CMOS CURRENT SENSE AMPLIFIERS

Abstract: A CMOS current sense amplifier circuit for providing a high speed of operation includes a sense amplifier, a dummy sense amplifier and an operational sense amplifier. A memory array is formed of a plurality of core transistors which are arranged in a plurality of rows of word lines and a plurality of columns of bit-lines. A dummy bit-line is formed of a plurality of core transistors which are arranged in parallel along the rows of word lines. A first pass transistor and a plurality of Y-pass transistors are coupled between the sense amplifier and the memory array. Second and third pass transistors are coupled between the dummy sense amplifier and the dummy bit-line. A plurality of N-channel MOS transistors are used to clamp all of the bit-lines in the array and dummy bit-line to a ground potential. The operational sense amplifier is responsive to the sense amplifier, dummy sense amplifier and the clamping transis­ tors for generating an output signal which has a fast response time when making a low-to-high transition (that is when selecting an unprogrammed memory cell).

Description: Current sense amplifiers for CMOS memory devices are generally well known in the prior art and are typically used in electronic circuits to read data out of semiconductor memories such as EPROMs and EEPROMs. However, these prior art sense amplifiers suffer from the shortcomings of having to trade a high-to-low transition with a low-to-high transition. Because of commercial demands for significant increases in access time for addressable memories fabricated on integrated circuit semiconductor chips, it has become very important to memory designers to have available a current sense amplifier circuit which operates at high speeds so as to improve efficiency of the overall memory circuit operation. It would therefore be desirable to provide a CMOS memory device having a current sense amplifier circuit whose design does not depend upon a trade-off between the high-to-low and low-to-high transitions. IEEE International Solid State Circuits Conference 27(1985) San Francisco, Febr. 22-24, Conf. 31, New York, vol. 27, no. 31, pages 136-137,327 describes bit-line clamping which is used to eliminate pattern sensitive access variation. Bit-lines not selected are actively held at ground by clamping transistors. EP 0136170 also describes bit-line clamping, which is designed to provide a shorter chip enable access time. Outputs of the bias circuits are clamped to a reference voltage. The present invention is set out in the appended claims According to one aspect of the present invention the low-to-high transition is made extremely fast by precharging all bit-lines to ground with the aid of clamping transistors, and a first capacitor is coupled to a first output of a sense amplifier, the capacitor being charged up while the bit-lines are clamped to ground potential and then discharged when a selected bit-line is released from its clamped condition. This ensures that all of the bit-lines charge up from ground and allows the circuit designer to optimise the sense amplifier for the high-to-low transition without deteriorating the low-to-high transition. A sense amplifier circuit embodying the invention may therefore have a low-to-high transition time of approximately 6 nanoseconds at a temperature of +30°C. In a preferred embodiment of the invention there is provided a CMOS memory device which includes a sense amplifier for providing a first output. A memory array is formed of core transistors which are arranged in n rows along word lines and m columns along bit-lines. A first pass transistor and a plurality of Y-pass transistors are coupled between the sense amplifier and the memory array. A dummy sense amplifier is used to provide a second output. A dummy bit-line has core transistors connected thereto which are arranged along the dummy bit-line. Second and third pass transistors are coupled between the dummy sense amplifier and the dummy bit-line. Clamping devices are coupled to the memory array and dummy bit-line for maintaining each column of bit-lines at a ground potential. The first pass transistor, second pass transistor and clamping devices are responsive to false and true clamping pull signals for permitting the quick discharge of first output of the sense amplifier. A differential sense amplifier is responsive to the first and second outputs for generating a third output at an output terminal which has a fast response time when making a low-to-high transition. In the accompanying drawings, by way of example only:- Figure 1 is a schematic circuit diagram of a CMOS memory device embodying the present invention; and Figures 2(a) through 2(e) are a set of waveform diagrams useful in understanding of the operation of the sense amplifier circuit in Figure 1. In Figure 1 a high speed CMOS memory device comprising a high speed CMOS current sense amplifier circuit 10 has a very fast response time to a logic "1" state (unprogrammed EPROM cell). As a result, the sense amplifier circuit 10 makes a low-to-high (zero to one) transition almost instantaneously. The sense amplifier circuit includes a sense amplifier arrangement 12, a dummy sense amplifier arrangement 14 and an operational amplifier arrangement 16. The sense amplifier arrangement 12 includes a pair of P-channel MOS transistors 18 and 20 having their gate electrodes connected together and to a ground potential during the read operation. The source of the transistor 18 is connected to a supply voltage or potential V CC . The drain of the transistor 18 is connected to the source of the transistor 20. The sense amplifier arrangement 12 further includes a pair of N-channel MOS transistors 22 and 24. The transistor 22 has its drain connected to the drain of the transistor 20 and to the gate of the transistor 24. The transistor 22 has its source connected to the ground potential and its gate electrode connected to the source of the transistor 24. The drain of the transistor 24 is connected to the supply potential V CC .The junction of the drain of the transistor 18 and the source of the transistor 20 is tied to the gate of an output transistor 26 formed of a N-channel MOS transistor. The source of the transistor 26 is connected to the source of the transistor 24. The drain of the transistor 26 is joined to the gate and drain electrodes of a load transistor 28 formed of a P-channel MOS transistor. The source of the transistor 28 is also connected to the supply potential V CC . The junction of the common drains of the transistors 26 and 28 serves as the output (node A) of the sense amplifier arrangement 12. The dummy sense amplifier arrangement 14 includes a pair of P-channel MOS transistors 30 and 32 having their gates connected together and to the ground potential during the read operation. The source of the transistor 30 is connected to the supply potential V CC . The drain of the transistor 30 is connected to the source of the transistor 32. The dummy sense amplifier arrangement further includes a pair of N-channel MOS transistors 34 and 36. The transistor 34 has its drain connected to the drain of the transistor 32. The transistor 34 has its source connected to the ground potential and its gate electrode connected to the source of the transistor 36. The drain of the transistor 36 is connected to the supply potential V CC .The junction of the drain of the transistor 30 and the source of the transistor 32 is tied to the gate of an output transistor 38 formed of a N-channel MOS transistor. The source of the transistor 38 is connected to the source of the transistor 36. The drain of the transistor 38 is joined to the gate and drain electrodes of a load transistor 40 formed of a P-channel MOS transistor. The source of the transistor 40 is also connected to the supply potential V CC . The junction of the common drains of the transistors 38 and 40 serves as the output (node B) of the dummy sense amplifier arrangement 14. The operational sense amplifier arrangement 16 includes current mirror N-channel MOS transistors 42 and 44 having their gate and source electrodes connected together. The common gates of the transistors 42 and 44 are connected to the drain of the transistor 42. The common sources of the transistors 42 and 44 are connected to the drain of a current source N-channel MOS transistor 46. The gate of the transistor 46 is connected to a reference voltage V REF . The source of the transistor 46 is tied to the ground potential. A load transistor 48 formed of a P-channel MOS transistor has its source connected to the supply potential V CC ,its gate connected to the output (node A) of the sense amplifier arrangement 12, and its drain connected to the drain of the transistor 42. A load transistor 50 formed of a P-channel MOS transistor has its source also connected to the supply potential V CC ,its gate connected to the output (node B) of the dummy sense amplifier arrangement 14, and its drain connected to the drain of the transistor 44. When the operational sense amplifier arrangement 16 is considered as functioning as a differential amplifier, the junction of the drain of the transistor 50 and the drain of the transistor 44 serves as the output (node C) of the differential amplifier. The operational sense amplifier arrangement 16 further includes an output buffer stage consisting of transistor 52 formed of a N-channel MOS transistor and a transistor 54 formed of a N-channel MOS transistor. The output (node C) of the differential amplifier is connected to the gate of the transistor 52. The transistor 52 has its drain connected to the supply potential V CC and its source connected to the drain of the transistor 54 and to the output V OUT (node D) of the sense amplifier circuit 10. The gate of the transistor 54 is connected to the reference voltage V REF , and the source of the transistor 54 is tied to the ground potential. The memory device further includes a memory array 56 formed of a plurality of core transistors which are arranged in n rows of word lines WL ₁, WL ₂ ... through WL n by m columns of bit-lines BL ₁ , BL ₂ ... through BL m . As can be seen, each of the core transistors Q ₁₁ ... Q 1m in the first row (n = 1) have their gate electrodes connected together and to an input data signal V in which is applied to the word line WL 1 . All of the sources of the transistors Q ₁₁ ... Q 1m are connected to the ground potential. The respective drains of the core transistors Q ₁₁ ... Q 1m are connected to corresponding bit-lines BL ₁ ... through BL m . Similarly, each of the core transistors Q ₂₁ ... Q 2m to Q n1 ... through Q nm in all of the remaining rows are likewise connected. The memory device also includes a dummy bit-line 58 formed of a plurality of core transistors. Specifically, the core transistors are arranged in parallel along the n rows of word lines WL₁ , WL₂ ... through WL n . In particular, each of the transistors Q₁₁'... through Q n1 'has its respective gate connected to the input data signal V in which is applied to the respective word lines WL₁, WL₂ ... through WL n . All of the sources of the core transistors Q₁₁'... through Q n1 'are connected to the ground potential. The respective drains of the transistors Q₁₁'... through Q n1 'are connected to bit-line BL1'. A first pass transistor 60 has its drain connected to the source of the transistor 26 of the sense amplifier arrangement 12, its source connected to all of the drains of Y-pass transistors Q D1 ... throughQ Dm , and its gate connected to a control terminal 62. A false clamp signal CP is applied to the control terminal 62 from a clamp pulse generator (not shown). A second pass transistor 64 has its drain connected to the source of the transistor 38 of the dummy sense amplifier arrangement 14, its source connected to drain of a third pass transistor 65 and its gate electrode connected to a control terminal 66. The false clamp pulse signal CP is also applied to the control terminal 66. A plurality of bit-line clamp transistors B/L are provided so that all of their sources are connected to the ground potential. All of the gates of the clamp transistors B/L are connected together and to a control terminal 68. A true clamp pulse signal CP is applied to the terminal 68 from the clamp pulse generator. Each of the drains of the transistors B/L are connected to respective bit-lines BL₁ ... through BL m and BL₁'of the respective memory array 56 and dummy bit-line 58. As previously stated, all of the Y-pass transistors Q D1 ...through Q Dm have their drains connected together and to the source of the first pass transistor 60. The respective sources of the transistors Q D1 ... through Q Dm are connected to corresponding bit-lines BL₁... through BL m . Each of the gates of the transistor Q D1 ... through Q Dm are connected to respective Y-decode signals Y D1 ... through Y Dm from a Y-decoder (not shown). The Y-decode signals are used to select an unprogrammed memory cell.The third pass transistor 65 has its drain connected to the source of the second pass transistor 64, its source connected the dummy bit-line BL₁', and its gate connected to the supply potential V CC . Parasitic capacitance represented by a capacitor C S has one of its ends connected to the output (node A) of the sense amplifier arrangement 12 and its other end connected to the ground potential. Parasitic capacitance represented by a capacitor C BL1 has one of its ends connected to the common drains of the core transistors which are connected to the bit-line BL₁. The other end of the capacitor C BL1 is connected to the ground potential. The capacitor C BL1 is used to facilitate the fast response of the sense amplifier circuit by clamping all of the bit-lines to the ground potential. Thus, this ensures that all of the bit-lines are required to be charged up from the ground potential. Also, this facilitates the quick discharge of the capacitance C S at the output of the sense amplifier arrangement during a low-to-high transition of the sense amplifier circuit 10. Reference is now made to the waveform diagrams shown in Figure 2(a) through 2(e) which are useful in understanding of the operation of the sense amplifier circuit 10 of Figure 1. For ease of explanation, it will be assumed that it is desired to select the bit-line BL₁ wherein the gate of the Y-pass transistor Q D1 receives the decode signal Y D1 being in the logic "1" state. As a result, the Y-pass transistor Q D1 will be turned on. Further, since the gate of the third transistor 65 is tied to the supply potential V CC ,it will always be turned on. Initially, at time t = 0, all of the bit-line clamp transistors B/L in the array 56 and dummy bit-line 58 have a true bit-line clamp pulse signal CP at a logic "1" level (Fig. 2b) applied to their gates via the control terminal 68 so as to cause them to be turned on. This results in the clamping of all of the bit-lines BL₁ ... through BL m and BL₁' to the ground potential. In other words, the capacitor C BL1 connected to the bit-line BL₁ will be completely discharged and will be required to charge up from the ground potential. Simultaneously, a false bit-line clamp pulse signal CP at a logic "0" level (Fig. 2c) will be applied to the gates of the pass transistors 60 and 64 via respective control terminals 62 and 66 so as to cause them to be turned off. Thus, the capacitor C S connected to the output of the sense amplifier arrangement 12 will be fully charged up. The outputs of the sense amplifier arrangement 12 at node A and the dummy sense amplifier arrangement 14 at node B are shown in curves 70 and 72 in Figure 2(c) respectively. The outputs of the differential amplifier at node C and the sense amplifier circuit 10 at node D are illustrated in curves 74 and 76 in Figure 2(e) respectively. Assuming that at time t = 1 the input data signal V in makes a transition from low-to-high (zero to one), the true clamp pulse signal CP will make a high-to-low transition, and the false clamp signal CP will make a low-to-high transition at a time t = 2. As a result, the bit-line transistor B/L connected to the first bit-line BL₁ will be turned off causing the capacitor C BL1 to start charging from the ground potential. Further, the pass transistors 60 and 64 will be turned on, permitting the outputs of the sense amplifier arrangement and dummy sense amplifier sense arrangement to discharge quickly. As a consequence, the output at node D of the sense amplifier circuit 10 responds substantially instantaneously causing the very fast low-to-high transition to appear as indicated by edge 78 in Figure 2(e). The sense amplifier circuit 10 has the following advantages over the conventional prior art sense amplifiers: (a) it uses a plurality of bit-line clamp transistors for grounding all of the bit-lines; (b) it provides a very fast response time when making a low-to-high transition; (c) it has a fast response due to the quick discharging of the capacitance C S connected to the output of a sense amplifier arrangement; and (d) the fast response during the low-to-high transition does not adversely affect the high-to-low transition. From the foregoing detailed description, it can thus be seen that the circuit 10 provides an improved current sense amplifier circuit for a CMOS memory device which has a fast response time when making a low-to-high transition. The current sense amplifier utilizes a plurality of bit-lines clamp transistors for grounding all of the bit-lines so as to require the bit-lines to charge up from the ground level. While there has been illustrated and described what is at present to be a preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention. Therefore, it is intended that this invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims: A CMOS memory device, comprising current sense amplifier means (12) for providing a first output; a memory array (56) formed of a plurality of core transistors (Q₁₁...Q nm ) which are arranged in n rows along word lines (WL) and m columns along bit-lines (BL); a first pass transistor (60) and a plurality of Y-pass transistors (QD) coupled between the first output of the sense amplifier means (12) and the memory array (56) each of said Y-pass transistors (QD) being coupled between a corresponding bit-line (BL) and said first pass transistor (60); dummy current sense amplifier means (14) for providing a second output; a dummy bit-line (58) having a plurality of core transistors (Q'₁₁, Q' n1 ) connected thereto and which are arranged along said dummy bit line (58); second and third pass transistors (64, 65) coupled between the second output of the dummy current sense amplifier means (14) and the dummy bit-line (58), clamping means (B/L) coupled to the memory array bit lines (BL) and to the dummy bit-line (58) for clamping the bit-lines (BL) at a ground potential; and operational sense amplifier means (16) responsive to the said first and second outputs for generating a third output at an output terminal (D), characterised in that the first pass transistor (60), second pass transistor (64) and clamping means (B/L) are responsive to false and true clamp pulse signals ( CP , CP), and a first capacitor (C S ) is coupled to the first output (node A) of the current sense amplifier means (12), the capacitor being charged while the bit-lines are clamped to ground potential prior to the selection of a word line (WL) and then discharged when a selected bit-line (BL) is released from its clamped condition. A CMOS memory device as claimed in claim 1, characterised in that the clamping means further comprises a second capacitor (C BL1...BLM ) coupled between each bit-line and ground potential, the respective second capacitor being charged from ground potential when the selected bit-line is released from its clamped condition. A CMOS memory device as claimed in claim 1 or claim 2, wherein the sense amplifier means (12) comprises a pair of P-channel MOS transistors (18, 20), a pair of N-channel MOS transistors (22, 24), a first output transistor (26) and a load transistor (28). A CMOS memory device as claimed in claim 3, wherein the dummy sense amplifier means (14) comprises a pair of P-channel MOS transistors (30, 32), a pair of N-channel MOS transistors (34, 36), a second output transistor (38), and a load transistor (40). A CMOS memory device as claimed in claim 4, wherein the first pass transistor (60) has its drain connected to the first output transistor (26), its source connected to the drains of the Y-pass transistors (QD), and its gate connected to a control terminal (62) for receiving the false clamp pulse signal ( CP ). A CMOS memory device as claimed in claim 4 or claim 5, wherein the second pass transistor (64) has its drain connected to the second output transistor (38), its source connected to the drain of the third pass transistor (65), and its gate connected to a control terminal (66) for receiving the false clamp pulse signal ( CP ). A CMOS memory device as claimed in any one of the preceding claims, wherein the clamping means comprises a plurality of N-channel MOS transistors (B/L), each transistor having its drain connected to a separate bit-line (BL) of the m columns in the memory array (56) and to the dummy bit-line (58), its source connected to the ground potential and its gate connected to a control terminal (68) for receiving the true clamp pulse signal (CP). A CMOS memory device according to any one of the preceding claims wherein the operational sense amplifier means (16) comprises a pair of current mirror transistors (42, 44) and a pair of load transistors (48, 50), a current source (46), and an output buffer stage (52, 54). A CMOS memory device according to any one of the preceding claims wherein the Y-pass transistors (QD) have each of their drains connected together and to the source of the first pass transistor (60), the sources of the Y-pass transistors being connected to a corresponding one of the bit-lines, respectively. A CMOS memory device according to claim 9 wherein the third pass transistor (65) has its drain connected to the source of the second pass transistor (64), its gate connected to a supply potential and its source connected to the dummy bit-line (58).

Inventors: VENKATESH, BHIMACHAR
Applicants: ADVANCED MICRO DEVICES, INC.
Application year: 1986
Main classification: G11C7
IPC codes: G11C7, G11C16, G11C17, G11C11