Switch circuit to control on/off of a high voltage source

1. A switch circuit, comprising:

a JFET transistor having a first terminal, a second and a third terminal, in which the first terminal is coupled to receive a high voltage source;

a first transistor having a drain terminal, a source terminal and a gate terminal, in which the drain terminal and the source terminal of the first transistor are connected to the second terminal of the JFET transistor and an output terminal of the switch circuit respectively;

a second transistor having a drain terminal, a source terminal and a gate terminal, in which the drain terminal of the second transistor is coupled to the gate terminal of the first transistor and the third terminal of the JFET transistor, the gate terminal of the second transistor is coupled to receive a control signal, and the source terminal of the second transistor is coupled to a ground; and

a resistive device coupled from the third terminal of the JFET transistor to the second terminal of the JFET transistor;

wherein the resistive device provides a bias voltage to turn on the first transistor and the JFET transistor when the second transistor is turned off, the control signal is coupled to turn on the second transistor for switching off the first transistor.

2. The switch circuit as claimed in claim 1 , in which the resistive device provides a negative bias to the JFET transistor once the second transistor is turned on.

3. The switch circuit as claimed in claim 1 , in which the JFET transistor has a negative threshold voltage.

4. The switch circuit as claimed in claim 1 , in which the JFET transistor is a voltage controlled resistance device.

5. The switch circuit as claimed in claim 1 , in which the JFET transistor still has a current flowed through the JFET transistor when the first transistor is turned off.

6. The switch circuit as claimed in claim 1 , in which the resistive device can be implemented by a resistor or a transistor.

7. A switch circuit comprising:

a JFET transistor coupled to receive a high voltage source;

a first transistor connected serially with the JFET transistor to output a voltage in response to the high voltage source;

a resistive device coupled to the JFET transistor and the first transistor to provide a bias voltage to turn on the JFET transistor and the first transistor; and

a second transistor coupled to the first transistor to turn off the first transistor;

wherein a control signal is coupled to the second transistor to control the second transistor.

8. The switch circuit as claimed in claim 7 , in which the second transistor provides a negative bias to the JFET transistor through the resistive device.

9. The switch circuit as claimed in claim 7 , in which the JFET transistor has a negative threshold voltage.

10. The switch circuit as claimed in claim 7 , in which the JFET transistor is a voltage controlled resistance device.

11. The switch circuit as claimed in claim 7 , in which the JFET transistor still has a current flowed through the JFET transistor when the first transistor is turned off.

12. The switch circuit as claimed in claim 7 , in which the resistive device can be implemented by a resistor or a transistor.

13. A switch circuit comprising:

a JFET transistor having a first terminal, a second terminal and a third terminal, in which the first terminal is coupled to receive a high voltage source;

a resistive device coupled from the third terminal of the JFET transistor to the second terminal of the JFET transistor to turn on/off the JFET transistor in response to a control signal; and

a second transistor having a drain terminal, a source terminal and a gate terminal, in which the drain terminal of the second transistor is coupled to the third terminal of the JFET transistor, the gate terminal of the second transistor is coupled to receive the control signal, and the source terminal of the second transistor is coupled to a ground, wherein the control signal is coupled to turn on the second transistor, the resistive device provides a negative bias to the JFET transistor once the second transistor is turned on;

wherein the JFET transistor is turned on for providing a voltage to a load in response to the high voltage source.

14. The switch circuit as claimed in claim 13 , in which the resistive device provides a negative bias to the JFET transistor to turn off the JFET transistor.

15. The switch circuit as claimed in claim 13 , in which the JFET transistor has a negative threshold voltage.

16. The switch circuit as claimed in claim 13 , in which the JFET transistor is a voltage controlled resistance device.

17. The switch circuit as claimed in claim 13 , in which the resistive device can be implemented by a resistor or a transistor.

18. The switch circuit as claimed in claim 13 , further comprising a first transistor having a drain terminal, a source terminal and a gate terminal, in which the drain terminal and the source terminal of the first transistor are connected to the second terminal of the JFET transistor and an output terminal of the switch circuit respectively, the source terminal of the first transistor providing the voltage to the load coupled to the output terminal, wherein the resistive device is further coupled from the gate terminal of the first transistor to the drain terminal of the first transistor to provide a bias voltage to turn on the first transistor.