000 -31.86919639 3000 -25.67993313
5000 -18.06179974 10000 -9.370421659 100000 -0.915149811 150000 -2.498774732 250000 -6.650940942 500000 -12.39577517 750000 -15.26421801 1000000 -16.71294288 5000000 -8.223365488 10000000 -16.89327925 For our third circuit, we used a 1.6 kΩ and a 1.3kΩ resistor, with two 10nF capacitors. We measured our results in figure 7-6. For both the experimental and theoretical data, there were no cutoff frequencies. There were some issues with finding data, partly because we didn’t have the 15nF that the directions told us to use. frequency (Hz) Amplitude (dB) 1000 -19.57621402 2000 -14.19930777 3000 -11.50236727 5000 -9.118639113 10000 -7.958800173 100000 -17.72113295 150000 -18.41637508 250000 -18.20189777 500000 -16.47817482
750000 -15.28943106 1000000 -14.8464285 5000000 -13.55561411 10000000 -20 Conclusion Overall, our graphs followed the same shape as the theoretical graphs. However, there were some large discrepancies with cutoff frequencies and overall values. This means that our circuits did follow the same characteristics as the theoretical data, but did not give us the same values. This could be because of component tolerances or components not being ideal.
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