Notch filters are a highly selective, high-Q, form of the band stop filter which can be used to reject a single or very small band of frequencies rather than a whole bandwidth of different frequencies. This type of filter design is called a “Notch Filter”. But we can also design and construct band stop filters to produce a much narrower frequency response to eliminate specific frequencies by increasing the selectivity of the filter. We have seen above that simple band stop filters can be made using first or second order low and high pass filters along with a non-inverting summing op-amp circuit to reject a wide band of frequencies. Find the geometric center frequency, -3dB bandwidth and Q of the circuit. Band Stop Filter Example No1ĭesign a basic wide-band, RC band stop filter with a lower cut-off frequency of 200Hz and a higher cut-off frequency of 800Hz. Now we understand the principle behind a Band Stop Filter, let us design one using the previous cut-off frequency values. The two non-inverting voltage followers can easily be converted into a basic non-inverting amplifier with a gain of Av = 1 + Rƒ/Rin by the addition of input and feedback resistors, as seen in our non-inverting op-amp tutorial.Īlso if we require a band stop filter to have its -3dB cut-off points at say, 1kHz and 10kHz and a stop band gain of -10dB in between, we can easily design a low-pass filter and a high-pass filter with these requirements and simply cascade them together to form our wide-band band-pass filter design. The use of operational amplifiers within the band stop filter design also allows us to introduce voltage gain into the basic filter circuit. We can show the effect of this frequency characteristic below. In other words a signal with a frequency of 200Hz or less and 800Hz and above would pass unaffected but a signal frequency of say 500Hz would be rejected as it is too high to be passed by the low-pass filter and too low to be passed by the high-pass filter. However, and input signal frequencies in-between these two frequency cut-off points of 200Hz and 800Hz, that is ƒ L to ƒ H would be rejected by either filter forming a notch in the filters output response. Likewise, all input frequencies above 800Hz would be passed unattenuated to the output by the high-pass filter. As the two filters are effectively connected in parallel, the input signal is applied to both filters simultaneously as shown above.Īll of the input frequencies below 200Hz would be passed unattenuated to the output by the low-pass filter. For example, suppose we have a first-order low-pass filter with a cut-off frequency, ƒ L of 200Hz connected in parallel with a first-order high-pass filter with a cut-off frequency, ƒ H of 800Hz. This is due to the fact that their start and ending frequencies are at different frequency points. The summing of the high pass and low pass filters means that their frequency responses do not overlap, unlike the band-pass filter. If this stop band is very narrow and highly attenuated over a few hertz, then the band stop filter is more commonly referred to as a notch filter, as its frequency response shows that of a deep notch with high selectivity (a steep-side curve) rather than a flattened wider band.Īlso, just like the band pass filter, the band stop (band reject or notch) filter is a second-order (two-pole) filter having two cut-off frequencies, commonly known as the -3dB or half-power points producing a wide stop band bandwidth between these two -3dB points. The band stop filter, also known as a band reject filter, passes all frequencies with the exception of those within a specified stop band which are greatly attenuated. The Band Stop Filter, (BSF) is another type of frequency selective circuit that functions in exactly the opposite way to the Band Pass Filter we looked at before. But we can also combine these low and high pass filter sections to produce another kind of RC filter network called a Band Stop Filter which can block or at least severely attenuate a band of frequencies within these two cut-off frequency points. By combining a basic RC low-pass filter with a RC high-pass filter we can form a simple band-pass filter that will pass a range or band of frequencies either side of two cut-off frequency points.
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