A band-stop filter is a circuit that attenuates a specific range of frequencies while letting frequencies below and above that range pass. In Electrical Circuits and Systems I, you use it to study frequency response, Bode plots, and noise rejection.
A band-stop filter is a circuit in Electrical Circuits and Systems I that blocks a chosen band of frequencies and lets the rest of the signal through. You will also hear it called a notch filter when the rejected band is very narrow. The main idea is simple: keep the useful parts of the signal, remove the unwanted slice.
That rejected slice is called the stopband. If the filter is centered near one frequency, the response on a Bode magnitude plot drops sharply around that center. Outside the stopband, the circuit should have much less attenuation, so the signal recovers on either side of the dip.
A band-stop filter can be built with passive parts like resistors, capacitors, and inductors, or with active parts such as op-amps. Passive designs are common when you want a straightforward frequency-shaping network. Active designs can give you sharper control, buffering, and sometimes better performance in a lab circuit because the op-amp isolates parts of the network and can boost the signal outside the rejected band.
The shape of the response matters just as much as the idea of rejection. A wide stopband removes a broader range of frequencies, while a high quality factor, or Q, means the dip is narrow and concentrated. That is why a notch filter is useful when you want to remove one problem frequency, like 60 Hz power-line hum, without flattening the rest of the audio or measurement signal.
In practice, you usually look at a band-stop filter in frequency-response language rather than time-domain language. A sinusoid near the stopband center comes out much smaller, while sinusoids far away from that band come out nearly unchanged. That makes the filter easy to spot on Bode plots and easy to reason about when you are designing circuits for audio, communication, or instrumentation.
Band-stop filters show up right where this course starts to connect circuit elements with real signal behavior. They are a clean example of frequency selectivity, which is one of the big goals of AC steady-state analysis and Bode plots. Instead of just asking whether a circuit has voltage or current, you ask which frequencies survive and which ones get suppressed.
That matters when a system has one annoying interference source sitting inside an otherwise useful signal. A measurement circuit might pick up a narrow hum from power wiring, and a communication circuit might need to reject a narrow jammer or spur. A band-stop filter lets you remove that narrow band without destroying everything around it.
The term also helps you compare filter types more clearly. If a low-pass filter keeps low frequencies and a high-pass filter keeps high frequencies, a band-stop filter keeps both sides and cuts out the middle. That comparison shows up often when you are choosing a circuit for a lab design or interpreting a plot from a homework problem.
It also gives you practice reading Bode magnitude plots correctly. A deep dip means strong attenuation in the stopband, while the steepness and width of that dip tell you something about Q and the filter’s selectivity. Once you can read that shape, you can move from a graph to a physical circuit goal pretty quickly.
Keep studying Electrical Circuits and Systems I Unit 9
Visual cheatsheet
view galleryLow-pass filter
A low-pass filter and a band-stop filter can both reduce unwanted high-frequency content, but they do it in different ways. A low-pass filter keeps the lower frequencies and gradually rolls off above cutoff, while a band-stop filter creates a dip only in a chosen middle range. When you compare their Bode plots, you are comparing a smooth cutoff shape with a selective notch.
High-pass filter
A high-pass filter is the mirror idea to a low-pass filter, since it rejects low frequencies and passes higher ones. A band-stop filter is different because it rejects a middle band while passing both low and high frequencies. In problem sets, this comparison helps you decide whether the circuit should remove slow drift, remove one interference tone, or do both.
cutoff frequency
Cutoff frequency helps define where attenuation begins or becomes significant, and band-stop filters usually have two cutoff points that mark the edges of the rejected band. Those edge values tell you how wide the stopband is. When you read or sketch a response curve, the cutoff frequencies tell you where the dip starts and ends.
Resonant frequency
A band-stop filter often has a center frequency where attenuation is deepest, and that center is closely tied to resonance in the circuit. For a notch filter, the resonant behavior of the network is what creates the sharp rejection. If you shift the resonant frequency, you shift the frequency that gets removed from the signal.
A quiz question may give you a Bode plot and ask which circuit matches it, so you identify the deep dip as a band-stop filter and describe the frequencies that are rejected. A problem set might ask you to design a circuit that removes 60 Hz hum, which means you choose a notch centered at that frequency and check that nearby signal content still passes. In lab work, you might measure the input and output amplitudes across a sweep of frequencies, then mark the stopband and estimate Q from the width of the dip. If the prompt asks for interpretation, say that the circuit suppresses a narrow interference band instead of filtering everything above or below one cutoff.
A band-pass filter does the opposite job: it passes only a middle range and rejects frequencies below and above that band. A band-stop filter rejects the middle range and passes the frequencies on both sides. They can look similar at a glance because both are described with a frequency band, but the output behavior is reversed.
A band-stop filter rejects a specific frequency range and passes frequencies outside that range.
In Electrical Circuits and Systems I, you usually identify it from a Bode plot that has a dip in the middle instead of a rise.
A narrow band-stop filter is often called a notch filter, especially when it removes one unwanted tone like 60 Hz hum.
The width of the rejected band and the quality factor Q tell you how selective the filter is.
You can build band-stop filters with passive RLC parts or with active op-amp circuits, depending on the design goal.
It is a circuit that reduces a chosen range of frequencies while allowing frequencies outside that range to pass. In this course, you usually meet it in frequency-response problems, Bode plots, and filter design examples.
They are closely related, but not always identical. A notch filter usually means a very narrow band-stop filter that targets one specific frequency, while band-stop can describe a wider rejected range.
Look for a dip in the magnitude response at the frequencies being rejected. The plot should be lower in the middle band and higher on both sides, which shows that the circuit is passing frequencies outside the stopband.
You use it when one unwanted frequency band is interfering with the signal you care about. A classic example is removing 60 Hz power-line hum from audio or measurement circuits without changing the rest of the signal very much.