DOUBLE STAGE SENSE AMPLIFIER FOR RANDOM ACCESS MEMORIES

Abstract: In combination with an electronic memory of the type having a plurality of memory cells (CA, ... CN) connected between two bit lines (BLT, BLC) having inherent bit line capacitances (C1, C2), there is disclosed an improved sense amplifier (15) comprised of two stages. A first stage (16) includes a first clocked latch (5) having an enable device (T5), gated by a first control signal (SSA) and bit switches (T6, T7) connected between the common nodes (6, 7) of said first clocked latch and said bit lines, and gated by a bit switch control signal (BS) to provide an output signal on first data lines (DLT, DLC). A second stage (17) includes a second clocked latch (20) having an enable device (T24) gated by a second control signal (SL) and data switches (T28, T29) connected between second data lines (DT, DC) at the same potential as data output nodes (21, 22) of said second clocked latch and said first data lines (DLT, DLC). Said data switches (T28, T29) are gated by a data switch control signal (DS) which is derived from the bit switch control signal (BS), so that said first and second stages (16, 17) operate sequentially to amplifly the data continuously along the sensing chain of the data path during a READ operation to provide a data output signal on said data output nodes. The data is then available for furhter processing at the output terminal (24) of the output driver (23).

Description: The present invention relates to Random Access Memories (RAM's) and more particularly to a double stage sense amplifier for reading data from the cells of such memories. In the design of a Random Access Memory, it is common practice to array a large number of memory cells in a matrix of rows and columns. Data is typically transferred to and from the memory cells of the same column by means of a pair of electrical conductors, often referred to as the column busses or bit lines. A specific cell among all of those connected between said pair of bit lines, is selected by a signal applied to a row address or word line which enables all of the memory cells in a row. The memory cells may be either of the static type (SRAM′s) or of the dynamic type (DRAM′s). In systems using SRAM′s, data are usually stored in cross coupled transistor latches where one or more paths to ground can be selectively switched on or off. In DRAM's, data are stored in capacitors by the operation of one or more transistors. In accessing or reading the static type cells, it has heretofore been necessary to permit one of the bit lines to be discharged substantially to ground by currents passing through the memory cells in order to detect a logic "1" (or "0") stored in the cell. Because of the high capacitance of the bit line, this has resulted in a relatively long time period before the bit line is discharged sufficiently for valid data to be read from the cell. In systems using DRAM's, where cells require destructive read cycles, data is read from the memory cell by detecting a voltage pulse on the column bus as the capacitor of the memory cell is either charged or discharged when addressed. Because of the relatively small capacitance of the cells compared to the capacitance of the bit line, the voltage swing is usually small. Detecting the small voltage swing represents a serious problem in designing a sense amplifier circuit which functions reliably. Generally speaking, as far as Random Access Memories are concerned, whichever its type, the output or sensing of information on the bit lines is difficult, because of the lack of a full swing logic level. On the other hand, short access times to stored data are particularly difficult to achieve in large scale MOSFET integrated circuits because of the capacitance of various nodes of the memory cells resulting from the manufacturing process. There have been numerous patents which employ different techniques for providing a full logic output from the data received from the bit lines. For example, see U.S. Patent No. 3 600 609 issued on August 17, 1971 to Christensen wherein a pair of cross coupled IGFET devices are connected in a race mode and combined with IGFET inverters to convert the differential double rail output of an IGFET memory into a full logic level double rail data output. However in the Christensen's reference, the "read" amplifier still tends to load the bit lines and does not isolate the bit lines when performing an output function. Additionally, it requires an additional stage of amplification to obtain a full logic output with appropriate levels. According to US Patent 3 879 621 to J.R. Cavaliere and assigned to the assignee of the present invention, there is described a single stage FET sense amplifier for converting a double rail differential memory output signal into a full logic output signal. The amplifier comprises first and second pairs of FETs coupled together to a pair of common nodes, to form a standard latch. A fith FET of a second type of conductivity connected to one of the pairs of FETs, is used as an enable device, controlled by a control clock signal. In a preferred embodiment, first and second FETs of the same conductivity type are connected to respective ones of the nodes to act as bit switches to provide good isolation between the bit lines and the latch. The first, second and third FETs are interconnected so that when the first and second transistors conduct, the third transistor is cut off, and vice versa. So implemented, the disclosed sense amplifier is therefore capable of providing a full logic level output while isolating the bit lines of the memory during sensing. It is a good example of a modern high performance clocked sense amplifier well adapted to CMOS technology. Fig. 1 of the present application describes the organization of a portion of a CMOS Static Random Access Memory circuit bearing reference 1, which utilizes such a single stage clocked sense amplifier provided with isolation means, to illustrate a typical example of the prior art known to the applicant. The clocked sense amplifier 2 comprises first and second pairs 3 and 4 of cross coupled transistors. The first pair 3 includes two PFET's T1 and T2, and the second pair 4 includes two NFET's T3 and T4. The source regions of transistors T1 and T2 are connected to the high voltage of a first power supply (VH) and source regions of transistors T3 and T4 are connected to the low voltage of a second power supply (GND), through an enabling device consisting of a NFET referenced T5, on the gate of which is applied a control clock signal SSA. The five devices T1 to T5 form a clocked latch bearing numeral 5. Negligible impedance means interconnect the pairs of FETs. In the illustrated instance, the drains of T1 and T3, and the drains of T2 and T4 form nodes 6 and 7 respectively, where the output signal from a selected one of the nodes is available for amplification by the sense amplifier. These nodes 6 and 7 are connected to the inputs of an output latch or a buffer 8, the output of which is applied to an output driver 9. Because nodes 6 and 7 are at the same potential as the data lines referenced DLT (Data Line True) and DLC (Data Line Complement), they will be often referred to as data out nodes, because the data is available there. Wires connecting said nodes to the output latch 8 are very short indeed. The data out signal or data DO to be subsequently used for further processing is available on data out terminal 10. Sense amplifier 2 also comprises means to isolate the clocked latch 5 and its related nodes 6 and 7, from the left or true bit line BLT and the right or complement bit line BLC, to permit said nodes to rise to full signal levels (full logic outputs), while not being loaded by bit lines. Said means essentially consists of switches so called bit switches. NFET's T6 and T7 are used to that end, to isolate nodes 6 and 7 from the bit lines BLT and BLC respectively, where desired. A control signal BS is applied on the gate electrodes of T6 and T7. A plurality of four transistor memory cells referenced CA to CN are connected to the bit lines BLT and BLC through two NFET's mounted in a transmission or transfer gate configuration and respectively referenced T8A to T8N and T9A to T9N to form an array 11. The gate electrodes of these FET's, e.g T8A and T9A for cell CA, are connected to the corresponding word line, e.g WLA, for READ and WRITE operations. Bit lines BLT and BLC are used as the input (WRITE) or output (READ) path for data movement in and out of the array portion. Those bit lines are very capacitive due to the summation of all diffusions of transfer devices (T8A to T8N and T9A to T9N) and because long metal wires physically form these bit lines. Resulting capacitances of a column are illustrated in Fig. 1 by capacitors C1 and C2 respectively for BLT and BLC. The stray capacitances of nodes 6 and 7 are represented by capacitors C3 and C4 and are kept as small as practical. Capacitances of the bit lines BLT and BLC, represented by capacitors C1 and C2 respectively, are inherently large when compared to capacitors C3 and C4. Optionally, it should be recognized that additional memory cells, which are connected to other pairs of bit lines, may be also coupled to nodes 6 and 7 of the sense amplifier 2 through bus 13, so that one sense amplifier services more than one cell column. The amount F of additional columns may vary from 0 to a number P which depends on the technology used, the memory size and its organization. To date, up to P = 15 bit line pairs, may be connected to the represented bit line pair consisting of BLT and BLC. In others words a single sense amplifier such as represented in Fig. 1 may service up to 16 memory cell columns. This number P which represents the dotting capabilities of the circuit is limited by the speed of the sense amplifier, which is necessarily a low gain amplifier in the implementation of Fig. 1. Of course, a separate clock control signal must be employed to permit appropriate gating into the nodes. Bit switches T6 and T7 are also used in cooperating to that purpose. Finally, the memory circuit 1 also includes a restore circuit 12 for pulling up the potential of bit lines BLT and BLC to the reference voltage VREF = VH - VT (VT being the threshold voltage of an NFET in a reference voltage generator not shown in Fig. 1). A suitable reference voltage generator is detailed in copending patent application N° 87480024.6 filed on 15 December 1987 and assigned to the same assignee as of the present invention. Circuit 12 comprises three PFETs, T10, T11 and T12, gated by the bit line restore signal BLR. T10 and T11 are coupling transistors with the voltage reference generator. T12 equalizes the charge of capacitors C1 and C2 and therefore the potential of the bit lines. However, it is important to note that nodes 6 and 7 of latch 5 are not equalized. As a result, potentials of said nodes may have opposite binary values compared to the cell content to be accessed when a READ operation is initiated, therefore slowing it down. The READ operation is as follows. Both bit lines BLT and BLC are charged to VREF. The particular cell is selected by raising the desired word line control signal WL to VH. The selected word line is maintained at this potential a sufficient time to discharge one of the bit lines by a predetermined amount. The clocked latch 5 is set, by bringing up the control clock signal SSA to VH, permitting a full swing logic output to be transmitted to the output latch 8. Clocked latch 5 has to be of the high gain type to ensure proper setting. During such a READ operation, the sense amplifier 2 amplifies the differential voltage developed first between the bit lines BLT and BLC, and then between the data lines DLT and DLC. This memory organization is limited in speed as it requires the development of a large differential voltage between the sense amplifier internal nodes 6 and 7, i.e between the two data lines. Moreover, because a full swing operation is needed in the sense amplifier 2, in order to supply the read data into the output latch 8, the access time is increased. In addition, the potentials of nodes 6 and 7 are not equalized, and may correspond to binary values opposite to the data being sensed on the bit lines when a memory cell is accessed. As a result, the setting of the clocked latch 5 has to be done later in order to be sure that the data has been correctly settled on the data lines. Finally, this READ operation is very power consuming when those nodes become highly capacitive with the growth of the memory size. In EP-A-0170285, there is described a multi-stage sense amplifier for a 6D static SRAM cell. According to that reference, the memory cells (M11, M21,...) are connected to a pair of bit lines (BA11, BB11) that are connected to a first amplifier (AX11) without the use of bit switches. The amplified signal is stored in a latch (L11) through switches (S). Those switches (S) controlled by signal (Φ1) do not operate either as bit switches or data switches but as multiplex switches, because the latch can be loaded with the signal appearing either on one of two sets of bit lines (BA11 or BB11; BA21, BB21). Once latched, in this first stage (L11) the data may wait as long as desired. Once the data has to be loaded in the output buffer (36), the content of the latch appears on first data lines (D1, D2) then on second data lines (BX1, BY1) through other switches (SC11, SD11) controlled by a signal (Φ2) that also operate as multiplex switches because data lines BX1, BY1 are common to different latches (L11, L12). Finally, after amplification in another sense amplifier (AY1) the data is sent to the output buffer (36) through still another set of switches (SF1, SG1) still of the multiplex type, because the output buffer (36) is also connected to the input buffer (34). In other words, data on the first data lines is transferred to the second data lines in response to the signal Φ2 only after the data on the first data lines have been fully latched, i.e; when amplification has ceased with respect to the first data lines. The amplifying operations of the first (L11) and second (AY1) stages do not overlap. Accordingly, it is a primary object of this invention to provide an improved sense amplifier to be used in electronic memory circuits which overcomes the shortcomings encountered with the single stage sense amplifier and associated circuitry of the prior art. It is a another object of this invention to provide an improved sense amplifier circuit which can provide fast access time to the memory cell. It is another object of this invention to provide an improved sense amplifier circuit which drastically reduces the consumed power. It is another object of this invention to provide an improved sense amplifier circuit having a short cycle time. It is still further object of this invention to provide an improved sense amplifier circuit with increased dotting capabilities. The present invention concerns a new high speed sensing scheme in electronic memories with superior performance. According to a primary aspect of the invention, there is provided an improved sense amplifier for a memory circuit of the type including a plurality of memory cells connected between two bit lines. This improved sense amplifier is basically constructed to have a double stage structure. The first stage is a low gain amplifier. It includes a first clocked latch having an enable device gated by a first control signal and bit switches connected between the nodes of said first clocked latch and said bit lines; said bit switches are gated by a bit switch control signal to provide an output signal on first data lines. The second stage is a high gain amplifier. It includes a second clocked latch having an enable device gated by a second control signal and data switches connected between the nodes of said second clocked latch and said first data lines; said data switches are gated by a data switch control signal to provide an output signal on second data lines at data output nodes. Said data switch control signal is derived from the bit switch control signal so that first and second stages operate sequentially, to get the differential voltage developed between the bit lines, continuously amplified along the sensing chain. The second stage operates not only as a high gain amplifier but also as a buffer. The data output nodes are connected to an output driver, at the output of which, the data out signal or data DO is available. According to another important aspect of the invention, restore and equalizing means are provided to said first and second stages to bring up the potentials of said first and second data lines and data output nodes at the high voltage VH during restoration. Still according to another important aspect of the invention, writing means are provided to the first stage of the improved sense amplifier so that a WRITE operation in a selected cell is allowed through the first stage while a READ operation is finishing in the second stage which is isolated therefrom. Briefly summarized, the invention may be described as the implementation of at least two successive toboggan-like stages in the sensing chain to decrease the amplification time. The use of such a double stage structure results in speeding up the data access, while full latching operation is performed at output level in the second stage. These at least two stages, are accessed sequentially in fast mode in order to get the differential voltage developed between the bit lines, continuously amplified and accelerated along the data lines of the sensing chain up to the output driver. The features of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may best be understood by reference to the following detailed description, read in conjunction with the accompanying drawings in which: Fig. 2 is a schematic diagram of a portion of a static random access memory circuit utilizing a double stage sense amplifier in accordance with the present invention. Fig. 3 is a timing chart used to explain the operation of the circuit shown in Fig. 2 during READ and WRITE operations. Fig. 4 shows curves representing variations of the potentials on the bit lines, first data lines and second data lines (data out nodes) during a READ operation. Circuit description We refer now to Fig. 2, which shows a typical memory organization referenced 14 utilizing the double stage sense amplifier circuit of the present invention to illustrate a preferred embodiment. Elements identical in Figs. 1 and 2 bear the same references. The sense amplifier of the present invention is referenced 15. It is comprised of two stages 16 and 17. First stage 16 is structurally quite similar to the sense amplifier 2 of Fig. 1, except in that a few functionally new devices are added. First, two PFET's T13 and T14 gated by control signal DLR are respectively connected to nodes 6 and 7, so that when they conduct, the potential of the first data lines DLT and DLC is raised to VH, this allows a better restoration of said first data lines. In addition, to each of the nodes 6 and 7, is connected a pair of NFET's T15, T16 and T17, T18 respectively. T15 and T17 are gated by the WRITE control signal WR, while T16 and T18 are gated by the data input signal DI and DI respectively. These transistors will be used during a WRITE operation. It is to be noted that first stage 16 operates as a low gain amplifier connected at nodes 6 and 7 which are loaded by relatively small capacitors C3 and C4, and amplifies the small differential voltage developed on said capacitances. Second stage 17 which is of a similar construction, consists of two pairs of FETs 18 and 19, properly cross coupled to implement the latching function. The first pair 18 includes two PFEt's T20 and T21, and the second pair 19 include two NEET's T22 and T23. There is also provided an enabling device: a NFET T24, the gate electrode of which receives the set latch control signal SL. FET's T20 to T24 form a clocked latch referenced 20. Nodes 21 and 22 of the clocked latch 20 are also the data output nodes of the second stage 17 and therefore also those of the improved sense amplifier 15. These nodes are loaded by PFET's 25 and T26 gated by the latch restore control signal LR to restore second data lines referenced DT and DC to VH when desired. An additional PFET T27 equalizes the charge of capacitors C5 and C6. Note that, a similar device could also be connected between nodes 6 and 7 of clocked latch 5. Transistors T13, T14 and T25, T26, T27 operate as restore and equalizing means to raise first and second pairs of data lines (and thus data output nodes) to VH at RESTORE. As a result, data lines are equalized and left floating until a READ/WRITE operation is initiated. According to an important aspect of the invention, the array 11 and the first stage 16 (and therefore the capacitors C3 and C4 associated thereto at nodes 6 and 7), are all isolated from the second stage 17 by two switches, called data lines switches. These switches are implemented with two transfer NFET's T28 and T29, the gate electrodes of which receive the data switch control signal DS. This allows to set the first stage with a small differential voltage developed on small capacitors C3 and C4, and next to get a large differential voltage on the small capacitors C5 and C6 and finally to set said second stage operating as a high gain differential amplifier. First and second stages amplify the differential signal developed first across the bit line pair and then across the data line pairs, in other words all along the sensing chain which also may be considered as the data path of the memory circuit. It is an additional feature of the present invention to have the second stage operating not only as an amplifier but also as the output latch 8 of Fig. 1. The data is available on the data output nodes 21 and 22 which are at the same potential as the second pair of data lines DT and DC. The latter are connected to an output driver 23 and data out signal or data DO, to be used for subsequent processing, is available at data out terminal 24. Said output driver 23 may simply be constituted by two separate inverters, each connected to a data output node, so that, DO and DO are individually available, if a symmetrical output is required. As explained above, up to P additional bit line pairs can be dotted on the bit line pair after the bit switches through bus 13. With the implementation shown in Fig. 2, number P can be higher when compared with implementation of Fig. 1. As an example, up to P = 31 bit line pairs can be added, so that. 32 cell columns may be serviced by first stage 16. However, it is an important feature of the present invention to add further dotting capabilities. Through bus 25, a plurality (F′ = O to M) of cell columns, each provided with their first stage (16′, 16″,...) can be dotted to the cell column depicted in Fig. 2. Practically, M may be as high as 3. Dotting is practically effected after the data switches as illustrated in Fig.2. All the dotted cell columns are then serviced by a single second stage 17, common to all. Circuit operation The operation of circuit of Fig. 2 will now be explained in conjunction with the timing chart of Fig. 3. During the RESTORE cycle, the BLR signal holds the bit lines at VREF = VH - VT, the DLR signal holds the first datalines high (VH) and the LR signal holds the second data lines high and equalized. READ cycle: During a READ cycle, one word line, e.g. WLA, is driven high by raising the word line control signal WL to VH, to enable all the memory cells of a row. A differential voltage appears between the bit lines. Bit line switch transistors T6 and T7 are opened (turned on) by bit switch control signal BS. Capacitances of bit lines BLT and BLC and first data lines DLT and DLC, start their discharge into the selected cell. A small potential difference (e.g 100 mv) appears between nodes 6 and 7 of the first stage 16 which is left floating. During this part of the discharge, data line switch transistors T28 and T29 remain off. Setting of the first stage of sense amplifier is done with control signal SSA, generated from a Dummy Word Line DWL rising, ensuring proper capture of expected differential voltage drop. The DWL signal acts as a worst case of any WL rising. As the SSA signal rises high, first stage 16 is enabled and the differential voltage existing between nodes 6 and 7 is amplified and stored in the clocked latch 5. Immediately after setting, data line switch transistors T28 and T29 are opened by the DS signal (derived from BS signal), to transfer data to latch 20 of the second stage. Remark that nodes 21 and 22 of latch 20 have been previously restored to VH and equalized through LR signal as explained above. In the very same manner as done with the first stage, latch 20 is floating because it is isolated from GND by T24 which is disabled by the SL signal. After RESTORE, those two nodes were left floating at VH level and are still floating at the time when data line switch transistors T28 and T29 are opened (turned on). At the opening of the data line switch transistors T28 and T29, a differential voltage appears promptly between nodes 21 and 22. At the appropriate time, latch 20 is set by SL signal (derived from SSA signal) which goes high, thus enabling latch 20 to store the data. Data line switch transistors T28 and T29 can then be turned off by pulling down the DS signal, thus isolating the second data lines from the first data lines. As a consequence, early restoration of the first stage is then allowed and may start at once. Nodes such as 6, 7 or 21, 22 have relatively small capacitances as compared to those of the bit lines. As a result, discharging relatively small capacitances of these nodes can be done at a much greater rate than the relatively large capacitances of the bit lines. Since the time required to discharge a capacitance with a given current flow, is directly related to the size of the capacitance, the small capacitances of these nodes permit valid data to appear on the data output nodes 21 and 22 very rapidly. As a result, sense amplifier 15 amplifies the differential signal developed across the sensing chain until the data is latched in the second stage. Cycle performances are greatly improved by use of those data line switch transistors T28 and T29. The specific operation of latch 20 allows output driving very early in the cycle, preventing from waiting for a full swing on nodes 21 and 22. WRITE cycle: During a WRITE cycle, the word line selection is the same as for a READ cycle. A WRITE signal WR enables writing of an input data "Data In" or DI and DI in the first stage when the word line is driven high through WL signal. Latch 5 is set by SSA signal as for a READ operation. Data line switch transistors T28 and T29 are kept off by maintaining the DS signal at a low level and thus no data is transferred to the output circuitry during a WRITE operation. When the BS signal goes high, transistors T6 and T7 are turned on and the data is written in the memory cell. This "half READ" operation improves the WRITE cycle and its sensitivity as the first stage amplifies and locks "Data In" while helping bit line discharge. General consideration As illustrated in Fig. 4 the inventive concept at the base of the present invention can be understood as the result of the action of two successive toboggans caused by the two stages of the disclosed improved sense amplifier depicted in Fig. 2. Figs. 4A, 4B and 4C respectively show potential variations on the bit lines, the first and second data lines (data output nodes) versus time. As it may be understood from Fig. 4A, which shows the potential variations on the bit lines, in the initial state, the word line signal WL is at a low level. Both bit lines are standing at the VREF potential. The bit switch control signal BS is high (see Fig. 3), transistors T6 and T7, are ready for conduction but still OFF (VGS = VT). A memory cell is selected at t0 by raising the corresponding word line signal WL to VH. The selected cell (e.g. CA), gently pulls down bit lines BLC and BLT in a differential manner and with a slope S1. Assuming that its binary content is such as transistor T9A is the conductive transistor, T9A pulls downwards potential of bit line BLC as represented in Fig. 4A. As soon as the differential voltage between BLT and BLC increases, bit switches are driven to conduction and a differential voltage develops between the first data lines DLT and DLC as clear from Fig. 4B. At time t1, the differential voltage between the first data lines has reached an amount considered to be sufficient to set the first stage by raising the SSA control signal to VH (time t1 also corresponds to the differential voltage ΔV1 between the bit lines). The differential voltage between the first data lines DLT and DLC significantly increases, and data line DLC presents a slope S2 more acute than S1. Shortly after time t1, the bit lines are restored using the DLR signal (falling edge). After time t1, data switches T28 and T29 are on the edge of conduction as explained above for T6 and T7, at time t2 they are driven to conduction, and a differential voltage is being developed between the second data lines DC and DT, as seen in Fig. 4C. When the differential voltage which is being developed between the second data lines DT and DC reaches a sufficient value, the second stage is set by raising control signal SL to VH. This is effected at time t3 which also corresponds to the differential voltage ΔV2 between the first data lines). A short time after time t3, data switches are turned off by pulling control signal DS to GND. At time t4, the first data lines are restored through signal DLR which goes down. As clear from Fig. 4C, the potential of the second data line DC develops very quickly towards the full swing with an increased slope S3. Therefore, as soon as the first stage has acquired speed amplification, the data is promptly transferred to the second stage. This second sensing device speeds up again the process and amplifies the slope of the falling data line as in a second toboggan. Toboggan action is drastically made clear from comparing the slopes S1, S2 and S3 of curves BLC, DLC and DC. Data out DO is then available shortly after t4 (Fig. 4C). Important rising or falling edges of different control clock signals with their corresponding timings have been reported in Fig. 3. This double effect toboggan makes feasible both fast access and short cycle time. The major advantages of this architecture are summarized below: a large improvement of the access time due to the small differential voltage necessary, for the READ operation; a reduction of consumed power (same reason as above); a cycletime improvement, as an operation which follows a READ operation (eg: RESTORE, WRITE...) can be started before the end of the READ operation. It also presents the following additional advantages: a WRITE operation of the DATA IN on the bit lines can be performed in a faster mode thanks to the first stage, as the WRITE operation is done with the help of the cross-coupled circuit discharging capacitances C1 and C2, C3 and C4; a PSEUDO-DUAL PORT operation can be performed thanks to the line overlap between 2 successive operations: the WRITE operation on the first stage overlapping with the READ operation being processed on the second stage. The apparent drawback of having a few additional clocks (SL, DS,...), which are necessary to properly operate the circuit of Fig. 2 when compared to the circuit of Fig. 1, is not very important, since these additional clocks are simple and are internally generated. Moreover, the addition of a certain number of devices necessary to implement the second stage is not really cumbersome when compared to the advantages obtained by the invention. As a whole, this "Double toboggan" architecture provides excellent performance to the design of SRAM's. Both in read and write cycles, so many improvements have been achieved that it can be worthwhile to implement the circuit of the present invention in a large variety of SRAM designs. However, the circuit of the present invention is of wide application and may find extensive use in other types of memories such as read only memories, in fact every time a high performance sense amplifier is required, the double stage sense amplifier of the present invention may be used with obvious benefits.

Claims: 1. An improved sense amplifier for an electronic memory circuit of the type including a plurality of memory cells (CA,... CN) connected between two bit lines (BLT, BLC) having inherent bit line capacitances; said improved sense amplifier including: a first stage (16) of a low gain type comprised of a first clocked latch (5) having a pair of nodes (6, 7), biased between a first power supply (VH) and a second power supply (GND) and having an enable device (T5) gated by a first control signal (SSA), and first switching means (T6, T7) connected between the nodes (6, 7) of said first clocked latch (5) and the bit lines (BLT, BLC) and gated by a bit switch control signal (BS); said first stage provides a differential output signal on first data lines (DLT, DLC) amplified with respect to the differential voltage produced on the bit lines when a cell has been selected; a second stage (17) of a high gain type including a second clocked latch (20) having a pair of nodes (21, 22), biased between a first power supply (VH) and a second power supply (GND) and having an enable device (T24) gated by a second control signal (SL) and second switching means (T28, T29) connected between the nodes (21, 22) of said second clocked latch (20) and said first data lines (DLT, DLC) and gated by a data switch control signal (DS); said second stage provides a data output signal on second data lines and associated output nodes (DT, DC); and first control means for generating the said first control signal (SSA) to activate the first stage; second control means for generating the said second control signal (SL) a predetermined period of time after the first control signal has been generated and while the first stage is still operating to amplify the data on the first data lines; and means for generating said bit switch and data switch control signals (BS, DS) whereby said first and second stages (16, 17) operate sequentially to amplify the data stored in the selected cell continuously along the sensing chain during a READ operation. 2. The improved sense amplifier of claim 1 wherein said first power supply supplies a high positive voltage VH and said second power supply supplies a less positive voltage GND. 3. The improved sense amplifier of claim 1 or 2 wherein each of said first and second switching means consists of two FET's mounted in a transfer configuration referred to as data switches. 4. The improved sense amplifier of claim 3 wherein said first stage (16) further includes first restore means (T13, T14) to raise the nodes of said first clocked latch (5) to said high voltage (VH) before accessing a memory cell. 5. The improved sense amplifier of claim 3 or 4 wherein said second stage (17) further includes second restore means (T25, T26, T27) to raise the nodes of said second clocked latch to said high voltage (VH) before accessing a memory cell. 6. The improved sense amplifier of claim 4 or 5 wherein said first restore means essentially consists of two FET's (T13, T14) each connected to a node (6, 7) of the first clocked latch (5) gated by a control clock signal (DLR); said second restore means essentially consists of two FET's (T25, T26) each connected to a node of the second clocked latch (20) and gated by a control clock signal (LR), both control clock signals (DLR, LR) being derived from the bit line restore signal (BLR). 7. The improved sense amplifier of claim 6 wherein said second stage (17) further includes equalization means consisting of a FET (T27) connected between the second clocked latch (20) nodes. 8. The improved sense amplifier of any above claim wherein said first stage (16) further includes writing means to force data in said first clocked latch (5). 9. The improved sense amplifier of claim 8 wherein said writing means are comprised of two pairs of FET's (T15, T16; T17, T18) respectively connected to each one of the nodes of said first clocked latch (5), each FET of a pair being respectively gated by the Data In and WRITE control clock signals. 10. The improved sense amplifier of any above claim wherein dotting means (25) are provided on the nodes of said second clocked latch (20) of the second stage (17), so that said latch may service a plurality of column cells, each provided with its own first stage (16′, 16″,...). 11. The improved sense amplifier of any above claim wherein said second clocked latch (20) is also used as a buffer to store the data to be outputted for further processing. 12. The improved sense amplifier of any above claim wherein an output driver (23) is connected to said output nodes. 13. A method for reading data from an electronic memory circuit (14) of the type having a plurality of memory cells (CA,... CN) and a double stage sense amplifier (15) connected between a pair of bit lines (BLT, BLC) forming the data path of said memory circuit comprising the steps of: addressing the selected memory cell by applying an appropriate potential to the selected word line (WLA) so that the selected cell (CA) develops a differential voltage between its bit lines (BLT, BLC); transferring said differential voltage to first data lines (DLT, DLC) of the first stage (16) of said sense amplifier which is of the low gain clocked latch type (5); setting said low gain clocked latch by a first control signal (SSA) when said differential voltage has reached a sufficient value so that the differential voltage is amplified and stored in said low gain clocked latch; transferring the amplified differential voltage to second data lines (DT, DC) of the second stage (17) of said sense amplifier, which is of the high gain clocked latch type (20); setting said high gain clocked latch by a second control signal (SL) a predetermined period of time after the first control signal has been generated and when said differential voltage on the second data lines has reached a sufficient value while the first stage is still operating to amplify the data on the first data lines so that the differential voltage is further amplified and stored therein; and reading the binary content of said high gain clocked latch available on the said second data lines. 14. The method of claim 13 wherein the steps of transferring said differential voltages is completed by turning on bit lines switch transistors and data line switch transistors located on the data path, respectively between the bit lines and the first stage and between first and second stages. 15. The method of claim 14 wherein the turning off of data line switch transistors substantially coincides with the bit line restoration.

Inventors: AKROUT, CHEKIB, COPPENS, PIERRE, DENIS, BERNARD, URENA, PIERRE-YVES
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1988
Main classification: G11C7
IPC codes: G11C7, G11C8, G11C11