Bias current supplying circuit

A bias current supplying circuit comprising:

• a differential amplifier for amplifying a difference between potentials at first and second modes and outputting the amplified difference voltage from a third node, said first node being connected to an output terminal of a bias voltage source for outputting a bias voltage, and said second node being connected to an output terminal from which a bias current is supplied to an external circuit;
• a negative feedback loop circuit for changing the potential at said second node in accordance with the potential at said third node of said differential amplifier;
• and
• potential dropping means connected to said second node for dropping the potential at said second node to intercept a supply of said bias current to said external circuit in the case where a control signal is input to said potential dropping means;
• and
• a control signal generator for generating the control signal and outputting the control signal to said potential dropping means in the case where said external circuit does not need said bias current.