Patent ID: 6717449
Filing Date: 2004-04-06
Classification: H03G,H03H

Abstract:
A variable resistance circuit comprising:a control circuit having a plurality of constant-current output terminals, with a constant-current output ratio of the output terminals varied by electrical signals to by electrical signals to be applied to a control terminal; an operational amplifier; a resistor; and a plurality of MOS transistors having gates thereof connected in common; wherein a first output terminal of said control circuit is connected to one end of said resistor and to an inverted input terminal of said operational amplifier, wherein a second output terminal of said control circuit is connected to a drain of a first of the plurality of MOS transistors and to a non-inverted input terminal of said operational amplifier, wherein a crate of the first MOS transistor is connected to an output terminal of said operational amplifier, wherein another end of said resistor and the source of said first MOS transistor are connected to a reference voltage terminal, and wherein said first MOS transistor is caused to operate in a triode region so as to use, as an equivalent resistance, the resistance between the source and drain of a second of the plurality of MOS transistors, the second MOS transistor having a gate connected to the output terminal of said operational amplifier and a source coupled directly to the reference voltage terminal.