Constant current source apparatus including two series depletion-type MOS transistors

1. A constant current source apparatus for supplying a load current to at least one load, comprising:

first and second output terminals, at least one of said first and second output terminals capable of being connected to said load; and

first and second depletion-type MOS transistors connected in series between said first and second output terminals,

wherein a source and a gate of said first depletion-type MOS transistor being connected to a gate of said second depletion-type MOS transistor, thereby to form said constant current source apparatus, and

wherein said first and second depletion-type MOS transistors interact such that, when a voltage applied to said constant current source apparatus fluctuates, then said first and second depletion-type MOS transistors suppress a resulting fluctuation of said load current due to a channel length modulation effect, and

wherein said channel-length modulation effect is thereby limited to be within the formula λ·(V thl -V th2 ), where:

λ is a channel-length modulation factor of said first and second depletion-type MOS transistors;

V th1 is a threshold voltage of said first depletion-type MOS transistor; and

V th2 is a threshold voltage of said second depletion-type MOS transistor.

2. The constant current source apparatus as set forth in claim 1 , wherein a drain-to-source breakdown voltage of said first depletion-type MOS transistor is larger than an absolute value of a threshold voltage of said second depletion-type MOS transistor.

3. The constant current source apparatus as set forth in claim 1 , wherein an absolute value of a threshold voltage of said first depletion-type MOS transistor is smaller than an absolute value of a threshold voltage of said second depletion-type MOS transistor.

4. The constant current apparatus as set forth in claim 1 , wherein a drain-to-source breakdown voltage of said first depletion-type MOS transistor is smaller than a drain-to-source breakdown voltage of said second depletion-type MOS transistor.

5. The constant current source apparatus as set forth in claim 1 , wherein back gates of said first and second depletion-type MOS transistors are connected to the source of said first depletion-type MOS transistor.

6. The constant current source apparatus as set forth in claim 1 , wherein a back gate of said first depletion-type MOS transistor is connected to the source of said first depletion-type MOS transistor, and a back gate of said second depletion-type MOS transistor is connected to the source of said second depletion-type MOS transistor.

7. The constant current source apparatus as set forth in claim 1 , wherein each of said first and second depletion-type MOS transistors comprises a depletion-type N-channel MOS transistor.

8. The constant current source apparatus as set forth in claim 1 , wherein each of said first and second depletion-type MOS transistors comprises a depletion-type P-channel MOS transistor.

9. A constant current source apparatus for supplying a load current to at least one load, comprising:

first and second output terminals, at least one of said first and second output terminals capable of being connected to said load; and

first and second depletion-type MOS transistors connected in series between said first and second output terminals,

a source and a gate of said first depletion-type MOS transistor being connected to a gate of said second depletion-type MOS transistor,

wherein an absolute value of a threshold voltage of said first depletion-type MOS transistor is smaller than an absolute value of a threshold voltage of said second depletion-type MOS transistor,

wherein a drain-to-source breakdown voltage of said first depletion-type MOS transistor is smaller than a drain-to-source breakdown voltage of said second depletion-type MOS transistor,

wherein said first and second depletion-type MOS transistors operate together to suppress a fluctuation of said load current due to a channel-length modulation effect, and

wherein said channel-length modulation effect is thereby limited by the formula λ·( V th1 -V th2 ), where:

λ is a channel-length modulation factor of said first and second depletion-type MOS transistors;

V th1 is a threshold voltage of said first depletion-type MOS transistor; and

V th2 is a threshold voltage of said second depletion-type MOS transistor.

10. A constant current source apparatus for supplying a load current to at least one load, comprising:

first and second output terminals, at least one of said first and second output terminals capable of being connected to said load; and

first and second depletion-type MOS transistors connected in series between said first and second output terminals,

wherein a source and a gate of said first depletion-type MOS transistor being connected to a gate of said second depletion-type MOS transistor, thereby to form said constant current source apparatus, and

wherein said first and second depletion-type MOS transistors interact such that, when a voltage applied to said constant current source apparatus fluctuates, then said first and second depletion-type MOS transistors suppress a resulting fluctuation of said load current due to a channel length modulation effect, and

wherein said channel-length modulation effect is limited by the formula λ·(V th1 -V ccs ), where:

λ is a channel-length modulation factor of said first and second depletion-type MOS transistors;

V th1 is a threshold voltage of said first depletion-type MOS transistor; and

V ccs is equal to a sum of a drain-to-source voltage of said first depletion-type MOS transistor and a drain-to-source voltage of said second depletion-type MOS transistor.