Filters of some sort are essential to the operation of most
Filters and Signals: What Does a
In circuit theory, a filter is an electrical network that alters the amplitude and/or phase characteristics of a signal with respect to frequency. Ideally, a filter will not add new frequencies to the input signal, nor will it change the component frequencies of that signal, but it will change the relative amplitudes of the various frequency components and/or their phase relationships. Filters are often used in electronic systems to emphasize signals in certain frequency ranges and reject signals in other frequency ranges. Such a filter has a gain which is dependent on signal frequency. The Basic Filter Types:
There are five basic filter types
Bandpass filters are used in electronic systems to separate a signal at one frequency or within a band of frequencies from signals at other frequencies. The number of possible
bandpass response characteristics is infinite, but they all
share the same basic form.
Notch or Band-Reject:
A filter with effectively the opposite function of the bandpass is the band-reject or notch filter.
A third filter type is the low-pass. A low-pass filter passes low frequency signals, and rejects signals at frequencies above
the filter's cutoff frequency.
The opposite of the low-pass is the high-pass filter, which
rejects signals below its cutoff frequency.
All-Pass or Phase-Shift:
The fifth and final filter response type has no effect on the amplitude of the signal at different frequencies. Instead, its function is to change the phase of the signal without affecting its amplitude. This type of filter is called an all-pass or phaseshift filter.
Approaches To Implementing Filters: Active, Passive, And Switched-Capacitor: Passive Filters:
The filters made up of passive components resistors, capacitors, and inductors are referred to as passive filters. A passive filter is simply a filter that uses no amplifying elements (transistors, operational amplifiers, etc.). In this respect, it is the simplest (in terms of the number of necessary components) implementation of a given transfer function. Passive filters have other advantages as well. Because they have no active components, passive filters require no power supplies. They can work well at very high frequencies.They can be used in applications involving larger current or voltage levels than can be handled by active devices. Passive filters also generate little noise when compared with circuits using active gain elements. The noise that they produce is simply the thermal noise from the resistive components and, with careful design, the amplitude of this noise can be very low.
Passive filters have some important disadvantages in certain applications, however. Since they use no active elements, they cannot provide signal gain. Input impedances can be lower than desirable, and output impedances can be higher the optimum for some applications, so buffer amplifiers maybe needed. Inductors are necessary for the synthesis of most useful passive filter characteristics, and these can be prohibitively expensive. Active Filters:
Active filters use amplifying elements, especially op amps,
with resistors and capacitors in their feedback loops,to synthesize the desired filter characteristics. Active filters can have high input impedance, low output impedance, and virtually any arbitrary gain. They are also usually easier to design than passive filters. Possibly their most important attribute is that they lack inductors, thereby reducing the problems associated with those components. Still, the problems of accuracy and value spacing also affect capacitors, although to a lesser degree. Performance at high frequencies is limited by the gain-bandwidth product of the amplifying elements, but within the...
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