Recent years has seen substantial development of multimode-compatible transceivers that can handle, in a single device, a plurality of wireless communication modes represented by, for example, IEEE (Institute of Electrical and Electronic Engineers) 802.11a/b/g and GSM (Global, System for Mobile Communications)/WCDMA (Wideband Code Division Multiple Access).
In such transceivers, design that uses micro-CMOS processing is indispensable for realizing complex signal processing at high speed while the limiting area and power consumption. However, adopting micro-CMOS processing raises the new problem of the increase of anomalies in performance or drops in the power-supply voltage.
A channel-selection filter that is incorporated as a filter circuit in such a multimode-compatible transceiver is required to feature small area and wide-range variability of such filter characteristics as bandwidth and gain, quality factor (Q), and order even under conditions of low voltage. In wireless standards that are Currently used in wireless communication according to the above-described wireless communication modes, channel bandwidth ranges from several kHz to several 100 MHz.
A Gm-C filter made up from a voltage-current converter (i.e., a Gm amplifier) and a capacitance element is typically used as a channel-selection filter (for example, see Non-Patent Document 1). In a multimode-compatible Gm-C filter, filter characteristics can be varied by controlling the voltage-current conversion gain of the Gm amplifier or the capacitance of the capacitance element. The method typically used for varying filter characteristics involves controlling the voltage-current conversion gain of the Gm amplifier by varying the bias conditions. However, the range of variability of bias voltage narrows with drops in the power-supply voltage, and the ability to vary the voltage-current conversion gain of a Gm amplifier over a wide range is therefore problematic.
The first example of the related art of a filter circuit for enabling variation of the voltage-current conversion gain over a wide range even under low voltage conditions is shown in FIG. 1A (for example, Non-Patent Document 2). The first example of the related art is a Gm-C filter made up from a voltage-current converter (i.e., a Gm amplifier) and a capacitance element C, the Gm amplifier being of a configuration that combines the voltage-current converter and the current-mirror circuit shown in FIG. 1B. In the first example of the related art, the voltage-current conversion gain of the Gm amplifier can be varied by switching paths inside the current-mirror circuit by means of switch circuits SW.
Similarly, the filter circuit of the second example of the related art is shown in FIG. 2 (for example, Non-Patent Document 3). The second example of the related art is of a configuration in which an active-RC filter constituted by operational amplifier OA, resistance elements R and Rx, and capacitance element C is further provided with switch circuits SW. In the second example of the related art, controlling the duty ratio of a clock that controls opening/closing of switch circuit SW enables variation of the effective resistance of resistor R, which carries out voltage-current conversion.
However, the filter circuits of the first and second examples of the related art have the following problems.
The problem of the first example of the related art is the difficulty of broadening the range of variability of the filter characteristics without bringing about an increase in the size of chip area. This problem arises because the first example of the related art requires the juxtaposition of a number of current-mirror circuits that is proportional to the range of variability, but because discrepancies in the threshold voltages of the MOSFETs must be limited, miniaturization of the area of each current-mirror circuit is difficult to achieve despite advances in micro-CMOS processing.
The problem of the second example of the related art is the inability to vary filter characteristics other than the bandwidth. Methods that can be considered for enabling variation of gain and the quality factor in this configuration include a method of juxtaposing resistors and then switching by means of switch circuits and a method of using MOSFETs as variable resistors, but the first method suffers from the problem of an increase in the size of the area and the second results in degraded linearity. Another problem of the second example of the related art is the need to also compensate for variations in performance by means of duty ratio control. In other words, because the range of variability realized by duty ratio control is split between two different types of objects, i.e., variation compensation and multi-mode compatibility, the range of variability for multi-mode compatibility is narrowed, and achieving a range of variability that is sufficient for multi-mode compatibility is therefore compromised.    Non-Patent Document 1: R. Schaumann and M. E. van Valkenburg, Design of Analog Fillers, Oxford University Press, 2001.    Non-Patent Document 2: IEEE JSSC, Vol. 37, No. 2, February 2002. pp. 125-136    Non-Patent. Document 3: IEEE ISCAS, Vol. 1, May 2004. pp. I-433-436