Voltage reference circuit and method therefor

1. A voltage reference circuit comprising:

a first transistor having a first active area, a first current carrying electrode, a second current carrying electrode, and a control electrode wherein the first active area is configured to form a first Vbe and wherein the first transistor is not coupled in a diode configuration;

a second transistor having a first current carrying electrode, a second current carrying electrode, a control electrode, and a second active area that is smaller than the first active area wherein the second active area is configured to form a second Vbe that is greater than the first Vbe and wherein the second transistor is not coupled in a diode configuration;

a first resistor coupled to receive a difference between the first Vbe and the second Vbe, the first resistor having first and second terminals; and

an operational amplifier having a first input coupled to the first current carrying electrode of the first transistor and a second input coupled to the first current carrying electrode of the second transistor.

2. The voltage reference circuit of claim 1 wherein the first transistor is a first bipolar transistor with the first input of the operational amplifier coupled to a collector of the first bipolar transistor, and wherein the second transistor is a second bipolar transistor with the second input of the operational amplifier coupled to a collector of the second bipolar transistor.

3. The voltage reference circuit of claim 1 further including a third transistor coupled in a diode configuration and having a control electrode commonly coupled to a first current carrying electrode of the third transistor, the control electrode of the first transistor, and the first terminal of the first resistor, the third transistor having a second current carrying electrode.

4. The voltage reference circuit of claim 3 further including a second resistor coupled in series with the first resistor, and a third resistor coupled in series with the first resistor.

5. The voltage reference circuit of claim 4 wherein the first, second, and third transistors are bipolar transistors.

6. The voltage reference circuit of claim 1 further including a current source coupled to the second current carrying electrode of the first transistor and to the second current carrying electrode of the second transistor.

7. The voltage reference circuit of claim 1 further including a second resistor coupled between the first current carrying electrode of the first transistor and an output of the voltage reference circuit and including a third resistor coupled between the first current carrying electrode of the second transistor and the output of the voltage reference circuit.

8. The voltage reference circuit of claim 1 further including a control transistor coupled to receive an output of the operational amplifier and control a current to flow through the first and second transistors.

9. The voltage reference circuit of claim 1 wherein the first resistor is coupled between the control electrode of the first transistor and the control electrode of the second transistor.

10. A method of forming a voltage reference circuit comprising:

coupling a first transistor and a second transistor in a differential pair configuration;

configuring the first transistor to have a first Vbe that is less than a second Vbe of the second transistor;

coupling a control electrode of the first transistor to a first terminal of a first resistor and coupling a control electrode of the second transistor to a second terminal of the first resistor wherein a difference between the first Vbe and the second Vbe forms a voltage across the first resistor and wherein neither the first nor second transistors are coupled in a diode configuration; and

coupling a control electrode of a third transistor to a control electrode of the first transistor.

11. The method of claim 10 further including coupling the first resistor to receive the first Vbe and the second Vbe and form a first current that is representative of a difference between the first Vbe and the second Vbe.

12. The method of claim 11 further including coupling a second resistor in series with the first resistor to receive the first current.

13. The method of claim 12 further including coupling a third resistor in series with the first resistor to receive the first current and coupling the third transistor in a diode configuration and in series with the first resistor.

14. The method of claim 10 wherein coupling the first transistor and the second transistor in the differential pair configuration includes coupling a current source to form a bias current through the first and second transistors.

15. The method of claim 10 wherein coupling the first transistor and the second transistor in the differential pair configuration includes coupling a first resistor between the first transistor and an output of the voltage reference circuit and coupling a second resistor between the second transistor and the output of the voltage reference circuit.

16. A method of forming a voltage reference circuit comprising:

coupling a first transistor and a second transistor in a differential pair configuration;

configuring the first transistor to have a first active area that is larger than a second active area of the second transistor wherein neither the first nor second transistors are coupled in a diode configuration;

coupling a control electrode of the first transistor to a first terminal of a first resistor;

coupling a control electrode of the second transistor to a second terminal of the first resistor and coupling a first current carrying electrode of the second transistor to a first current carrying electrode of the first transistor;

coupling a first input of a differential amplifier to a second current carrying electrode of the first transistor, coupling a second input of the differential amplifier to a second current carrying electrode of the second transistor; and

coupling an output of the differential amplifier to a current source to control current flow through the first and second transistors.

17. The method of claim 16 wherein configuring the first transistor to have the first active area that is larger than the second active area includes configuring the first transistor to form a first Vbe that is less than a second Vbe of the second transistor wherein a difference between the first Vbe and the second Vbe forms a voltage across the first resistor, and coupling a current source to form a bias current through the first and second transistors.

18. The method of claim 17 further including coupling the first resistor to receive the difference between the first Vbe and the second Vbe and form a first current that is representative of the difference between the first Vbe and the second Vbe.

19. The method of claim 16 wherein coupling the output of the differential amplifier to the current source includes coupling first and second transistors of the differential amplifier in a differential pair configuration, and coupling a control electrode of a third transistor of the differential amplifier to a current carrying electrode of the second transistor of the differential amplifier and coupling a first current carrying electrode of the third transistor of the differential amplifier to the output of the differential amplifier.