This invention relates generally to current mode filters, and more particularly to current mode filters implementing finite zeros.
Current-mode filters have been used in radiocommunication architectures to detect digital signals. In general, current mode filters use current mirrors to amplify current. Previously, voltage mode filters were used but were limited in their speeds, becoming less useful at higher operating frequencies. This is because, in the voltage mode, signals were sent as voltage levels thus being affected by parasitic capacitances and due to this effect, limiting higher frequency signals. In contrast, current-mode filters are able to operate at higher frequencies with less bandwidth limitations, since signals are sent as currents that are not sensitive to the parasitic capacitances present in a typical IC layout. This is used to advantage in low-noise baseband matched filters.
Typically, in any receiver line-up there will be either an anti-aliasing filter or a matched filter in front of an A/D converter. In previous radio communication devices the filter was implemented either using a GmC continuous time filter or Active RC topologies, as are known in the art. GmC filters are known for having a high equivalent input noise, which in turn requires a high take-over gain in the previous receiver stages. In addition, this high gain reduces the 3rd-order intermodulation product (IP3) of the receiver line-up. On the other hand, Active-RC filters have very good noise properties, but require a higher bias current to move the non-dominant poles away from the operating frequencies of radio communication device. This becomes more critical as the bandwidth of the filter is increased.
GmC and Active RC filters have been implemented with finite zeros to improve performance. Most cellular telephone channel filters employ elliptic approximations that, incorporate some form of subcircuit to generate the complex zeros necessary to implement the elliptic transfer function. This normally leads to floating capacitors in the case of GmC filters. However, this results in parasitic capacitances due to interconnection and other devices being added to each side of this floating capacitor making the transfer function dependent on the parasitic capacitance. The techniques used to create finite zeros in a GmC topology cannot be applied to current-mode filters.
Most prior art current-mode filters only describe all-poles filters. However, one alternative approach for implementing a zero in a current-mode topology uses an algebraic manipulation of the state equation to eliminate the effect of floating capacitors. This manipulation requires the use of a grounded capacitor and requires further circuit complexity by the addition of more current mirror circuits. Unfortunately, additional current mirror circuits drain more current in the radio communication device. Moreover, this alternative approach utilizes integer current mirrors ratios to adjust the transfer function, which limits the accuracy of adjustment.
What is needed is a current-mode filter with finite zeros that drains a lower current and is not limited by the use of integer-ratio current mirrors. In particular, it is desirable to save circuit power by reducing the need to use any additional current mirror circuits. It would also be beneficial to provide this improvement while also increasing performance.