Capacitive coupled sense amplifier biased at maximum gain point
A sense amplifier includes a first inverter including a first input node and a first output node, the first input node coupled to a first bitline through a first capacitor, the first output node coupled to a second bitline through a second capacitor, a second inverter including a second input node and a second output node, the second input node coupled to the second bitline through the second capacitor, the second output node to the first bitline through the first capacitor, a first transmission gate switch coupled between the first input node and the second input node, a second transmission gate switch coupled between a first common node of the first and second inverters and a second common node of the first and second inverters. The sense amplifier is maintained at a maximum gain point in a read cycle.
1. A sense amplifier coupled between a complementary pair of first and second bitlines that are coupled to respective first and second global bitlines in a static random access memory (SRAM) cell, the sense amplifier comprising:
a first inverter including a first input node and a first output node, the first input node coupled to the first bitline through a first capacitor, the first output node coupled to the second bitline through a second capacitor;
a second inverter including a second input node and a second output node, the second input node coupled to the second bitline through the second capacitor, the second output node to the first bitline through the first capacitor;
a first transmission gate switch coupled between the first input node and the second input node; and
a second transmission gate switch coupled between a first common node of the first and second inverters and a second common node of the first and second inverters,
wherein the sense amplifier is maintained at a maximum gain point in a read cycle of the SRAM cell.
2. The sense amplifier of claim 1 , further comprising a common node PMOS transistor at the first common node and a common node NMOS transistor at the second common node.
3. The sense amplifier of claim 1 , wherein the maximum gain point is a half of a drain supply voltage (V dd /2).
4. The sense amplifier of claim 1 , wherein the first and second transmission gate switches are formed by a NMOS transistor and a PMOS transistor in parallel.
5. The sense amplifier of claim 1 , wherein the first and second bitlines are precharged to a drain supply voltage (V dd ) with a precharging circuit.
6. The sense amplifier of claim 5 , wherein the precharging circuit includes three PMOS transistors in series, and the three PMOS transistors are triggered together with a charging signal.
7. The sense amplifier of claim 1 , wherein the first and second bitlines are coupled to the first and second global bitlines by a respective multiplexer PMOS transistor.
8. The sense amplifier of claim 1 , wherein the internal nodes are maintained at a maximum gain point in a read cycle time.
9. A method for sensing a bitline voltage difference with a sense amplifier in a static random access memory (SRAM), the method comprising the steps of:
starting a default state of the sense amplifier with precharging a complimentary pair of bitlines to a drain supply voltage (V dd ), connecting the bitlines to corresponding global bitlines, switching transmission gate switches OFF and switching common node transistors of the sense amplifier ON, wherein each bitline is coupled to the corresponding global bitline with a multiplexer transistor and coupled to the sense amplifier with a corresponding capacitor;
when a read operation starts, executing an equalizing state of the sense amplifier with the transmission gate switches ON and the common node transistors OFF, wherein the voltages of internal nodes of the sense amplifier are equalized and maintained at a half of the drain supply voltage (V dd /2);
moving to the evaluation state of the sense amplifier, the wordline is switched ON, the precharge transistors on the bitlines are switched OFF, the Global Bitlines starts to discharge, which being connected to bitlines make them also discharge; and
in the evaluation state, when a voltage difference (V diff ) from the bitlines transfers from the capacitor to internal nodes of the sense amplifier to a certain point, switching the common node transistors ON and turning OFF the transmission gates, wherein at the certain point, the internal nodes reach to the stable states by travelling V dd /2, resulting in the sense amplifier biased at a maximum gain point in a read/write cycle time.
10. The method of claim 9 , wherein the maximum gain point is a half of the drain supply voltage (V dd /2).
11. The method of claim 9 , wherein the certain point is a point that a voltage difference is greater than an OFFSET of the sense amplifier, wherein the OFFSET is a minimum voltage difference between the internal nodes of the sense amplifier to resolve the sense amplifier in a correct and intended direction.
12. The method of claim 9 , wherein the internal nodes reach to the stable states with one internal node falling to 0 and the other internal node reaching to V dd .
13. The method of claim 9 , wherein the transmission gate switches are formed by a NMOS transistor and a PMOS transistor in parallel, respectively.
14. The method of claim 9 , wherein the complimentary pair of bitlines are coupled to a complimentary pair of global bitlines by a respective multiplexer PMOS transistor, respectively.
15. An integrated circuit comprising:
a complimentary pair of bitlines coupled to a respective complimentary pair of global bitlines;
a first inverter including a first input node and a first output node, the first input node coupled to a first bitline through a first capacitor, the first output node coupled to a second bitline through a second capacitor;
a second inverter including a second input node and a second output node, the second input node coupled to the second bitline through the second capacitor, the second output node to the first bitline through the first capacitor;
a first transmission gate switch coupled between the first input node and the second input node; and
a second transmission gate switch coupled between a first common node of the first and second inverters and a second common node of the first and second inverters,
wherein the first input node and the second output node are maintained at a maximum gain point in a read cycle.
16. The integrated circuit of claim 15 , further comprising a common node PMOS transistor at the first common node and a common node NMOS transistor at the second common node.
17. The integrated circuit of claim 15 , wherein the maximum gain point is a half of a drain supply voltage (V dd /2).
18. The integrated circuit of claim 15 , wherein the first and second transmission gate switches are formed by a NMOS transistor and a PMOS transistor in parallel.
19. The integrated circuit of claim 15 , wherein the first and second bitlines are precharged to a drain supply voltage (V dd ) with a precharging circuit.
20. The integrated circuit of claim 15 , wherein the first and second bitlines are coupled to the first and second global bitlines by a respective multiplexer PMOS transistor.