Current drive device

1. A current drive device comprising:

a current source transistor for allowing a preset drive current to flow to a drain;

a cascode transistor cascode-connected to the current source transistor;

a switch circuit for switching ON/OFF flow of the drive current through a drain of the cascode transistor and a circuit to be driven; and

a bypass circuit for allowing the drive current to flow therethrough to bypass the switch circuit and the circuit to be driven when the switch circuit is OFF,

wherein the bypass circuit comprises a bypass transistor connected to the drain of the cascode transistor at its source and receiving a predetermined fixed gate voltage at its gate, and

the predetermined fixed gate voltage is set so that the bypass transistor is ON with respect to a source voltage given when the switch circuit is OFF and OFF with respect to a source voltage given when the switch circuit is ON.

2. A current drive device comprising:

a current source transistor for allowing a preset drive current to flow to a drain;

a cascode transistor cascode-connected to the current source transistor;

a switch circuit for switching ON/OFF flow of the drive current through a drain of the cascode transistor and a circuit to be driven; and

a bypass circuit for allowing the drive current to flow therethrough to bypass the switch circuit and the circuit to be driven when the switch circuit is OFF,

wherein a plurality of current source transistors are provided,

the switch circuit comprises individual switch transistors provided for the respective current source transistors for selectively switching ON/OFF flow of drive currents to the current source transistors,

the bypass circuit comprises a plurality of individual bypass transistors corresponding to the current source transistors, and

the currents flowing through the individual switch transistors are summed to be outputted as a summed drive current.

3. The current drive device of claim 2 , further comprising a summed drive current switch transistor for switching ON/OFF flow of the summed drive current.

4. The current drive device of claim 3 , wherein each of the individual bypass transistors is turned ON when at least either the corresponding individual switch transistor or the summed drive current switch transistor is turned OFF.

5. The current drive device of claim 3 , wherein the bypass circuit further comprises a summed drive current bypass transistor for allowing the summed drive current to flow therethrough to bypass the summed drive current switch transistor and a circuit to be driven,

the individual bypass transistors are turned ON when the corresponding individual switch transistors are turned OFF, and

the summed drive current bypass transistor is turned ON when the summed drive current switch transistor is turned OFF.