Active Filter Types

Types of Active Filters Butterworth, Chebyshev, Bessel and Elliptic filters. There are basically 4 types of active  filters. They are butterworth, Chebyshev, Bessel and Elliptic filters. Butterworth Filter: This filter is also called as maximally flat or flat flat filter. This class of filters approximates the ideal filter well in the pass band. Frequency response curves of different types of filters are shown in figure. The Butterworth filter has an essentially flat amplitude-fre­quency response upto the cut­off frequency. The sharpness of the cut-off can be seen in the figure. It is to be noted that all the three filters reach a…

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State Variable Filters

With the advancement in IC technology, a number of manufacturers now offer universal filters having simultaneous low-pass, high-pass, and band-pass output responses. Notch and all-pass functions are also available by combining these output responses in the uncommitted op-amp. Because of its versatility, this filter is called the universal filter. It provides the user with easy control of the gain and Q-factor. It is also called a state-variable filter.   The filters we have discussed so far are relatively simple single op-amp circuits or several single op-amp circuits cascaded. The state-variable filter, however, makes use of three or four op-amps and…

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All pass filters

An all-pass filter is that which passes all frequency components of the input signal without attenuation but provides predictable phase shifts for different frequencies of the input signals. The all-pass filters are also called delay equalizers or phase correctors. An all-pass filter with the output lagging behind the input is illustrated in figure.   The output voltage vout of the filter circuit shown in fig. (a) can be obtained by using the superposition theorem vout = -vin +[ -jXC/R-jXC]2vin Substituting -jXC = [1/j2∏fc] in the above equation, we have vout ­= vin [-1 +( 2/ j2∏Rfc)] or vout / vin…

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Band Stop Filter

The bandpass fil­ter passes one set of frequen­cies while reject­ing all others. The band-stop filter does just the opposite. It rejects a band of frequencies, while passing all others. This is also called a band-reject or band-elimination filter. Like band­pass filters, band-stop filters may also be classified as (i) wide-band and (ii) narrow band reject filters. The narrow band reject filter is also called a notch filter. Because of its higher Q, which exceeds 10, the bandwidth of the narrow band reject filter is much smaller than that of a wide band reject filter. Wide Band-Stop (or Reject) Filter.  …

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Band Pass Filters

Band Pass Filter A band-pass filter is a circuit which is designed to pass signals only in a certain band of frequencies while attenuating all signals outside this band. The parameters of importance in a bandpass filter are the high and low cut-off frequencies (fH and fl), the bandwidth (BW), the centre frequency fc, centre-frequency gain, and the selectivity or Q. There are basically two types of bandpass filters viz wide bandpass and narrow bandpass filters. Unfortunately, there is no set dividing line between the two. However, a bandpass filter is defined as a wide bandpass if its figure of…

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Higher Order Filters

Higher Order Filters From the discussion made so far on the filters, it may be concluded that in the stopband the gain of the filter changes at the rate of 20 db/decade for first-order filters and 40 db/decade for second-order filters. This means that as the order of the filter is increased, the actual stopband response of the filter approaches its ideal stopband characteristics. In general, a third-order filter produces 60 db/decade, a fourth-order filter produces 80 db/decade and so on. Higher-order filters, such as third, fourth, fifth, and so on, are built simply by using the first and second-order…

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Butterworth Filter

In many low-pass filter applications it is necessary that the closed-loop gain is as close to unity as possible within the passband. The Butterworth filter is best suited for such applications. This filter is also called a maximally flat or flat-flat filter. Ideal and the practical frequency responses for three types of Butterworth low-pass filters are depicted in fig. (a) by solid line and dashed lines respectively. As the roll-off becomes steeper, they approach the ideal filter characteristics more closely. Butterworth Filters The frequency responses for three types of high-pass Butterworth filters are shown in fig. (b). By contrast, for…

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Classification of Active Filters

The most widely used active filters are (i)                 low-pass (ii)                 high-pass (iii)                band-pass (iv)               band-stop or band reject (also called the band-elimination or notch) and (v)                 all-pass filters. All of these filters use op-amps as the active elements and R-C networks. Although the 741 type op-amp operates satisfactorily in these filter circuits, high­-speed op-amps like the LM 318 or ICL 8017 improve the performance of the filter circuits through their increased slew rates and higher unity GBW. Low Pass Filter: A low-pass filter has a constant gain from 0 Hz to a high cut-off frequency fH. Therefore the bandwidth is…

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Active and Passive filters

Active and Passive filters – A Comparison: The simplest approach to building a filter is with passive components (resistors, capacitors, and inductors). In the R-F range it works quite well but with the lower frequencies, inductors create problems. AF inductors are physically larger and heavier, and therefore expensive. For lower frequencies the inductance is to be increased which needs more turns of wire. It adds to the series resistance which degrades the inductor’s performance. Input and output impedances of passive filters are both a problem, especially below RF. The input impedance is low, that loads the source, and it varies…

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Filter Circuits – Active Filters

Introduction to Filters An electric filter is a network designed to attenuate certain frequencies but pass others without attenuation. A filter circuit, therefore, possesses at least one pass band — a band of frequencies in which the output is approximately equal to the input (that is, attenuation is zero) and an attenuation band in which output is zero (that is, attenuation is infinite). The frequencies that separate the different pass and attenuation bands are called the cut-off frequencies. Filters may be of any type such as electrical, mechanical, pneumatic, hydraulic, acous­tical etc. but the most commonly used filters are of…

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