Methods and apparatus for generating voltage references using transistor threshold differences
Methods and apparatus are described that develop a reference voltage that is based on a difference between a threshold voltage of a first transistor and a threshold voltage of a second transistor, and further based on a difference between a gate overdrive voltage of the first transistor and a gate overdrive voltage of the second transistor.
1. A method comprising:
developing a reference voltage that is based on a difference between a threshold voltage of a first transistor and a threshold voltage of a second transistor, and further based on a difference between a gate overdrive voltage of the first transistor and a gate overdrive voltage of the second transistor, wherein:
the difference between the threshold voltage of the first transistor and the threshold voltage of the second transistor is a conversely proportional to absolute temperature voltage;
the difference between the gate overdrive voltage of the first transistor and the gate overdrive voltage of the second transistor is a proportional to absolute temperature voltage;
the reference voltage is substantially independent of temperature variations; and
the first transistor has a gate terminal coupled to a gate terminal of the second transistor.
2. The method as recited in claim 1 , wherein the reference voltage is substantially independent of a power supply voltage operably applied to the circuit, for values of the power supply voltage greater than a predetermined value.
3. The method as recited in claim 1 , wherein the first and second transistors comprise transistors having identical conductivity type.
4. The method as recited in claim 1 , wherein the first and second transistors comprise N-channel transistors having respective threshold voltages that are nominally non-negative in value.
5. The method as recited in claim 1 , further comprising comparing the reference voltage to a voltage derived from a power supply voltage, and for generating a signal to indicate that the derived voltage exceeds the reference voltage.
6. A method for making a product incorporating a voltage reference circuit, the method comprising:
forming a reference voltage circuit configured to generate a reference voltage based on a difference between a threshold voltage of a first transistor and a threshold voltage of a second transistor, and further based on a difference between a gate overdrive voltage of the first transistor and a gate overdrive voltage of the second transistor, wherein:
the difference between the threshold voltage of the first transistor and the threshold voltage of the second transistor is a conversely proportional to absolute temperature voltage;
the difference between the gate overdrive voltage of the first transistor and the gate overdrive voltage of the second transistor is a proportional to absolute temperature voltage;
the reference voltage is substantially independent of temperature variations; and
the first transistor has a gate terminal coupled to a gate terminal of the second transistor.
7. The method of claim 6 , further comprising selecting component values to achieve a voltage reference that is stable for values of its power supply voltage greater than about 1.4 volts.
8. The method of claim 6 , further comprising selecting component values to achieve a voltage reference that is stable for values of its power supply voltage greater than about 800 millivolts.
9. A reference voltage circuit comprising:
a first transistor and a second transistor, wherein the first transistor has a gate terminal coupled to a gate terminal of the second transistor; and
a means for generating a reference voltage based on a difference between a threshold voltage of the first transistor and a threshold voltage of the second transistor, and further based on a difference between a gate overdrive voltage of the first transistor and a gate overdrive voltage of the second transistor, wherein:
the difference between the threshold voltage of the first transistor and the threshold voltage of the second transistor is a conversely proportional to absolute temperature voltage;
the difference between the gate overdrive voltage of the first transistor and the gate overdrive voltage of the second transistor is a proportional to absolute temperature voltage; and
the reference voltage is substantially independent of temperature variations.
10. The circuit of claim 9 , wherein the reference voltage is substantially independent of a power supply voltage operably applied to the circuit, for values of the power supply voltage greater than a predetermined value.
11. The circuit of claim 9 , wherein the first and second transistors comprise transistors having identical conductivity type.
12. The circuit of claim 9 , wherein the first and second transistors comprise N-channel transistors having respective threshold voltages that are nominally non-negative in value.
13. The circuit of claim 9 , further comprising a comparator adapted to compare the reference voltage to a voltage derived from a power supply voltage, and to generate a signal to indicate that the derived voltage exceeds the reference voltage.
14. The circuit of claim 9 , embodied in a computer-readable form suitable for design, test, or manufacturing of an integrated circuit.
15. A circuit comprising:
a reference voltage output node; and
means for developing a reference voltage on the output node that is based on a difference in a respective threshold voltage of a first transistor and a second transistor, and further based on a difference in a respective gate overdrive voltage of the first transistor and the second transistor, wherein:
the difference between the threshold voltage of the first transistor and the threshold voltage of the second transistor is a conversely proportional to absolute temperature voltage;
the difference between the gate overdrive voltage of the first transistor and the gate overdrive voltage of the second transistor is a proportional to absolute temperature voltage;
the reference voltage is substantially independent of temperature variations; and
the first transistor has a gate terminal coupled to a gate terminal of the second transistor.
16. A circuit comprising:
first and second supply nodes;
a first circuit leg comprising a first transistor coupled between the first supply node and the second supply node, wherein the first circuit leg conducts a current; and
a second circuit leg comprising a second transistor coupled between the first supply node and the second supply node, wherein the second circuit leg conducts substantially the current;
wherein:
the second transistor comprises a first output node that provides a reference voltage that is based on a difference between respective threshold voltages of the first transistor and the second transistor, and further based on a difference between respective gate overdrive voltages of the first transistor and the second transistor;
the difference between the threshold voltage of the first transistor and the threshold voltage of the second transistor is a conversely proportional to absolute temperature voltage;
the difference between the gate overdrive voltage of the first transistor and the gate overdrive voltage of the second transistor is a proportional to absolute temperature voltage;
the reference voltage is substantially independent of temperature variations; and
the first transistor has a gate terminal coupled to a gate terminal of the second transistor.
17. The circuit of claim 16 , wherein:
the first transistor is of a first conductivity type, and has a first threshold voltage; and
the second transistor is of the first conductivity type, and has a second threshold voltage less than the first threshold voltage.
18. The circuit of claim 16 , further comprising:
a third circuit leg comprising a third transistor coupled between the first supply node and the second supply node, wherein the third circuit leg conducts substantially the current;
wherein the third transistor comprises a second output node that provides a second reference voltage that is based on a difference between respective threshold voltages of the first transistor and the second transistor, and further based on a difference between respective gate overdrive voltages of the first transistor and the second transistor.
19. The circuit of claim 18 , wherein the second reference voltage varies by less than about 1% over a process and temperature range from 25 to +100 degrees Celsius.
20. The circuit of claim 18 , further comprising a comparator circuit for comparing the second reference voltage to a voltage derived from the voltage conveyed on the first supply node, and for generating a signal to indicate that the derived voltage exceeds the second reference voltage.