A transconductance amplifier is an amplifier for supplying output current in proportion to input voltage, and has stable gain in general. In other words, when varying input voltage over a predetermined operating input range, the ratio between output current and input voltage is stable, that is, the output current is linear with respect to the input voltage.
As a transconductance amplifier with good linearity between input voltage and output current over a predetermined operating input range, one using a differential pair composed of source grounded MOS transistors as shown in FIG. 1 is known (see Non-Patent Document 1, for example). The transconductance amplifier as shown in FIG. 1 comprises a differential pair composed of source grounded MOS transistors 111 and 112; a MOS transistor 113 with its source terminal connected to a drain terminal of the MOS transistor 111; a MOS transistor 114 with its source terminal connected to a drain terminal of the MOS transistor 112, and its gate terminal connected to a gate terminal of the MOS transistor 113; a voltage generating circuit 100 for outputting tuning voltage Vctrl to be supplied (good?) to the gate terminals of the MOS transistors 113 and 114; a voltage generator for outputting common voltage Vcm for generating voltages Vip and Vin to be input to the differential pair; and a differential pair input voltage generating circuit 120 which receives input voltage Vinput and the common voltage Vcm to supply the voltage Vip to the gate terminal of the MOS transistor 111 and the voltage Vin to the gate terminal of the MOS transistor 112. The transistor size (the ratio between channel width and channel length) of the individual MOS transistors, and the tuning voltage Vctrl and the common voltage Vcm are controlled so as to operate the MOS transistors 111 and 112 constituting the differential pair in triode region, and the MOS transistors 113 and 114 in saturation region. In addition, the voltages Vip and Vin satisfy the relationshipVin=2×Vcm−Vip,and the difference between Vip and Vin is determined by Vinput.
In such a configuration, when Vip−Vin is seen as input voltage and Ip−In as output current, the circuit as shown in FIG. 1 functions as a transconductance amplifier. FIG. 2 illustrates the transconductance Gm obtained by differentiating the output current by the input voltage. The transconductance Gm is constant near Vip−Vin=0, and it can be seen that the output current is proportional to the input voltage. In addition to the good linearity between the input voltage and the output current, tuning of the transconductance Gm is possible by controlling the tuning voltage Vctrl. The transconductances Gm under different tuning voltage conditions are shown wherein the tuning voltage Vctrl is varied from a middle level to a small and large level.
In a conventional transconductance amplifier as shown in FIG. 1, however, the linearity between input voltage and output current of the transconductance amplifier deteriorates when the tuning voltage Vctrl is varied for the purpose of tuning transconductance. Assume that the range in which transconductance Gm is constant before tuning (when the Vctrl is middle in the example of FIG. 2) is the operating input range when using the transconductance amplifier. Then, increased tuning voltage Vctrl diminishes the transconductance Gm near the upper limit and lower limit of the operating input range, thereby narrowing the range in which the transconductance Gm is constant. Hence, it would become impossible to supply output current in proportion to input voltage. From a different point of view, to achieve tuning with the linearity between input voltage and output current maintained over the entire operating input range, the range in which the tuning voltage Vctrl is variable for tuning the transconductance is narrowed.
The present invention is directed to the foregoing problem, and an object of the present invention is to provide a transconductance amplifier capable of tuning transconductance in a broader range with the linearity between input voltage and output current maintained over a predetermined operating input range.
Non-Patent Document 1: Chun-Sup Kim, “A CMOS 4× Speed DVD Read Channel IC,” IEEE Journal of Solid-State Circuits, vol. 33, No. 8, August 1998.