TUNING CIRCUIT

A tuning circuit is provided to compensate for the changed characteristics, when the characteristics are changed above constant range in comparison with the characteristic set at initial design and the cut-off frequency is changed. A tuning circuit comprises a current generator(210) including a first transistor(MN21) and a resistor(R) and a capacitor compensating unit(220) including a second transistor(MN22), an up/down counter(222) and a capacitor unit(223). The first transistor(MN21) has three terminals and changes the current from a second terminal to a third terminal in proportion to a voltage applied to a first terminal. The resistor(R) is connected to the first transistor(MN21) in serial. The current passing through the resistor(R) is changed according to the resistance value. The second transistor(MN22) has three terminals and configures a current-mirror circuit(212) together with the first transistor(MN21). A mirror current passes from a second terminal to a third terminal of the second transistor(MN22) in proportion to a voltage applied to a first terminal of the second transistor(MN22). The capacitor unit(223) is connected to the third terminal of the second transistor(MN22) and varies the time constant of the capacitance of capacitor(C). The up/down counter(222) provides a control signal to vary the capacitance of capacitor(C).