Clearly for a pass band filter to function correctly, the cut-off frequency of the low pass filter must be higher than the cut-off frequency for the high pass filter.
Then for widely spread frequencies, we can simply define the term “bandwidth”, BW as being the difference between the lower cut-off frequency ( ƒc LOWER ) and the higher cut-off frequency ( ƒc HIGHER ) points. This band of frequencies can be any width and is commonly known as the filters Bandwidth.īandwidth is commonly defined as the frequency range that exists between two specified frequency cut-off points ( ƒc ), that are 3dB below the maximum centre or resonant peak while attenuating or weakening the others outside of these two points.
Unlike the low pass filter which only pass signals of a low frequency range or the high pass filter which pass signals of a higher frequency range, a Band Pass Filters passes signals within a certain “band” or “spread” of frequencies without distorting the input signal or introducing extra noise. This new type of passive filter arrangement produces a frequency selective filter known commonly as a Band Pass Filter or BPF for short. Sometimes it is necessary to only pass a certain range of frequencies that do not begin at 0Hz, (DC) or end at some upper high frequency point but are within a certain range or band of frequencies, either narrow or wide.īy connecting or “cascading” together a single Low Pass Filter circuit with a High Pass Filter circuit, we can produce another type of passive RC filter that passes a selected range or “band” of frequencies that can be either narrow or wide while attenuating all those outside of this range. One simple use for these types of passive filters is in audio amplifier applications or circuits such as in loudspeaker crossover filters or pre-amplifier tone controls. The cut-off frequency or ƒc point in a simple RC passive filter can be accurately controlled using just a single resistor in series with a non-polarized capacitor, and depending upon which way around they are connected, we have seen that either a Low Pass or a High Pass filter is obtained. The Passive Band Pass Filter can be used to isolate or filter out certain frequencies that lie within a particular band or range of frequencies.
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