Describe the basic operation of a notch (band-stop) filter and a common implementation using an op-amp.

• A. A notch filter attenuates a narrow frequency band; implemented with parallel RLC tuned to the target frequency or via all-pass structures with a notch in the transfer function.
• B. A notch filter amplifies a narrow frequency band.
• C. A notch filter attenuates all frequencies equally.
• D. A notch filter uses a high-pass with a defined Q.

Answer: (A)

A notch filter is designed to make a very sharp dip in the output at a specific frequency while leaving other frequencies mostly untouched. That means it attenuates a narrow band around the target frequency and preserves the rest of the spectrum.

A common way to realize this in hardware is with a parallel resonant circuit (a resistor in parallel with LC). At the resonant frequency, the LC branch presents a very high impedance, which creates a zero in the transfer function and pulls the output toward zero at that frequency. The amount of damping—often set by the parallel resistor—controls how wide or sharp the notch (the Q) is. In op-amp implementations, that parallel RLC can be placed in the feedback path or the signal path to produce the same notch effect in an active circuit. Another active approach uses all-pass structures arranged so the overall transfer function has a notch: the all-pass network shifts phase in a frequency-dependent way, and when combined with a direct path, destructive interference cancels the signal at the target frequency.

So the best description is that a notch filter attenuates a narrow frequency band and is commonly implemented with a parallel RLC tuned to the target frequency or with all-pass-based structures that create a notch in the transfer function. The other options don’t describe this selective attenuation behavior.