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A voltage generator is provided which is reliable, self starting and only requires a few components. The voltage generator comprises a first stage that provides a current to a second stage. The first stage has a temperature coefficient of one sign, such as positive, and the second stage has an oppos

A voltage generator is provided which is reliable, self starting and only requires a few components. The voltage generator comprises a first stage that provides a current to a second stage. The first stage has a temperature coefficient of one sign, such as positive, and the second stage has an opposing temperature coefficient, e.g. negative. The responses are summed such that the overall temperature coefficient is reduced.

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1. A voltage generator comprising: first and second coupled stages, wherein the first stage has a voltage versus temperature characteristic which is opposite to a voltage versus temperature characteristic of the second stage,wherein the first stage comprises: a first transistor having a gate, a drai

1. A voltage generator comprising: first and second coupled stages, wherein the first stage has a voltage versus temperature characteristic which is opposite to a voltage versus temperature characteristic of the second stage,wherein the first stage comprises: a first transistor having a gate, a drain and a source, the first transistor configured to pass a current when its gate voltage is approximately the same as its source voltage, anda first resistive element having a first node and a second node, the first node being connected to the source of the first transistor and the second node being connected to the gate of the first transistor;wherein the second stage comprises an enhancement mode transistor, andwherein a node at which the first stage is coupled to the second stage is configured to provide a temperature compensated voltage due at least partly to a temperature coefficient of the first transistor and a temperature coefficient of the enhancement mode transistor. 2. A voltage generator as claimed in claim 1, in which the enhancement mode transistor has a gate, a drain and a source, and wherein the gate of the enhancement mode transistor is connected to the drain of the enhancement mode transistor. 3. A voltage generator as claimed in claim 2, in which the second stage further comprises a second resistive element in series with the drain of the enhancement mode transistor. 4. A voltage generator as claimed in claim 2, in which the enhancement mode transistor comprises a plurality of series connected transistors. 5. A voltage generator as claimed in claim 2, further comprising a third stage connected in series with the second stage. 6. A voltage generator as claimed in claim 5, wherein the third stage comprises at least one of a resistor and a diode connected field effect transistor. 7. A voltage generator as claimed in claim 1, in which the first transistor is a native transistor or a depletion mode transistor. 8. A voltage generator as claimed in claim 1, further comprising a cascode transistor coupled between the first transistor and a supply rail for the first transistor. 9. A voltage generator as claimed in claim 1, in which the first and second stages are connected in series, and the second stage is connected to the second node of the first resistive element. 10. A voltage generator as claimed in claim 1, in which an output is connected to the first node of the first resistive element. 11. A voltage generator as claimed in claim 1, in which the temperature coefficient of the enhancement mode transistor is negative. 12. A voltage generator as claimed in claim 1, in which the first and second stages are coupled via a current mirror such that the current flowing in the second stage is proportional to the current flowing in the first stage. 13. A voltage generator as claimed in claim 12, in which the second stage further comprises a second resistive element coupled between the enhancement mode transistor and an output node of the current mirror. 14. A voltage generator as claimed in claim 13, in which the second resistive element comprises a semiconductor device. 15. A voltage generator as claimed in claim 1 further comprising an output stage to buffer an output voltage provided by the first and second coupled stages. 16. A voltage generator as claimed in claim 15 in which the second stage is coupled to the first stage by a current mirror, and the enhancement mode transistor of the second stage is a slave transistor in a current mirror configuration with a transistor of the output stage. 17. A voltage generator as claimed in a claim 16, in which the output stage comprises a first output stage transistor and a second output stage transistor connected by a resistor, and a gate of the first output stage transistor is connected to a drain of the slave transistor, and the second output stage transistor is in a diode connected configuration and has its gate connected to the gate of the slave transistor. 18. A voltage generator as claimed in claim 1 in which a current flowing through the first stage is mirrored to the second stage and to a first output device, and the current flowing in the second stage is used by a feedback circuit to generate a second stage control current which is mirrored to a second output device, and where the currents of the first and second output devices have complementary temperature coefficients, and are combined to generate an output signal. 19. A voltage generator as claimed in claim 1, in which the first resistive element is a resistor or a transistor. 20. A voltage generator as claimed in claim 1, in which the first transistor is formed by a plurality of series connected transistors. 21. A voltage generator as claimed in claim 1, in which a temperature coefficient of an output voltage at an output of the voltage generator is less than a temperature coefficient of the first and second stages. 22. A voltage generator as claimed in claim 21 in which the output voltage has a temperature coefficient of less than 20% of the coefficient of one of the first and second stages. 23. A power up reset circuit comprising the voltage generator of claim 1. 24. A power up reset circuit as claimed in claim 23, wherein the second stage comprises a diode or a diode connected transistor. 25. A power up reset circuit as claimed in claim 23, further comprising a comparator having a current therethrough controlled by a current source or sink, wherein the enhancement mode transistor is diode connected, and wherein the current source or sink comprises a device of the same technology as the enhancement mode transistor. 26. A power up reset circuit as claimed in claim 23, further including a reference limb comprising a voltage translation circuit. 27. A method of generating a reference voltage, the method comprising providing a voltage to a reference generator comprising first and second stages in current flow communication, wherein the first stage has a temperature coefficient of a first sign and the second stage has a temperature coefficient of a second sign, and wherein the first stage comprises a first resistive element and first transistor having a gate, a drain and a source, wherein a first node of the first resistive element is connected to the source of the first transistor, a second node of the first resistive element is connected to a gate of the first transistor, and the first transistor is configured to pass a current when its gate voltage is approximately the same as its source voltage, wherein the second stage comprises an enhancement mode transistor, and wherein a node at which the first stage is coupled to the second stage is configured to provide a temperature compensated voltage due at least partly to a temperature coefficient of the first transistor and a temperature coefficient of the enhancement mode transistor. 28. A method as claimed in claim 27, in which the first stage has a temperature coefficient which is positive, and the second stage has a temperature coefficient which is negative and a current flow communication sums the responses of the first and second stages. 29. A method as claimed in claim 27, in which a summed result is translated to an output voltage by observing the voltage difference across a transistor in the second stage or a sum of the gate-source voltages of transistors in the first and second stages. 30. A voltage generator comprising: a first stage comprising: a first transistor having a gate, a drain and a source, the first transistor configured to pass a current when its gate voltage is approximately the same as its source voltage;a first resistive element having a first node and a second node, the first node being connected to the source of the first transistor and the second node being connected to the gate of the first transistor; anda cascode transistor coupled between the first transistor and a supply rail for the first transistor, the cascode transistor having a gate configured to receive a reference voltage output by the voltage generator; anda second stage coupled to the first stage, wherein the first stage has a voltage versus temperature characteristic which is opposite to a voltage versus temperature characteristic of the second stage. 31. A voltage generator as claimed in claim 30, wherein the second stage comprises a diode connected enhancement mode transistor, and wherein a node at which the first stage is coupled to the second stage is configured to provide a temperature compensated voltage due at least partly to a temperature coefficient of the first transistor and a temperature coefficient of the enhancement mode transistor.