Band-stop fllters

Chapter 8.5 Band-stop fllters

Lessons In Electric Circuits Volume II – AC Book
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Lessons In Electric Circuits Volume II – AC Book

  • 200CHAPTER 8. FILTERSsections will always be blocking signals to some extent, and their combined effort makes for anattenuated (reduced amplitude) signal at best, even at the peak of the “pass-band” frequency range.Notice the curve peak on the previous SPICE analysis: the load voltage of this filter never risesabove 0.59 volts, although the source voltage is a full volt. This signal attenuation becomes morepronounced if the filter is designed to be more selective (steeper curve, narrower band of passablefrequencies).There are other methods to achieve band-pass operation without sacrificing signal strength withinthe pass-band. We will discuss those methods a little later in this chapter.• REVIEW:• A band-pass filter works to screen out frequencies that are too low or too high, giving easypassage only to frequencies within a certain range.• Band-pass filters can be made by stacking a low-pass filter on the end of a high-pass filter, orvice versa.• “Attenuate” means to reduce or diminish in amplitude. When you turn down the volumecontrol on your stereo, you are “attenuating” the signal being sent to the speakers.8.5Band-stop filtersAlso called band-elimination, band-reject, or notch filters, this kind of filter passes all frequenciesabove and below a particular range set by the component values. Not surprisingly, it can be madeout of a low-pass and a high-pass filter, just like the band-pass design, except that this time weconnect the two filter sections in parallel with each other instead of in series. (Figure 209,8.17)Low-pass filterHigh-pass filterSignalinputSignaloutputpasses low frequenciespasses high frequenciesFigure 8.17: System level block diagram of a band-stop filter.Constructed using two capacitive filter sections, it looks something like (Figure 210,8.18).The low-pass filter section is comprised of R1, R2, and C1 in a “T” configuration. The high-pass filter section is comprised of C2, C3, and R3 in a “T” configuration as well. Together, thisarrangement is commonly known as a “Twin-T” filter, giving sharp response when the componentvalues are chosen in the following ratios: