Abstract:
An electrical circuit which includes a filter bypass mode. The circuit includes an amplifier including an inverting terminal, a noninverting terminal and an output terminal, at least one first capacitor coupled to the inverting terminal of the amplifier through at least one first switch, and, at least one second capacitor coupled to the noninverting terminal of the amplifier through at least one second switch. The electrical circuit provides filtering when the first and second switches are in a first state, and when the first and second switches are in a second state, the electrical circuit provides substantially no filtering.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrical filter circuit, and in particular, an electrical filter circuit which includes a bypass mode for bypassing a filtering portion of the circuit. 
     DESCRIPTION OF THE RELATED ART 
     Filter circuits are well known in the art and are used in various applications. Filters are often used widely in telecommunications applications. For instance, wireless (e.g. cellular) communications equipment include various filters for performing the to different functions of the equipment. An example may include a reconstruction filter which follows a digital to analog (D/A) conversion device in either of the transmit or receive channels of the wireless equipment. 
     One well known filter circuit is the Salen and Key filter. The Salen and Key filter is an active filter which includes two basic elements: a filtering element and a amplification element. FIG. 1 shows a conventional second order low pass Salen and Key filter  10 . The filter  10  includes an amplifier  20  for providing amplification, and an R-C network including two capacitors  30 ,  40  and two resistors  50 ,  55  for providing filtering. The filter  10  also includes resistors  60  and  65  which set the gain of the amplifier  20 . The filter  10  includes an input terminal  12  and an output terminal  14 . Signals which are presented at the input terminal  12  of the filter  10  are attenuated (and appear as such as output terminal  14 ) according to the passband of the low pass filter formed by capacitors  30 ,  40  and resistors  50 ,  55 . Signals which are within the passband are amplified by amplifier  20  with a particular gain set by resistors  60  and  65 . 
     Active filters such as Salen and Key filter  10  described above may be used for a variety of different reasons, including: (1) the filter function is required to multiple order, (2) large signal swing requirements, (3) linearity requirements, and (4) output drive requirements. It should be noted that the component values of the capacitors (e.g. capacitors  30 ,  40 ) of the filter  10  can be adjusted to more accurately tune the passband of the filter. This is typically accomplished by making one of the capacitors  30 ,  40  a variable capacitor, tuning that capacitor and a resistor (e.g. resistor  50  or  55 ) to a known time reference, and then tuning the other elements in accordance therewith. 
     Other examples of conventional active filter structures are the second order Rausch filter and the “leaky” integrator filter shown in FIGS. 2 and 3, respectively. The Rausch filter  100  includes an amplifier  120  for providing amplification, and an R-C network including two capacitors  130 ,  140  and three resistors  150 ,  155 ,  160  for providing filtering. The “leaky” integrator filter  200  shown in FIG. 3 includes an amplifier  220  and an R-C network including one capacitor  230  and two resistors  250 ,  260  for providing filtering. It will be noted that the filters shown in FIGS. 2 and 3 (e.g. Rausch and “leaky” integrator) have similar construction to the Salen and Key filter (shown in FIG.  1 ), but are slightly different as to the placement of the resistors and capacitors. However, all three conventional filters described above include capacitors which contribute to the filtering functions of the circuits. 
     Another conventional filter circuit  600  is shown in FIG.  4 . FIG. 4 shows a full-differential third order Rausch filter. The filter  600  includes an amplifier  620  for amplification, and an R-C network including capacitors  630 ,  635 ,  640 , and  645  and resistors  650 - 657  for providing filtering. The filter  600  differs from the other conventional filter circuits  10 ,  100 , and  200  described above in that it includes balanced input  610 ,  611  and output  612 ,  613  terminals rather than unbalanced input and output terminals, and the amplifier  620  includes two output terminals, instead of just one. 
     In a conventional filter circuit applications, such as those described above, a mode may exist where the filtering function of the filter circuit is not desired, but the loading conditions on the output remain unchanged. Previous solutions to this problem required at least two buffer circuits, one with filtering elements and one without filtering elements, to accommodate both modes of operation. The requirement of two buffers requires additional area on either the silicon die or the circuit board on which the filter circuit is disposed. 
     Thus, there is currently a need for a combined filter and buffer circuit which includes a filter bypass mode. 
     SUMMARY OF THE INVENTION 
     The present invention is method and apparatus for providing an electrical circuit which includes a filter bypass mode. The method includes the steps of: providing at least one switched filtering element in a filtering circuit, and switching the filtering element out of the filter circuit in order to bypass the filter circuit. 
     The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional Salen and Key filter circuit. 
     FIG. 2 shows a conventional second order Rausch filter circuit. 
     FIG. 3 shows a conventional “leaky” integrator circuit. 
     FIG. 4 shows a conventional fully differential third order Rausch filter circuit. 
     FIG. 5 shows a filter circuit according to a first exemplary embodiment of the present invention. 
     FIG. 6 shows a filter circuit according to a second exemplary embodiment of the present invention. 
     FIG. 7 shows a filter circuit according to a third exemplary embodiment of the present invention. 
     FIG. 8 shows a filter circuit according to a fourth exemplary embodiment of the present invention. 
     FIG. 9 shows a telecommunications circuit including one or more of the filter circuits shown in FIGS.  5 - 8 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 5, there is shown a filter circuit  300 , according to a first exemplary embodiment of the present invention. The filter circuit  300  includes an input terminal  301 , an output terminal  302 , and an amplifier  310  which includes an inverting terminal  311  (−), a noninverting terminal  312  (+), and an output terminal  313 . The amplifier  310  is coupled, through its noninverting terminal, to a first plurality of capacitors  320 - 335  (also labeled C 1  through Cn) through a first plurality of switches  340 - 355  (also labeled S 1  through Sn). A second plurality of capacitors  360 - 375  (also labeled Cf 1  through Cfn) and a second plurality of switches  380 - 395  (also labeled Sf 1  through Sfn) are coupled between the noninverting terminal and the inverting terminal. The first plurality of capacitors  320 - 335  are coupled in parallel with each other between the noninverting terminal and ground. The second plurality of capacitors  360 - 375  are also coupled in parallel with each other between the noninverting and inverting terminals. 
     The first and second pluralities of switches  340 - 355 ,  380 - 395  allow any one (or none) of the first and second pluralities of capacitors  320 - 335 ,  360 - 375  to be coupled to the amplifier  310 , as explained below. Also coupled between the inverting terminal and the noninverting terminal is a first resistor  315 . Resistor  315  connects the inverting terminal  311  of the amplifier  310  to the second plurality of capacitors  360 - 375 . A second resistor  316  is coupled between the inverting terminal and ground. Resistors  315  and  316  set the gain of the amplifier, as explained below. Two additional resistors  317  and  318  are coupled in series with the noninverting terminal  312  of the amplifier  310 . The second plurality of capacitors  360 - 375  are connected to a point between the resistors  317  and  318 . The switches  340 - 355  and  380 - 395  may be, for example, MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) switches controlled by a control circuit (not shown). Alternatively, the switches  340 - 355  and  380 - 395  may be any type of switch known to those skilled in the art (e.g. bipolar junction transistor (BJT) switches, Silicon Germanium (SiGe) semiconductor switches, Gallium Arsenide (GaAs) semiconductor switches, etc.). 
     The operation of the filter circuit  300  will next be explained. During normal filtering operation, at least one of the first and second pluralities of switches  340 - 355 ,  380 - 395  are closed, and thus at least one of the first and second pluralities of capacitors  320 - 335 ,  360 - 375  provide filtering to a signal present at the input terminal  301  of the filter circuit  300 . When it is determined, by a control circuit or otherwise, that filtering is no longer required or a different degree of filtering is required, the control circuit (not shown) opens switches  340 - 355 ,  380 - 395  accordingly. For example, if no filtering is required, the control circuit opens all the switches  340 - 355 ,  380 - 395 . When this occurs, the entire circuit  300  becomes nothing more than a buffer circuit, as signals pass from the input  301  to the output  302  without experiencing any filtering. Alternatively, if a different degree of filtering is required, the control circuit opens and closes only some of the switches  340 - 355 ,  380 - 395  so that some of the capacitors  320 - 335 ,  360 - 375  remain coupled to the circuit  300 . As different one of the capacitors  320 - 335  and  360 - 375  are switched in and out of the circuit  300 , the passband of the filter circuit is changed. Thus, the filter circuit  300  provides both a variable filter and a buffer circuit in one. 
     FIG. 6 shows a filter circuit  400 , according to a second exemplary embodiment of the present invention. The filter circuit  400  is similar to the filter circuit  300 , and like reference numerals denote like elements. As with the circuit  300 , the circuit  400  includes a plurality of capacitors  420 ,  425 , and  460 ,  465  which are switched in and out of the circuit by switches  440 ,  445  and  480 ,  485 , respectively. The operation of the filter circuit  400  is similar to the operation of the filter circuit  300 , therefore a detailed explanation is omitted here. Although only two sets of capacitors are coupled to the input  401  and output  402  of the circuit  400 , it should be noted that any number of capacitors may be so coupled without departing from the scope of the invention. 
     FIG. 7 shows a filter circuit  500 , according to a second exemplary embodiment of the present invention. The filter circuit  500  is similar to the filter circuit  300 , and like reference numerals denote like elements. As with the circuit  300 , the circuit  500  includes a plurality of capacitors  520 ,  525 , and  560 ,  565  which are switched in and out of the circuit by switches  540 ,  545  and  580 ,  585 , respectively. The operation of the filter circuit  500  is similar to the operation of the filter circuit  300 , therefore a detailed explanation is omitted here. Although only two capacitors are coupled to the input  501  and output  502  of the circuit  500 , it should be noted that any number of capacitors may be so coupled without departing from the scope of the invention. 
     FIG. 8 shows a filter circuit  700 , according to a fourth exemplary embodiment of the present invention. The circuit  700  includes input terminals  710 ,  711 , output terminals  712 ,  713 , and a plurality of capacitors  730 ,  731 ,  735 ,  736 ,  740 ,  741 ,  745 ,  746 , and  747  which are switched in and out of the circuit by switches  760 ,  761 ,  765 ,  766 ,  770 ,  771 ,  775 ,  776 , and  777 , respectively. The circuit  700  also includes a plurality of resistors  751 - 757 . The operation of the filter circuit  700  is similar to the operation of the filter circuits  300 ,  400  and  500 , and therefore a detailed explanation is omitted here. 
     The filter circuits  300 ,  400 ,  500  and  700  described above may be used, for example, as baseband filters in a wireless communications apparatus. 
     FIG. 9 shows a telecommunications circuit  800  which may utilize any one of the filter circuits  300 ,  400 ,  500  or  700 . The telecommunications circuit  800  may be, for example, a portion of the circuitry utilized in a wireless communications device, such as a cellular telephone or a cordless telephone. The circuit  800  includes a transmission antenna  801  with associated up-conversion circuitry  805 , and a reception antenna  802  with associated down-conversion circuitry  806 . The up-conversion circuitry  805  is coupled to a reconstruction filter  810  and a digital to analog (D/A) converter  811  of a transmission baseband unit  812 . The down-conversion  806  is coupled to an anti-aliasing filter  815  and an analog to digital (A/D) converter  816  of a reception baseband unit  817 . An auxiliary channel unit  820  is coupled to both the up-conversion circuitry  805  and the down-conversion circuitry  806 , and includes a deglitching filter  821  and a D/A converter  822 . The transmission baseband unit  812 , the reception baseband unit  817  and the auxiliary channel unit  820  are all coupled to a baseband processing unit  830 . The baseband processing unit  830  is, in turn, coupled to voiceband processing unit  840 . The voiceband processing unit  840  is coupled to a microphone  841  for picking up signals to be transmitted, and a speaker  842  for presenting signals received by the telecommunications circuit  800 . Filter circuits  300 ,  400 ,  500  or  700  may be used as, for example, the reconstruction filter  810  or the deglitching filter  821  of the above-described telecommunications circuit  800 . 
     It should be noted that the filter circuits  300 ,  400 ,  500  and  600  shown in FIGS. 5-8 above are only exemplary. Those of ordinary skill in the art will understand that the present invention has application in any filter circuit. More particularly, the present invention has utility in filter circuits in which filtering is sometimes not required. Further, it should be noted that the above filter circuits  300 ,  400 ,  500  and  600  may be manufactured either as integrated circuits (e.g. digital signal processor, microprocessor, microcontroller) or from discrete components. 
     Although the above description is directed to active filter circuits, it will be understood by those skilled in the art that the techniques described above are equally applicable to passive filter circuits. 
     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.