The present invention relates to electronic filters, and more particularly to devices and methods for filtering a signal in which the rating requirements of resistive and capacitive components in the filter are substantially reduced.
Electronic filters typically include resistive and capacitive components arranged in a circuit to pass particular frequencies (e.g., high pass, low pass, band pass, band rejection). By way of example, FIG. 1 depicts a low pass filter typical of the prior art; a first order filter that includes a resistor R and capacitor C that may be grounded if the filter is ground referenced, and in which ##EQU1## Filters may also include inductive components, but their expense and size usually limits their use. However, even the size and expense of the resistive and capacitive components may present difficulties in the design and manufacture of electronic filters, and to this end it is desirable to reduce the size and cost of these components. As component size is generally equated to the component's capability--its rating, a typical solution has been to tradeoff capability or size in order to meet design requirements.
The size and cost of a resistor is a function of the power it must dissipate, and the size and cost of a capacitor is a function of the maximum voltage that will appear across the capacitor. Electronic systems with low voltage levels and small current levels can use small and relatively inexpensive resistors and capacitors. Other systems, such as a subscriber line interface circuit (SLIC) for a telephone system, are not so fortunate. A SLIC may operate with voltages as high as 60 volts and currents up to tens of milliamps. Further, filters for SLICs must operate in the audio band that further increases the size and capability required of their resistive and capacitive components.
Accordingly, it is an object of the present invention to provide a novel filter and method that obviates the problems of the prior art.
It is another object of the present invention to provide a novel filter and method in which the rating of resistive and capacitive components in the filter is less than would be required to accommodate the input signal.
It is yet another object of the present invention to provide a novel filter and method in which an input signal is converted to its logarithm before the input signal is filtered to thereby reduce the voltage and power requirements of filter components.
The present invention may find application in SLICs for telephone systems, and more particularly to current mirrors that are used in telephone systems. Current mirrors duplicate an input current by providing one or more reflections of the input current. Such devices are known and illustrated, by way of example, in FIG. 2 in which a pair of transistors Q1 and Q2 are mated to provide a single reflection, Iout. Current mirrors may be used in telephone system SLICs for a variety of purposes.
Accordingly, it is still another object of the present invention to provide a novel combination of a filter and a current mirror in which an input current is converted to a voltage proportional to the logarithm of the input current before being filtered, and thereafter reconverted to an output current proportional to the inverse logarithm of the filtered voltage.
It is a further object of the present invention to provide a novel filter for a telephone system subscriber line interface circuit that is embedded in a current mirror.
It is yet a further object of the present invention to provide a novel method and filter for an input current (a) in which a first bipolar junction transistor with a collector connected to receive the input current, a grounded emitter, and a base connected to an input of the filter and to the collector of the first transistor converts the input current to a first voltage that is proportional to the logarithm of the input current, (b) in which an RC filter with components not rated to handle the input current filters the first voltage, and (c) in which a second bipolar junction transistor with a base connected to receive the filtered first voltage from the RC filter, a grounded emitter, and a collector connected to an output, provides a filtered output current that is proportional to the inverse logarithm of the filtered first voltage.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.