![]() Now that we understand the basic concept of a Band Stop Filter, let’s create one using the previous cut-off frequency values.Also, if we need a band-stop filter with -3dB cut-off points at 1kHz and 10kHz and a stop band gain of -10dB in between, we can easily design a low-pass and a high-pass filter with these specifications and cascade them together to form our wide-band band-pass filter design.By simply adding input and feedback resistors, the two non-inverting voltage followers can be easily converted into a basic non-inverting amplifier with a gain of Av = 1 + R/Rin. The incorporation of operational amplifiers into the band stop filter design also allows us to incorporate voltage gain into the basic filter circuit.The summing amplifier combines the output of the low-pass and high-pass filters. As we can see, the circuit combines a low-pass filter and a high-pass filter. ![]() As we can see, it is an active notch filter circuit with operational amplifiers. A notch filter circuit diagram is shown below.However, input signal frequencies between 200Hz and 800Hz, that is, L to H, would be rejected by either filter, resulting in a notch in the filter’s output response. Similarly, all input frequencies above 800Hz would be unattenuated and passed to the output by the high-pass filter. The low-pass filter would pass all input frequencies below 200Hz unattenuated to the output.Because the two filters are effectively connected in parallel, the input signal is applied to both filters at the same time, as illustrated below. Assume a first-order low-pass filter with a cut-off frequency of 200Hz is connected in parallel with a first-order high-pass filter with a cut-off frequency of 800Hz. This is because their starting and ending frequencies are at different frequency points. In contrast to the band-pass filter, the summing of the high pass and low pass filters means that their frequency responses do not overlap. ![]() This is accomplished by running the signals through a notch filter. In many applications, the narrowband signal must be removed without changing the band energy. Signal processing techniques’ important applications include the detection and filtering of narrowband signals in the presence of noise.And thereby, the function of a Notch Filter is to pass all frequencies from zero (DC) up to the lower cut-off frequency (fL) and above the higher cut-off frequency (fH), while rejecting all frequencies that fall within the bandwidth region, i.e., BW= FH-fL. ![]() As a result, the notch filter is a supplement to the bandpass filter. A band-stop filter has the exact opposite characteristics of a bandpass filter.
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