1. Field of the Invention
The present invention generally relates to an equivalent inductance circuit. More specifically, the present invention relates to an equivalent inductance circuit which includes semiconductor devices so as to be built in an IC (integrated circuit).
2. Description of the Prior Art
One example of a conventional technique of such a kind of equivalent inductance circuit is disclosed in Japanese Patent Laying-Open No. 53-18362 laid-open on Feb. 20, 1978, and Japanese Patent Laying-Open No. 1-208011 laid-open on Aug. 22, 1989.
The former is a semiconductor inductance element in which a current being in proportion to an input voltage is outputted by a voltage-current conversion circuit, and an output current is integrated, and an integrated output is converted into a current by a further voltage-current conversion circuit so as to feed the current back to an input signal source.
The latter is a current-controlled gyrator in which two differential amplifying circuits are connected in parallel to each other, and a capacitor is connected between inputs of one differential amplifying circuit and a voltage-current conversion circuit is provided at an output of any one of the differential amplifying circuits.
In such equivalent inductance circuits, it is necessary to adjust an equivalent inductance due to a change of a temperature characteristic of the semiconductor devices, variation or dispersion of the devices, and etc. In such a case, in the former, since the equivalent inductance is primarily decided by a resistor and a capacitor, it is impossible to control or adjust the equivalent inductance externally. In the latter, the equivalent inductance is given by the 42nd formula described in the official gazette of Japanese Patent Laying-Open No. 1-208011, the equivalent inductance can be changed by I.sub.0 /I.sub.1, but a linear control characteristic can not be obtained according to the circumstances because a differential resistor re is dependent on I.sub.0. That is, the equivalent inductance is changed in a linear manner with respect to the current I.sub.0 when resistors R.sub.1 and R.sub.2 in the 42nd formula are sufficiently larger than the differential resistor 2re; however, in a case where the equivalent inductance is to be set smaller, since it is impossible to make the resisters R.sub.1 and R.sub.2 larger, an influence of the differential resistor 2re with respect to the inductance becomes larger, and therefore, the control characteristic of the equivalent inductance is changed.