BandpassFilter

Bandpass filter is an electronic filter that allows signals whose frequencies lie within a specified range to pass while attenuating frequencies outside that range. The passband is centered around a center frequency fc and has a bandwidth BW, with the quality factor Q = fc/BW indicating its selectivity. Bandpass behavior can be realized by combining high-pass and low-pass responses or by resonant circuits that selectively amplify a narrow band around fc.

Analog implementations commonly use resonant LC circuits, as in RLC networks or cascaded stages, and can be active with operational amplifiers to improve gain and control. Digital implementations realize bandpass filtering through FIR or IIR structures, performing convolution or difference equations to shape the spectrum. The design order determines the sharpness of the transition between passband and stopbands and the amount of passband ripple and stopband attenuation.

A bandpass filter is characterized by its frequency response, including the flatness or ripple within the passband, the attenuation in the stopbands, and the phase or group delay. In some applications linear phase is desirable, influencing the choice of filter type. Typical parameters include center frequency, bandwidth, passband ripple, and stopband attenuation.

Applications span communications receivers, RF front-ends, audio processing, instrumentation, and scientific measurement. Design considerations include impedance matching, component tolerances and parasitics for analog filters, sampling rate and quantization effects for digital filters, and stability and complexity versus performance for both implementations.