Patent ID: 7057448

Claim:
A variable output-type constant current source circuit comprising: a first self-biasing type constant current circuit including a resistor, a first transistor connected in series to said resistor, a second transistor for supplying a given voltage across said resistor in cooperation with said first transistor to generate a reference current, a third transistor connected in series to said first transistor, and a fourth transistor connected in series to said second transistor and associated with said third transistor to form a current mirror circuit; a switching circuit including a fifth transistor connected in series with said resistor, the switching circuit being arranged so as to cut off current flow through said first transistor without preventing current flow through said second transistor; and a second constant current circuit comprising a sixth transistor which is a depletion-type MOSFET, wherein a main current path of said sixth transistor is connected in parallel to a series circuit of said second transistor, said resistor and said fifth transistor, and wherein a gate and a source of said sixth transistor are shorted; wherein when said fifth transistor is turned off, a current passing through said second and fourth transistors becomes a relatively small value corresponding to current flowing through said second constant current circuit, and when said fifth transistor is turned on, a current passing through said second and fourth transistors becomes a relatively large value corresponding to a combined current flowing through said first and second constant current circuits.