Programmable reference voltage regulator

1. A voltage regulator, comprising:

a bias circuit coupled to an output node;

a first operational amplifier having a first input coupled to the output node, a second input coupled to a reference voltage node, and an output coupled to a first node;

a second operational amplifier having a first input coupled to the output node, a second input coupled to the reference voltage node, and an output coupled to a second node;

an output transistor coupled between the output node and a ground node, the output transistor including a gate;

first, second, and third stacked transistor pairs each serially coupled between the output node and the ground node, each transistor of the first, second, and third stacked transistor pairs including a gate; and

switch circuits configured to selectively couple: the gates of the first and second stacked transistor pairs to the second node; and the gate of the output transistor to the first node.

2. The voltage regulator of claim 1 , wherein the switch circuits are further configured to selectively couple: a first voltage node to the gates of the output transistor and each transistor of the first, second, and third stacked transistor pairs; a second voltage node to gates of top transistors in each of the first, second, and third stacked transistor pairs; and a ground voltage node to the gates of the output transistor and bottom transistors in each of the first, second, and third stacked transistor pairs.

3. The voltage regulator of claim 2 , wherein the first voltage node is coupled to a first voltage source, the second voltage node is coupled to a second voltage source, and the ground node is coupled to an electrical ground.

4. The voltage regulator of claim 1 , further comprising:

a capacitor and a resistor coupled in series between the output node and a third node;

wherein the switch circuits are further configured to selectively couple the third node to the first node or the second node.

5. The voltage regulator of claim 1 , further comprising:

a capacitor coupled between a first voltage node and the output node.

6. The voltage regulator of claim 1 , wherein the bias circuit comprises a resistor divider circuit coupled between a first voltage node and a ground node.

7. The voltage regulator of claim 1 , wherein the output transistor includes a thicker gate oxide than the transistors of the first, second, and third stacked transistor pairs.

8. A transmitter, comprising:

a pre-driver coupled between a first voltage node and a regulated ground voltage node; and

a voltage regulator having an output node coupled to regulated ground voltage node, the voltage regulator including:

a bias circuit coupled to the output node;

a first operational amplifier having a first input coupled to the output node, a second input coupled to a reference voltage node, and an output coupled to a first node;

a second operational amplifier having a first input coupled to the output node, a second input coupled to the reference voltage node, and an output coupled to a second node;

an output transistor coupled between the output node and a ground node, the output transistor including a gate;

first, second, and third stacked transistor pairs each serially coupled between the output node and the ground node, each transistor of the first, second, and third stacked transistor pairs including a gate; and

switch circuits configured to selectively couple: the gates of the first and second stacked transistor pairs to the second node; and the gate of the output transistor to the first node.

9. The transmitter of claim 8 , wherein the switch circuits are further configured to selectively couple: the first voltage node to the gates of the output transistor and each transistor of the first, second, and third stacked transistor pairs; a second voltage node to gates of top transistors in each of the first, second, and third stacked transistor pairs; and a ground voltage node to the gates of the output transistor and bottom transistors in each of the first, second, and third stacked transistor pairs.

10. The transmitter of claim 9 , wherein the first voltage node is coupled to a first voltage source, the second voltage node is coupled to a second voltage source, and the ground node is coupled to an electrical ground.

11. The transmitter of claim 8 , further comprising:

a capacitor and a resistor coupled in series between the output node and a third node;

wherein the switch circuits are further configured to selectively couple the third node to the first node or the second node.

12. The transmitter of claim 8 , further comprising:

a capacitor coupled between a first voltage node and the output node.

13. The transmitter of claim 8 , wherein the bias circuit comprises a resistor divider circuit coupled between a first voltage node and a ground node.

14. The transmitter of claim 8 , wherein the output transistor includes a thicker gate oxide than the transistors of the first, second, and third stacked transistor pairs.

15. A method of voltage regulation, comprising:

controlling voltage of an output node using a bias circuit, wherein an output transistor is coupled between the output node and a ground node, the output transistor including a gate, and wherein first, second, and third stacked transistor pairs are each serially coupled between the output node and the ground node, each transistor of the first, second, and third stacked transistor pairs including a gate;

controlling voltage of a first node using an first operational amplifier that compares the voltage of the output node with a reference voltage;

controlling voltage of a second node using a second operational amplifier that compares the voltage of the output node with the reference voltage; and

controlling switch circuits that selectively couple: the gates of the first and second stacked transistor pairs to the second node; and the gate of the output transistor to the first node.

16. The method of claim 15 , wherein the switch circuits are further configured to selectively couple: a first voltage node to the gates of the output transistor and each transistor of the first, second, and third stacked transistor pairs; a second voltage node to gates of top transistors in each of the first, second, and third stacked transistor pairs; and a ground voltage node to the gates of the output transistor and bottom transistors in each of the first, second, and third stacked transistor pairs.

17. The method of claim 16 , wherein the first voltage node is coupled to a first voltage source, the second voltage node is coupled to a second voltage source, and the ground node is coupled to an electrical ground.

18. The method of claim 16 , wherein the step of controlling the switch circuits includes:

controlling, in a power-up mode, the switch circuits to: couple the gate of the output transistor to the ground node; the gates of the top transistors to the second voltage node; and the gates of the bottom transistors to the ground node.

19. The method of claim 16 , wherein the step of controlling the switch circuits includes:

controlling the switch circuits to: couple the gate of the output transistor to the ground node; the gates of the transistors in the first and second stacked transistor pairs to the second node; the gate of the top transistor in the third stacked transistor pair to the second voltage node; and the gate of the bottom transistor in the third stacked transistor pair to the ground node.

20. The method of claim 16 , wherein the step of controlling the switch circuits includes:

controlling the switch circuits to: couple the gate of the output transistor to the first node; the gates of the top transistors to the second voltage node; and the gates of the bottom transistors to the ground node.