text: Patent US5072200 - Combination of active and passive filters - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsA combination of passive electrical filters and active electrical filters in a series arrangement forming active passive band pass or high pass or low pass or band reject filters. The preferred embodiment includes an amplifier stage to include high input impedance. These filter combinations are free...http://www.google.com/patents/US5072200?utm_source=gb-gplus-sharePatent US5072200 - Combination of active and passive filtersAdvanced Patent SearchPublication numberUS5072200 APublication typeGrantApplication numberUS 07/608,240Publication dateDec 10, 1991Filing dateNov 2, 1990Priority dateNov 24, 1989Fee statusLapsedPublication number07608240, 608240, US 5072200 A, US 5072200A, US-A-5072200, US5072200 A, US5072200AInventorsBela RankyOriginal AssigneeBela RankyExport CitationBiBTeX, EndNote, RefManPatent Citations (7), Non-Patent Citations (2), Referenced by (27), Classifications (9), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetCombination of active and passive filtersUS 5072200 AAbstract A combination of passive electrical filters and active electrical filters in a series arrangement forming active passive band pass or high pass or low pass or band reject filters. The preferred embodiment includes an amplifier stage to include high input impedance. These filter combinations are free of internal electrical noise.
  I claim: 1. An electrical filter for attenuating part of the frequency spectrum of an input signal, comprising:at least one active amplifier means; comprising resistors and an operational amplifier element operatively connected together; and providing a high impedance input and low impendance output; at least one passive filter means; comprising inductors, and capacitor elements operatively connected together to form at least one of a lowpass, highpass, bandpass and bandreject filter; at least one active filter means; comprising resistors, capacitors, and an operational amplifier element operatively connected together to form at least one of a lowpass, highpass, bandpass and bandreject filter; wherein said active amplifier means, said passive filter means, and said active filter means are respectively coupled in a series arrangement, the input of said active amplifier means is connected to a filter input, the output of said active amplifier means is connected to an input of said passive filter means, an output of said passive filter means is connected to an input of said active filter means, an output of said active filter means is connected to a filter output. 2. The filter recited in claim 1 comprises small dimensions, on the order of, 5 cm.�4 cm.�1.5 cm.
  BACKGROUND OF THE INVENTION This is a continuation-in-part of application Ser. No. 440,699 filed Nov. 24, 1989. now abandoned.
  FIELD OF INVENTION This invention relates generally to electronic filters, particularly a combination of passive and active electronic filters.
  PRIOR ART Passive filters, so-called because they operate without a source of outside power, are known in the art. Generally, these filters comprise capacitors and inductors and make use of the reactive impedance associated with each of these types of components, to attenuate electrical signals of a certain frequency. Passive filters also attenuate, to a lesser degree, signals having frequencies which are not to be filtered, i.e., the frequency band which is to be passed. This causes a loss of energy in the desired signals. Passive filters, even those designed properly to match the source and load impedances of the system, all attenuate the pass band to a certain extent. This loss is called "insertion loss".
  SUMMARY OF THE INVENTION It is a general object of this invention to provide a filter for electrical signals.
  The disclosed series combination of a high impedance amplifier section, a passive low-pass filter section, and an active high-pass filter section provides a band-pass filter without the problems which exist when using an all-active filter or an all-passive filter to attenuate a broad frequency spectrum of electrical signals both above and below the pass-band. By using a passive low pass filter to filter relatively high frequencies from a signal, noise is not introduced into the filtered signal. An active high pass filter section to filter relatively low frequencies maintains compactness of the overall filter size by eliminating the need for large inductors and protective shielding. The use of the amplifier section allows the passive section to be operated at high impedances, further allowing small-sized inductors and capacitors to be chosen for the low-pass section. A filter according to the disclosed design is dimensioned, for example as small as 5 cm�4 cm�1.5 cm, or smaller. This is six times smaller than a passive filter alone and two times smaller than an active filter alone performing comparable filter functions.
  BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a preferred embodiment of the invention showing a series connected active amplifier, passive low pass filter and active high pass filter.
  DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, the signal to be filtered is coupled to system 9 at input 10. The signal is forwarded to active high impedance amplifier stage 11. As seen in FIG. 2, the high impedance amplifier stage 11 comprises resistors 15 and 16 connected in combination with operational amplifier 17 in the common inverting amplifier configuration as shown. Operational amplifier 17, by choice of appropriate values for resistors 15 and 16, can add some gain to increase the signal level. It may also be desirable in some instances for resistors 15 and 16 to be of such value so that no gain is imparted, the active high-impedance stage 11 providing only high input impedance at the input to system 9.
  A filter according to the disclosed design is dimensioned, for example as small as 5 cm�4 cm�1.5 cm, or smaller. This is six times smaller than a passive filter alone and two times smaller than an active filter alone performing comparable filter functions.
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