# Patent

## Title
Sense amplifiers with high voltage swing

## Abstract

A sense amplifier includes a reference voltage generator for generating a reference output voltage and a core output voltage generator for generating a core output voltage. The core output voltage generator includes a core front-end stage and a core back-end stage or includes a plurality of amplifier transistors each conducting a portion of a core current through a current conducting device such as core cell. The sizes and/or connections of transistors of such components result in high voltage swing and thus high sensitivity of the sense amplifier.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
The present application is a divisional of an earlier filed patent application with Ser. No. 11/024,257 filed on Dec. 28, 2004, issued as U.S. Pat. No. 7,312,641 on Dec. 25, 2007, for which priority is claimed. This earlier filed patent application with Ser. No. 11/024,257 is in its entirety incorporated herewith by reference.


TECHNICAL FIELD 
The present invention relates generally to sense amplifiers, and more particularly, to sense amplifier circuits with high voltage swing for enhanced sensitivity.
BACKGROUND OF THE INVENTION 
Referring to FIG. 1, a sense amplifier 100 of the prior art is used for determining the bit data of a core cell 102 that is typically part of a memory device. A current level (IR+Δi) through the core cell 102 varies depending on the bit data stored therein. A core bit voltage VCBIT is generated at a source of a first NMOSFET (N-channel metal oxide semiconductor field effect transistor) 104 from the core cell 102.
The source of the first NMOSFET 104 and the core cell 102 are coupled to a negative input 108 of a first differential amplifier 106 that compares the core bit voltage VCBIT with a regulation reference voltage VREG—REF applied on a positive input 110 of the first differential amplifier 106. The output of the first differential amplifier 106 is coupled to a gate of the first NMOSFET 104 for stably maintaining the core bit voltage VCBIT.
A drain of the first NMOSFET 104 is coupled to a positive voltage supply VCC via a first resistor 112. A core output voltage VCORE is generated at the drain of the first NMOSFET 104 and is applied on a negative input of a comparator 120.
The sense amplifier 100 also includes a second NMOSFET 122 having a source coupled to a reference cell 124. A current level IR flows through the reference cell 124, and a reference bit voltage VRBIT is generated at the source of the second NMOSFET 122 from the reference cell 124.
The source of the second NMOSFET 122 and the reference cell 124 are coupled to a negative input 126 of a second differential amplifier 128 that compares the reference bit voltage VRBIT with the regulation reference voltage VREG—REF applied on a positive input 130 of the second differential amplifier 128. The output of the second differential amplifier 130 is coupled to a gate of the second NMOSFET 122 for stably maintaining the reference bit voltage VRBIT.
A drain of the second NMOSFET 122 is coupled to a positive voltage supply VCC via a second resistor 132. A reference output voltage VREF is generated at the drain of the second NMOSFET 122 and is applied on a positive input of the comparator 120.
The output of the comparator generates an output signal OUT that is a logical high state or a logical low state depending on the core output voltage VCORE compared to the reference output voltage VREF. Such a logical high or low state of the output signal OUT indicates the bit data stored within the core cell 102.
The current (IR+Δi) through the core cell has a current offset component Δi from the reference current IR through the reference cell 124 that varies depending on the bit data stored within the core cell 102. Such a variable current offset component Δi determines the core output voltage VCORE which in turn determines the logical state of the output signal OUT.
Unfortunately, the core output voltage VCORE in the sense amplifier 100 of the prior art has limited voltage swing because the core bit voltage VCBIT is relatively high and substantially close to the positive supply voltage VCC for proper operation of the core cell 102. For example, when the positive supply voltage VCC is about 1.8 Volts, the core bit voltage VCBIT is about 1.5 Volts for proper operation of the core cell 102. In addition, a voltage drop is generated across the first resistor 112. Thus for such example voltages, the core output voltage VCORE has a voltage swing of from about 0.2 Volts to about 0.3 Volts for maintaining the first NMOSFET 104 in saturation.
Such a low voltage swing of the core output voltage VCORE disadvantageously results in low sensitivity of the sense amplifier 100 in the prior art. Thus, sense amplifiers having higher voltage swing are desired for higher sensitivity.
SUMMARY OF THE INVENTION 
In one embodiment of the present invention, a sense amplifier includes a reference voltage generator for generating a reference output voltage. In addition, the sense amplifier also includes a core output voltage generator for generating a core output voltage. The core output voltage generator includes a core front-end stage and a core back-end stage. The core front-end stage is coupled to a current conducting device for converting a core current through the current conducting device to a core bit voltage. The core back-end stage is coupled to the core front-end stage for converting the core bit voltage to the core output voltage having higher voltage swing from the core bit voltage.
The sense amplifier may be used to particular advantage when the current conducting device is a core cell of a memory device. However, the present invention may also be used for sensing the current level through any type of current conducting device.
In another embodiment of the present invention, the core output voltage generator includes a plurality of amplifier transistors each conducting a portion of a core current through the current conducting device. A gate of a selected one of the amplifier transistors has the core output voltage generated thereon. A width to length (W/L) ratio of the selected amplifier transistor is minimized such that the core output voltage has high voltage swing.
In this manner, such sense amplifiers have high voltage swing for high sensitivity. These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.



BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows a sense amplifier having lower voltage swing according to the prior art;
FIG. 2 shows a sense amplifier with front-end and back-end stages for high voltage swing, according to an embodiment of the present invention;
FIG. 3 shows modification to the sense amplifier of FIG. 2 using bias resistors instead of transistors, according to another embodiment of the present invention;
FIG. 4 shows modification of the sense amplifier of FIG. 2 with alternative connectivity of transistors, according to another embodiment of the present invention;
FIG. 5 shows modification of the sense amplifier of FIG. 4 with added source resistors for reducing effect of noise and mismatch of transistors, according to another embodiment of the present invention;
FIG. 6 shows a sense amplifier with a smaller sized transistor for high voltage swing, according to another embodiment of the present invention;
FIG. 7 shows modification of the sense amplifier of FIG. 2 for coupling a reference voltage generator to a plurality of core voltage generators, according to another embodiment of the present invention; and
FIG. 8 shows modification of the sense amplifier of FIG. 3 for coupling a reference voltage generator to a plurality of core voltage generators, according to another embodiment of the present invention.



The figures referred to herein are drawn for clarity of illustration and are not necessarily drawn to scale. Elements having the same reference number in FIGS. 1, 2, 3, 4, 5, 6, 7, and 8 refer to elements having similar structure and function.
DETAILED DESCRIPTION 
FIG. 2 shows a circuit diagram of a sense amplifier 200 having high voltage swing according to an embodiment of the present invention. The sense amplifier 200 includes a reference voltage generator 202 for generating a reference output voltage VREF, and includes a core output voltage generator 204 for generating a core output voltage VCORE.
The reference voltage generator 202 includes a reference front-end stage 206 comprised of a reference regulation transistor MRP1 coupled to a reference cell 208 having a reference current IR flowing there-through. A reference bit voltage VRBIT is generated at a node for coupling a drain of the first reference PMOSFET (P-channel metal oxide semiconductor field effect transistor) MRP1 and the reference cell 208. A source of the first reference PMOSFET MRP1 is coupled to a positive power supply VCC.
The reference voltage generator 202 also includes a reference back-end stage 210 coupled to the reference front-end stage 206. The reference back-end stage 210 is comprised of a second reference PMOSFET MRP2 and a first reference NMOSFET (N-channel metal oxide semiconductor field effect transistor) MRN1. The second reference PMOSFET MRP2 has a gate coupled to the gate of the first reference PMOSFET MRP1, and has a source coupled to the positive power supply VCC. A drain of the second reference PMOSFET MRP2 is coupled to a drain of the first reference NMOSFET MRN1 at a node for generating the reference output voltage VREF.
A gate and the drain of the first reference NMOSFET MRN1 are coupled together, and a source of the first reference NMOSFET MRN1 is coupled to a low power supply such as a ground node. The first and second reference PMOSFETs MRP1 and MRP2 and the first reference NMOSFET MRN1 each have the reference current IR flowing there-through.
The gate and drain of the first reference PMOSFET MRP1 are coupled to a reference feed-back regulator 212 that stabilizes the reference bit voltage VRBIT. In the example embodiment of FIG. 2, the reference feed-back regulator 212 is a differential amplifier comprised of second, third, and fourth reference NMOSFETs MRN2, MRN3, and MRN4, respectively, and third and fourth reference PMOSFETs MRP3 and MRP4, respectively.
Sources of the third and fourth reference PMOSFETs MRP3 and MRP4 are coupled to the positive power supply VCC, and gates of the third and fourth reference PMOSFETs MRP3 and MRP4 are coupled together. A drain of the third reference PMOSFET MRP3 is coupled to a drain of the second reference NMOSFET MRN2, and a drain of the fourth reference PMOSFET MRP4 is coupled to a drain of the third reference NMOSFET MRN3. The gate and the drain of the fourth reference PMOSFET MRP4 are coupled together.
The drains of the third reference PMOSFET MRP3 and the second reference NMOSFET MRN2 are coupled to the gates of the first and second reference PMOSFETs MRP1 and MRP2. A gate of the second reference NMOSFET MRN2 has a regulation reference voltage VREG_REF applied thereon. A gate of the third reference NMOSFET MRN3 is coupled to the reference cell 208 such that the reference bit voltage VRBIT is applied thereon. Sources of the second and third reference NMOSFETs MRN2 and MRN3 are coupled together to a drain of the fourth reference NMOSFET MRN4.
The gate of the fourth reference NMOSFET MRN4 has a bias voltage VBIAS applied thereon, and the source of the fourth reference NMOSFET MRN4 is coupled to a low power supply such as the ground node. The bias voltage VBIAS at the gate of the fourth reference NMOSFET MRN4 sets the bias current through the reference MOSFETs MRP3, MRP4, MRN2, and MRN3 of the differential amplifier 212. In addition, the differential amplifier 212 acts to stabilize the reference bit voltage VRBIT by feed-back.
The core output voltage generator 204 includes a core front-end stage 216 comprised of an amplifier regulation transistor MCP1 coupled to a core cell 218 having a core current (IR+Δi) flowing there-through. The core current has a current deviation component Δi offset from the reference current IR. Such a current deviation component Δi depends on the bit data stored in the core cell 218.
The core cell 218 is typically part of a memory device for example. A core bit voltage VCBIT is generated at a node for coupling a drain of the first amplifier PMOSFET MCP1 and the core cell 218. A source of the first amplifier PMOSFET MCP1 is coupled to a positive power supply VCC.
The core output voltage generator 204 also includes a core back-end stage 220 coupled to the core front-end stage 216. The core back-end stage 220 is comprised of a second amplifier PMOSFET MCP2 and a first amplifier NMOSFET MCN1. The second amplifier PMOSFET MCP2 has a gate coupled to the gate of the first amplifier PMOSFET MCP1, and has a source coupled to the positive power supply VCC. A drain of the second amplifier PMOSFET MCP2 is coupled to a drain of the first amplifier NMOSFET MCN1 at a node for generating the core output voltage VCORE.
A gate and the drain of the first amplifier NMOSFET MCN1 are coupled together, and a source of the first amplifier NMOSFET MCN1 is coupled to a low power supply such as the ground node. The first and second amplifier PMOSFETs MCP1 and MCP2 and the first amplifier NMOSFET MCN1 each have the core current (IR+Δi) flowing there-through.
The gate and drain of the first amplifier PMOSFET MCP1 are coupled to a core feed-back regulator 222 that stabilizes the core bit voltage VCBIT. In the example embodiment of FIG. 2, the core feed-back regulator 222 is a differential amplifier comprised of second, third, and fourth amplifier NMOSFETs MCN2, MCN3, and MCN4, respectively, and third and fourth amplifier PMOSFETs MCP3 and MCP4, respectively.
Sources of the third and fourth amplifier PMOSFETs MCP3 and MCP4 are coupled to the positive power supply VCC, and gates of the third and fourth amplifier PMOSFETs MCP3 and MCP4 are coupled together. A drain of the third amplifier PMOSFET MCP3 is coupled to a drain of the second amplifier NMOSFET MCN2, and a drain of the fourth amplifier PMOSFET MCP4 is coupled to a drain of the third amplifier NMOSFET MCN3. The gate and the drain of the fourth amplifier PMOSFET MCP4 are coupled together.
The drains of the third amplifier PMOSFET MCP3 and the second amplifier NMOSFET MCN2 are coupled to the gates of the first and second amplifier PMOSFETs MCP1 and MCP2. A gate of the second amplifier NMOSFET MCN2 has the regulation reference voltage VREG_REF applied thereon. A gate of the third amplifier NMOSFET MCN3 is coupled to the core cell 218 such that the core bit voltage VCBIT is applied thereon. Sources of the second and third amplifier NMOSFETs MCN2 and MCN3 are coupled together to a drain of the fourth amplifier NMOSFET MCN4.
The gate of the fourth amplifier NMOSFET MCN4 has a bias voltage VBIAS applied thereon, and the source of the fourth amplifier NMOSFET MCN4 is coupled to a low power supply such as the ground node. The bias voltage VBIAS at the gate of the fourth amplifier NMOSFET MCN4 sets the bias current through the core MOSFETs MCP3, MCP4, MCN2, and MCN3 of the differential amplifier 222. In addition, the differential amplifier 222 acts to stabilize the core bit voltage VCBIT by feed-back.
Furthermore, the sense amplifier 200 includes a comparator 230 which is implemented as a differential amplifier for example. The comparator 230 has a positive input with the reference output voltage VREF applied thereon, and has a negative input with the core output voltage VCORE applied thereon. The comparator 230 generates the output signal OUT from comparing the core output voltage VCORE with the reference output voltage VREF. The logical state of the output signal OUT indicates the bit data stored in the core cell 218.
In this manner, referring to FIG. 2, note that the core front-end stage 216 does not have any resistor in series with the voltage regulation transistor MCP1. Thus, the core bit voltage VCBIT may be relatively high near the high power supply voltage VCC with the voltage regulation transistor MCP1 still operating in saturation. In addition, the core output voltage VCORE is generated at the subsequent back-end stage 220 that is not directly coupled to the core cell 218. Thus, the core output voltage VCORE is not restricted from the core bit voltage VCBIT such that the core output voltage VCORE has higher voltage swing with the MOSFETs MCP1 and MCN1 still operating in saturation. Such higher voltage swing is advantageous for enhanced sensitivity of the sense amplifier 200.
In the sense amplifier 200 of FIG. 2:

ΔV=VREF−VCORE;

and

ΔV=66 i/gml,

with gml being the transconductance of each of the first reference and amplifier NMOSFETs MRN1 and MCN1.

In addition, the sense amplifier 200 of FIG. 2 has high signal to noise ratio for both balanced and unbalanced power-supply noise within the reference voltage generator 202 and the core output voltage generator 204. Balanced power-supply noise refers to noise at the power-supply that is present substantially equally within each of the reference voltage generator 202 and the core output voltage generator 204. Unbalanced power-supply noise refers to noise at the power-supply that is present unequally only in one of the reference voltage generator 202 and the core output voltage generator 204.
Referring to FIG. 2, if either balanced or unbalanced noise is present at the power supply VCC of the reference voltage generator 202, the effect of such noise appears at the source of the second reference PMOSMFET MRP2. In addition, a similar effect from such noise appears at the gate of the second reference PMOSMFET MRP2 through the reference feed-back regulator 212. Such effects at the source and the gate of the second reference PMOSFET MRP2 cancel each-other away to not have substantial effect on the reference output voltage VREF.
Similarly, if either balanced or unbalanced noise is present at the power supply VCC of the core output voltage generator 204, the effect of such noise appears at the source of the second amplifier PMOSMFET MCP2. In addition, a similar effect from such noise appears at the gate of the second amplifier PMOSMFET MCP2 through the amplifier feed-back regulator 222. Such effects at the source and the gate of the second amplifier PMOSFET MCP2 cancel each-other away to not have substantial effect on the core output voltage VCORE.
Furthermore, referring to FIG. 7, the MOSFETs of the reference voltage generator 202 may be appropriately sized such that the reference voltage generator 202 provides the reference output voltage VREF for a plurality of core output voltage generators. Referring to FIG. 7, the second reference PMOSFET MRP2 is sized with a W/L (width to length) ratio that is N times the W/L ratio of the first reference PMOSFET MRP1. Thus, a current of NxIR flows through the second reference PMOSFET MRP2.
Further referring to FIG. 7, the reference output voltage VREF from the reference voltage generator 202 is coupled to a respective positive input of N comparators 230_1, 230_2, . . . , and 230_N. Each of the N comparators 230_1, 230_2, . . . , and 230_N has a respective negative input coupled to a respective one of the core output voltage generators 204_1, 204_2, . . . , and 204_N, respectively. Each of the core output voltage generators 204_1, 204_2, . . . , and 204_N is implemented similarly to the core output voltage generator 204 of FIG. 2 to generate a respective one of the core output voltages VCORE—1, VCORE—2, . . . , and VCORE—N, respectively. Each of the comparators 230_1, 230_2, . . . , and 230_N compares a respective one of the core output voltages VCORE—1, VCORE—2, . . . , and VCORE—N, respectively, to the reference output voltage VREF for generating a respective output signal OUT1, OUT2, . . . , and OUTN.
In this manner, the reference voltage generator 202 is used for the plurality of core output voltage generators 204_1, 204_2, . . . , and 204_N for saving space of the integrated circuit of the sense amplifier 200 of FIG. 7. Furthermore, because a higher level of current NxIR flows through the second reference PMOSFET MRP2, the sense amplifier 200 of FIG. 7 operates at higher speed.
FIG. 3 shows a circuit diagram of a sense amplifier 200A according to another embodiment of the present invention. Elements having the same reference number in FIGS. 2 and 3 refer to elements having similar structure and function. Thus, the reference and core front-end stages 206 and 216 and the reference and core feed-back regulators 212 and 222 are substantially similar in FIGS. 2 and 3.
However, the reference and core back-end stages 210A and 220A of FIG. 3 are different from the reference and core back-end stages 210 and 220 of FIG. 2. Referring to FIG. 3, reference and amplifier bias resistors RBR and RBC, respectively, replace the first reference and amplifier NMOSFETs MRN1 and MCN1, respectively. Thus, the reference bias resistor RBR is coupled between the drain of the second reference PMOSFET MRP2 and the ground node, and the amplifier bias resistor RBC is coupled between the drain of the second amplifier PMOSFET MCP2 and the ground node.
The sense amplifier 200A of FIG. 3 operates similarly to the sense amplifier 200 of FIG. 2. If each of the reference and amplifier bias resistors RBR and RBC has a resistance value of R, then in the sense amplifier 200A of FIG. 3:

ΔV=VREF−VCORE;

and

ΔV=Δi*R. 

Similar to the sense amplifier 200 of FIG. 2, the sense amplifier 200A of FIG. 3 has high signal to noise ratio for both balanced and unbalanced noise within the reference voltage generator 202 and the core output voltage generator 204. Furthermore, referring to FIG. 8, the reference voltage generator 202 may be used for the plurality of core output voltage generators 204_1, 204_2, . . . , and 204_N. In FIG. 8, each of the core output voltage generators 204_1, 204_2, . . . , and 204_N is implemented similarly to the core output voltage generator 204 of FIG. 3.
In addition, the second reference PMOSFET MRP2 is sized with a W/L (width to length) ratio that is N times the W/L ratio of the first reference PMOSFET MRP1. In addition, the resistance value of the reference bias resistor RBR is R/N when the resistance value of the amplifier bias resistor RBC within each of the core output voltage generators 204_1, 204_2, . . . , and 204_N is R. Thus, a current of NxIR flows through the second reference PMOSFET MRP2 in FIG. 8.
In this manner, the reference voltage generator 202 is used for the plurality of core output voltage generators 204_1, 204_2, . . . , and 204_N for saving space of the integrated circuit of the sense amplifier 200A of FIG. 8. Furthermore, because a higher level of current NxIR flows through the second reference PMOSFET MRP2, the sense amplifier 200A of FIG. 8 operates at higher speed.
FIG. 4 shows a circuit diagram of a sense amplifier 200B according to another embodiment of the present invention. Elements having the same reference number in FIGS. 2 and 4 refer to elements having similar structure and function. Thus, the reference and core front-end stages 206 and 216 and the reference and core feed-back regulators 212 and 222 are substantially similar in FIGS. 2 and 4.
However, the reference and core back-end stages 210B and 220B of FIG. 4 are different from the reference and core back-end stages 210 and 220 of FIG. 2. Referring to FIG. 4, the reference output voltage VREF from the reference back-end stage 210B is also coupled to the gate of the first amplifier NMOSFET MCN1 of the core back-end stage 220B. In addition, the gate and the drain of the first amplifier NMOSFET MCN1 are not coupled together.
Instead, the drains of the first amplifier NMOSFET MCN1 and the second amplifier PMOSFET MCP2 are coupled together at a node that generates the core output voltage VCORE in FIG. 4. Such a node is coupled to the negative input of the comparator 230. Similar to the sense amplifier 200 of FIG. 2, the sense amplifier 200B of FIG. 4 also generates an output signal OUT at the output of the comparator 230 with a logical state that indicates the bit data stored in the core cell 218.
In addition referring to FIG. 4, the core front-end stage 216 does not have any resistor in series with the voltage regulation transistor MCP1. Thus, the core bit voltage VCBIT may be relatively high near the high power supply voltage VCC with the voltage regulation transistor MCP1 still operating in saturation. In addition, the core output voltage VCORE is generated at the subsequent back-end stage 220 that is not directly coupled to the core cell 218. Thus, the core output voltage VCORE is not restricted from the core bit voltage VCBIT such that the core output voltage VCORE has higher voltage swing with the MOSFETs MCP1 and MCN1 still operating in saturation. Such higher voltage swing is advantageous for enhanced sensitivity of the sense amplifier 200B.
FIG. 5 shows a circuit diagram of a sense amplifier 200C according to another embodiment of the present invention. Elements having the same reference number in FIGS. 4 and 5 refer to elements having similar structure and function. Thus, the reference and core front-end stages 206 and 216 and the reference and core feed-back regulators 212 and 222 are substantially similar in FIGS. 4 and 5.
However, the reference and core back-end stages 210C and 220C of FIG. 5 are different from the reference and core back-end stages 210B and 220B of FIG. 4. Referring to FIGS. 4 and 5, in the sense amplifier 200C of FIG. 5, a reference noise immunity resistor RNR is coupled between the source of the first reference NMOSFET MRN1 and the ground node 223. Furthermore in the sense amplifier 200C of FIG. 5, an amplifier noise immunity resistor RNC is coupled between the source of the first amplifier NMOSFET MCN1 and the ground node 223.
Including such resistors RNR and RNC is advantageous for suppressing a noise or mismatch component Δi′ of Δi, resulting from noise at the ground node 223 or mismatch between the NMOSFETs MRN1 and MCN1. For example, assume that noise at the ground node 223 results in a voltage bounce of 20 milli-Volts at the ground node 223. In addition, assume that the transconductance gml of each of the NMOSFETs MRN1 and MCN1 is about 1 milli-Amps/Volt. In that case, the noise component Δi′ from such voltage bounce is as follows in the sense amplifier 200B of FIG. 4:

Δi′=gml×20 milli-Volts=20 μA. 

In contrast, for the same voltage bounce at the ground node 223 in the sense amplifier 200C of FIG. 5, the noise component Δi′ is reduced from the resistors RNR and RNC as follows:

Δi′≈20 milli-Volts/15 kilo-Ohms=1.3 μA. 

with each of the resistors RNR and RNC having a resistance value of 15 kilo-Ohms for example in FIG. 5. Thus, such resistors RNR and RNC reduce the effect of noise on Δi in FIG. 5. The sense amplifier 200C of FIG. 5 may also be implemented with the ground node 223 being a low supply voltage source VSS instead. In that case, the resistors RNR and RNC reduce the effect of noise at the low supply voltage source VSS in FIG. 5.

Similarly, the NMOSFETs MRN1 and MCN1 may be mismatched from processing variations to have a difference in threshold voltage ΔVth which may be 20 milli-Volts for example. The mismatch component Δi′ is similar as described above for the voltage bounce. Thus, the resistors RNR and RNC reduce the effect of such MOSFET mismatch on Δi in FIG. 5. Otherwise, the sense amplifier 200C of FIG. 5 operates similarly to the sense amplifier 200B of FIG. 4.
FIG. 6 shows a circuit diagram of a sense amplifier 300 according to another embodiment of the present invention. Elements having the same reference number in FIGS. 2 and 6 refer to elements having similar structure and function. Thus, the reference and core feed-back regulators 212 and 222 are substantially similar in FIGS. 2 and 6.
However, the sense amplifier 300 of FIG. 6 is not implemented with front-end and back-end stages. Rather, referring to FIG. 6, the reference voltage generator 202 includes a plurality of reference PMOSFETs MRP1 and MRP2 for collectively conducting the reference current IR through the reference cell 208. Thus, the sum of the currents through the reference PMOSFETs MRP1 and MRP2 is the reference current IR through the reference cell 208. The reference feed-back regulator 212 is coupled between the gates of the reference PMOSFETs MRP1 and MRP2 and the reference cell 208 for stabilizing the reference bit voltage VRBIT.
Additionally referring to FIG. 6, the core output voltage generator 204 includes a plurality of amplifier PMOSFETs MCP1 and MCP2 for collectively conducting the core current (IR+Δi) through the core cell 218. Thus, the sum of the currents through the amplifier PMOSFETs MCP1 and MCP2 is the core current (IR+Δi) through the core cell 218.
Furthermore, the gates of the reference PMOSFETs MRP1 and MRP2 are coupled together to generate the reference voltage VREF applied on the positive input of the comparator 230 and on the gate of the second amplifier PMOSFET MCP2. The first amplifier PMOSFET MCP1 is a selected one of the plurality of amplifier PMOSFETs MCP1 and MCP2 having a gate for generating the core output voltage VCORE applied on the negative input of the comparator 230. The core feed-back regulator 222 is coupled between the gate of the first amplifier PMOSFET MCP1 and the core cell 218 for stabilizing the core bit voltage VCBIT.
In an example embodiment of the present invention, the W/L ratio of the first reference and amplifier PMOSFETs MRP1 and MCP1 in FIG. 6 is minimized for higher voltage swing of the reference and core output voltages VREF and VCORE. The equation for current conduction through a MOSFET is as follows:

I=k(W/L)(VGS−Vth)2 

with k being a constant, W/L being the width to length ratio, VGS being the gate to source voltage, and Vth being the threshold voltage, of the MOSFET. When the W/L of the first amplifier PMOSFET MCP1 is minimized, the VGS of the first amplifier PMOSFET MCP1 changes more drastically with variation of the core current (IR+Δi) through the core cell 218. Such higher variation of the VGS of the first amplifier PMOSFET MCP1 results in higher voltage swing of the core output voltage VCORE for the sense amplifier 300. Such higher voltage swing is advantageous for enhanced sensitivity of the sense amplifier 300.

In the sense amplifier 300 of FIG. 6:

ΔV=VREF−VCORE;

and

ΔV=Δi/gml,

with gml being the transconductance of each of the first reference and amplifier PMOSFETs MRP1 and MCP1.

Similar to the sense amplifier 200 of FIG. 2, the sense amplifier 300 of FIG. 6 also generates an output signal OUT at the output of the comparator 230 with a logical state that indicates the bit data stored in the core cell 218. In addition referring to FIG. 6, the plurality of amplifier PMOSFETs MCP1 and MCP2 doe not have any resistor in series such that the core bit voltage VCBIT may be relatively high near the high power supply voltage VCC with the plurality of transistors MCP1 and MCP2 still operating in saturation.
The foregoing is by way of example only and is not intended to be limiting. For example, the present invention is described for sensing the current level through the core cell 218 of a memory device. However, the present invention may also be used for sensing the current level when the core cell 218 is any other type of current conducting device. Thus, the terms “core current”, “core output voltage generator”, “core front-end state”, “core back-end stage”, “core feed-back regulator”, and “core output voltage” as used herein is hereby generalized for sensing the current level through any type of current conducting device with the core cell 218 of a memory device being just one example.
In addition, the present invention herein is described for specific connections of the transistors in the example circuits of FIGS. 2, 3, 4, 5, 6, 7, and 8. However, the present invention may also be implemented with variation in the specific connections of the drain, source, and gate of the transistors. Furthermore, any dimensions or parameters specified herein are by way of example only. The present invention is limited only as defined in the following claims and equivalents thereof.

## Claims

1. A sense amplifier comprising:
a reference voltage generator for generating a reference output voltage; and
a core output voltage generator for generating a core output voltage, the core output voltage generator including:
a plurality of amplifier transistors each conducting a portion of a core current through a current conducting device;
wherein a gate of a selected one of the amplifier transistors has the core output voltage generated thereon;
and wherein the reference voltage generator includes:
a plurality of reference transistors each conducting a portion of a reference current through a reference cell;
wherein gates of the reference transistors are coupled together with the reference output voltage generated thereon, and wherein drains of the reference transistors are coupled together at a reference bit node having a reference bit voltage generated thereon, and wherein sources of the reference transistors are coupled to a power supply.

2. The sense amplifier of claim 1, further including:
a reference feed-back regulator coupled to the gates and the drains of the reference transistors for maintaining the reference bit voltage.

3. A sense amplifier comprising:
a reference voltage generator for generating a reference output voltage; and
a core output voltage generator for generating a core output voltage, the core output voltage generator including:
a plurality of amplifier transistors each conducting a portion of a core current through a current conducting device;
wherein a gate of a selected one of the amplifier transistors has the core output voltage generated thereon,
and wherein another one of the amplifier transistors has a gate with the reference output voltage applied thereon.

4. The sense amplifier of claim 3, wherein the current conducting device is a core cell of a memory device.

5. The sense amplifier of claim 3, wherein a width to length (W/L) ratio of the selected amplifier transistor is minimized.

6. The sense amplifier of claim 3, wherein drains of the amplifier transistors are coupled together at a bit node having a core bit voltage generated thereon, and wherein sources of the amplifier transistors are coupled to a power supply.

7. The sense amplifier of claim 6, further including:
a core feed-back regulator coupled to the gate and the drain of the selected amplifier transistor for maintaining the core bit voltage.

8. The sense amplifier of claim 3, wherein the reference voltage generator includes:
a plurality of reference transistors each conducting a portion of a reference current through a reference cell;
wherein gates of the reference transistors are coupled together with the reference output voltage generated thereon, and wherein drains of the reference transistors are coupled together at a reference bit node having a reference bit voltage generated thereon,
and wherein sources of the reference transistors are coupled to a power supply.

9. The sense amplifier of claim 8, further including:
a reference feed-back regulator coupled to the gates and the drains of the reference transistors for maintaining the reference bit voltage.

10. The sense amplifier of claim 1, wherein the current conducting device is a core cell of a memory device.

11. The sense amplifier of claim 1, wherein a width to length (W/L) ratio of the selected amplifier transistor is minimized.

12. The sense amplifier of claim 1, wherein drains of the amplifier transistors are coupled together at a bit node having a core bit voltage generated thereon, and wherein sources of the amplifier transistors are coupled to a power supply.

13. The sense amplifier of claim 12, further including:
a core feed-back regulator coupled to the gate and the drain of the selected amplifier transistor for maintaining the core bit voltage.

14. The sense amplifier of claim 1, wherein another one of the amplifier transistors has the reference output voltage applied thereon.

15. The sense amplifier of claim 14, wherein a gate of said another one of the amplifier transistors has the reference output voltage applied thereon.