Active band pass filters are used to attenuate frequencies below and above a range of frequencies (i.e., the bandwidth or passband of the filter). Any signal with a frequency within that band pass range passes easily through the filter. Any frequency outside of the band pass is attenuated or reduced. There are two basic types of active band pass filters: switched capacitor and continuous. Each type of filter is available with first, second, third, fourth, fifth, sixth, seventh or eighth order filtering. Switched capacitor active band pass filters are clocked devices. The input signal is sampled at a high rate and processed discretely instead of on a continuous-time basis. Continuous active band pass filters have a continuous time operation.
Active band pass filters carry performance specifications for cutoff or center frequency, bandwidth, power dissipation, operating temperature, supply voltage and supply current. Like other types of active filters, they also differ in terms of features and applications. For example, some active band pass filters feature protection against electrostatic discharge (ESD), short circuits, and over current conditions. Others are radiation-tolerant or can operate with only one power supply. In terms of applications, active band pass filters may be rated for audio/video, automotive, avionics, commercial, communications, computers, data acquisition, general industrial, medical, or military applications. Some active band pass filters are also used in portable devices such as cell phones and personal digital assistants (PDA).
Filter characteristic is an important specification to consider when selecting active band pass filters. Choices include Bessel, Butterworth, Chebyshev, elliptic, Gaussian, and Legendre. Bessel filters are active band pass filters with a passband that maximizes the group delay at zero frequency, thus showing a constant group delay in the passband. Butterworth filters are designed so that the frequency response is flat in the passband. Chebyshev filters feature a very steep roll-off, but have ripples in the passband. Elliptic filters exhibit equalized ripple in both the passband and the stopband. Gaussian filters produce no overshoot in response to an input step. Instead, they optimize the rise and fall times. Legendre filters are designed to produce the maximum roll-off rate for a given order and a flat frequency response in the passband.