Bitcell current sense device and method thereof
A memory device includes a sense amplifier to sense the state of a bitcell. The sense amplifier includes two input terminals connected via a switch. One of the input terminals is connected to a node, whereby a current through the node represents a difference in current drawn by a bitcell and a reference current. During a first phase, the switch between the input terminals of the sense amplifier is closed, so that a common voltage is applied to both input terminals. During a second phase, the switch is opened, and the sense amplifier senses a state of information stored at the bitcell based on the current through the node. By using the switch to connect and disconnect the inputs of the sense amplifier in the two phases, the accuracy and speed with which the state of the information stored at the bitcell can be determined is improved.
1. A device, comprising:
a first bitcell comprising a first terminal configured to allow a first current at the first terminal during a read cycle, the first current representative of information stored at the first bitcell;
a second bitcell comprising a second terminal configured to allow a second current at the second terminal, the second current representative of a first reference current;
a first node coupled to the first terminal of the first bitcell and the second terminal of the second bitcell;
a sense amplifier comprising a first input coupled to the first node and a second input configured to receive a voltage; and
a switch comprising a first terminal coupled to the first input of the sense amplifier, a second terminal coupled to the second input of the sense amplifier, and a control input, a voltage at the first node based on the first current and the second current when the switch is open.
2. The device of claim 1 , wherein the control input of the switch is configured to receive a control signal, the control signal configured to close the switch during a first phase of the read cycle, and open the switch during a second phase of the read cycle.
3. The device of claim 2 , wherein the first phase corresponds to a precharge phase of the read cycle, and the second phase corresponds to a read phase of the read cycle.
4. The device of claim 1 , further comprising a current mirror coupled between the first terminal of the first bitcell and the first node.
5. The device of claim 4 , wherein the current mirror comprises:
an operational amplifier comprising an input configured to be charged to a bitline voltage associated with a bitline of the first bitcell, a second input coupled to the output of the first bitcell, and an output;
a first transistor comprising a first current electrode coupled to a first voltage reference, a second current electrode coupled to the first terminal of the first bitcell, and a control electrode coupled to the output of the operational amplifier;
a second transistor comprising a first current electrode coupled to the first voltage reference, a second current electrode coupled to the first node, and a control electrode coupled to the output of the operational amplifier.
6. The device of claim 1 , further comprising:
a current mirror coupled between the second terminal of the second bitcell and the first node.
7. The device of claim 6 , wherein the current mirror comprises:
a first operational amplifier comprising an input configured to receive a bitline voltage associated with a bitline of the second bitcell, a second input coupled to the output of the second bitcell, and an output;
a first transistor comprising a first current electrode coupled to a first voltage reference, a second current electrode coupled to the second terminal of the second bitcell, and a control electrode coupled to the output of the first operational amplifier; and
a second transistor comprising a first current electrode coupled to the first voltage reference, a second current electrode coupled to the first node, and a control electrode coupled to the output of the first operational amplifier.
8. The device of claim 7 , wherein the second current mirror further comprises:
a third transistor comprising a first current electrode coupled to the second current electrode of the second transistor, a second current electrode coupled to a second voltage reference, and a control electrode coupled to the first current electrode of the third transistor.
9. The device of claim 8 , wherein the second current mirror further comprises:
a first resistor coupled between the second current electrode of the third transistor and the second voltage reference.
10. The device of claim 9 , wherein the second current mirror further comprises:
a second operational amplifier coupled between the second current electrode of the second transistor and the first node, the second operational amplifier comprising a first input coupled to the second current electrode of the second transistor, a second input coupled to first node, and an output coupled to the second input.
11. The device of claim 10 , wherein the current mirror further comprises:
a fourth transistor coupled between the second input of the second operational amplifier and the first node, the fourth transistor comprising a first current electrode coupled to the first node, a second current electrode coupled to the second voltage reference, and a control electrode coupled to the second input of the second operational amplifier.
12. The device of claim 11 , wherein the second input of the sense amplifier is coupled to the output of the second operational amplifier.
13. The device of claim 11 , further comprising a second resistor coupled between the second current electrode of the fourth transistor and the second voltage reference.
14. The device of claim 7 , wherein the current mirror further comprises:
a second operational amplifier coupled between the second current electrode of the second transistor and the first node, the second operational amplifier comprising a first input coupled to the second current electrode of the second transistor, a second input configured to receive the bitline voltage, and an output coupled to the first node.
15. The device of claim 14 , wherein the second input of the sense amplifier is configured to receive the bitline voltage.
16. The device of claim 14 , wherein the current mirror further comprises:
a third transistor comprising a first current electrode coupled to the second current electrode of the second transistor, a second current electrode coupled to a second voltage reference, and a control electrode coupled to the output of the second operational amplifier; and
a fourth transistor coupled between the output of the second operational amplifier and the first node, the fourth transistor comprising a first current electrode coupled to the first node, a second current electrode coupled to the second voltage reference, and a control electrode coupled to the output of the second operational amplifier.
17. The device of claim 1 , further comprising:
a third bitcell comprising a third terminal, a current through the third terminal representative of a second reference current; and
a current mirror coupled between the second terminal of the second bitcell and the first node, the current mirror comprising a first terminal coupled to the second terminal of the second bitcell and the third terminal of the third bitcell, and a second terminal coupled to the first node, a current through the second terminal representative of an average of the first reference current and the second reference current.
18. The device of claim 17 , wherein the current mirror comprises a third terminal coupled to the second input of the sense amplifier.
19. The device of claim 17 , wherein the second input of the sense amplifier is configured to receive a bitline voltage associated with a bitline of the second bitcell.
20. A method, comprising:
during a first phase, coupling a first input terminal of a sense amplifier to a second input terminal of the sense amplifier;
during a second phase, decoupling the first input terminal of the sense amplifier from the second input terminal of the sense amplifier;
during the second phase, determining a difference between a first current associated with the state of a first bitcell and a second current associated with the state of a reference bitcell based on a voltage at a node coupled to the first bitcell and the reference bitcell; and
determining a state of the first bitcell based on the difference.
21. The method of claim 20 , wherein the first phase substantially corresponds with a precharge phase of a bitline of the first bitcell.