Abstract:
A sense amplifier having an amplifier stage with decreased gain is described. The sense amplifier includes a first input/output (“I/O”) node and a second complementary I/O node. The sense amplifier includes two amplifier stages, each for amplifying a signal on one of the I/O nodes. The first amplifier stage, having a first conductivity-type, amplifies one of the I/O node towards a first voltage. The second amplifier stage, having a second conductivity-type, amplifies the other I/O node towards a second voltage. The sense amplifier also includes a resistance circuit coupled to the second amplifier stage to reduce the gain of the second amplifier stage thereby reducing the rate of amplification of the signal on the corresponding I/O node.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/646,735, filed Dec. 27, 2006. This application is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This invention, in various embodiments, relates generally to integrated circuits and, more specifically, to array sense amplifiers in memory devices. 
     BACKGROUND OF THE INVENTION 
     Memory devices, such as static random access memory (“SRAM”) devices and dynamic random access memory (“DRAM”) devices are in common use in a wide variety of electronic systems, such as personal computers. Memory devices include one or more arrays of memory cells, which in DRAM devices, are small capacitors that are arranged in rows and columns. Data is represented by the presence or absence of a charge on the capacitor in the memory cell. Data can be stored in the memory cells during a write operation or retrieved from the memory cells during a read operation. If the capacitor in the addressed or selected memory cell is charged, then the capacitor discharges onto a digit line associated with the memory cell, which causes a change in the voltage on the digit line. On the other hand, if the capacitor in the selected memory cell is not charged, then the voltage on the digit line associated with the memory cell remains constant. The change in voltage (or lack of change) on the digit line can be detected to determine the state of the capacitor in the selected memory cell, which indicates the value of the data bit stored in the memory cell. 
     Sense amplifiers are used to improve the accuracy of determining the state of the capacitor in selected memory cells. As known in the art, when the memory cell array is accessed, a row of memory cells are activated, and the sense amplifiers amplify data for the respective column of memory cells by coupling each of the digit lines of the selected column to voltage supplies such that the digit lines have complementary logic levels. A conventional sense amplifier  100  of a DRAM memory array is shown in  FIG. 1 . The sense amplifier  100  is coupled to a pair of complementary digit lines DIGIT and DIGIT_ to which a large number of memory cells (not shown) are connected. The sense amplifier  100  includes a pair of cross-coupled PMOS transistors  102 ,  104 . The sources of the PMOS transistors  102 ,  104  share a common node to which a PMOS activation signal ACT is coupled during operation. The ACT signal is typically provided by a power supply voltage (not shown) during operation. The sense amplifier  100  also includes a pair of cross-coupled NMOS transistors  112 ,  114 . The drains of the NMOS transistors  112 ,  114  also share a common node to which an NMOS activation signal RNL_ is coupled during operation. The RNL_ signal is typically provided by being connected to ground (not shown) during operation. The sense amplifier  100  is configured as a pair of cross-coupled inverters in which the ACT and RNL_ signals provide power and ground, respectively. The digit lines DIGIT and DIGIT_ are additionally coupled together by an equilibration transistor  110  having a gate coupled to receive a control signal EQ. 
     In operation, the sense amplifier  100  equilibrates the digit lines DIGIT and DIGIT_, senses a differential voltage that develops between the digit lines DIGIT and DIGIT_, and then drives the digit lines to corresponding logic levels. In response to an active HIGH EQ signal the equilibration transistor  110  turns ON, connecting the digit lines DIGIT and DIGIT_ to each other and equilibrating the digit lines to the same voltage. The digit lines are typically equilibrated to V CC /2, which keeps the PMOS transistors  102 ,  104  and the NMOS transistors  112 ,  114  turned OFF. After the differential voltage between the digit lines DIGIT and DIGIT_ has reached substantially zero volts, the EQ signal transitions LOW to turn OFF the transistor  110 . 
     When a memory cell is accessed, the voltage of one of the digit lines DIGIT or DIGIT_ increases slightly, resulting in a voltage differential between the digit lines. While one digit line contains a charge from the accessed cell, the other digit line does not and serves as a reference for the sensing operation. Assuming, for example, the voltage on the DIGIT line increases, the voltage level of the DIGIT line increases slightly above V CC /2 causing the gate-to-source voltage of the NMOS transistor  114  to be greater than the NMOS transistor  112 . The RNL signal is activated, driving the common node of the NMOS transistors  112 ,  114  to ground, switching the NMOS transistor  114  ON. As a result, the complementary digit line DIGIT_ is coupled to the active RNL_ signal and is pulled to ground. In response to the low voltage level of the DIGIT_ line, the gate-to-source voltage of the PMOS transistor  102  increases, and in response to activation of the ACT signal, is turned ON due to the gate-to-source voltage being larger than the PMOS transistor  104 . The DIGIT line is consequently coupled to the power supply voltage as provided by the active ACT signal, and the PMOS transistor  102  drives the digit line DIGIT towards the power supply voltage. Thereafter, the voltage on the digit line DIGIT further increases and the voltage on the complementary digit line DIGIT_ further decreases. At the end of the sensing period, the NMOS transistor  114  has driven complementary digit line DIGIT_ to ground by the active RNL_ signal and the PMOS transistor  102  has driven the digit line DIGIT to the power supply voltage V CC  by the active ACT signal. 
     Random threshold voltage mismatch of transistor components in conventional sense amplifiers  100  are undesirable because deviations of the threshold voltage may abruptly cause an imbalance in the sense amplifier that can erroneously amplify input signals in the wrong direction. For example, the offset due to a threshold voltage mismatch of the sense amplifier  100  may be amplified by the large gain of the NMOS transistors  112 ,  114 , as will be understood by one skilled in the art. Consequently, the sense amplifier  100  would likely amplify the signal on the asserted digit line incorrectly, resulting in reading the incorrect data. Errors and delays due to mismatched threshold voltages in sense amplifiers ultimately affect the overall accuracy of memory operations. While efforts have been made to compensate for threshold voltage offsets, such compensation methods typically increase memory access time, occupy chip space and increase power consumption. 
     Therefore, there is a need for a sense amplifier designed to have tolerance to voltage threshold mismatch of transistor components included in the sense amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a conventional sense amplifier. 
         FIG. 2  is a schematic drawing of a sense amplifier according to an embodiment of the invention. 
         FIG. 3  is a schematic drawing of a sense amplifier according to another embodiment of the invention. 
         FIG. 4  is a schematic drawing of a sense amplifier according to another embodiment of the invention. 
         FIG. 5  is a functional block diagram of a memory device including a sense amplifier according to an embodiment of the invention. 
         FIG. 6  is a functional block diagram of a computer system including the memory device of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Certain details are set forth below to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, and timing protocols have not been shown in detail in order to avoid unnecessarily obscuring the invention. 
       FIG. 2  illustrates a sense amplifier  200  according to an embodiment of the invention. Components and signals that were previously described with reference to  FIG. 1  have been given the same reference numbers in  FIG. 2 . The sense amplifier  200  of  FIG. 2  includes an NMOS amplifier stage  201  having a pair of NMOS transistors  222 ,  224  coupled to the sources of the NMOS transistors  112 ,  114  to provide source degeneration. The drains to each of the NMOS transistors  222 ,  224  are coupled to the sources of the NMOS transistors  112 ,  114 , and the sources of the NMOS transistors  222 ,  224  are coupled to a common node coupled to ground. The gates of the NMOS transistors  222 ,  224  are coupled together and receive a control signal SLAT that provides a voltage signal to the respective gates. In the source degenerate configuration, the NMOS transistors  222 ,  224  provide a resistance on the sources of the NMOS transistors  112 ,  114 . The effect of adding resistance at the NMOS amplifier stage  201  reduces the gain of the NMOS transistors  112 ,  114 . As a result, an offset that typically would have been amplified due to a threshold voltage mismatch is reduced, which in turn minimizes its interference with amplifying the digit line signal. The resistance provided by the NMOS transistors  222 ,  224  may be changed by adjusting the voltage of the SLAT signal. The SLAT signal may be predetermined for the sense amplifier  200  by design or be an adjustable control signal by a user. 
     In operation, the DIGIT and DIGIT_ lines are precharged to Vcc/2 and the voltages of the digit lines are equilibrated by activating the EQ signal and coupling the two digit lines together through the transistor  110 . The EQ signal is then deactivated to isolate the DIGIT and DIGIT_ lines in preparation for a sense operation. A word line (not shown) of the memory cell array is activated to couple a row of memory cells to a respective digit line and to a respective sense amplifier  200 . As previously described, coupling a memory cell to the respective digit line causes a voltage differential between the DIGIT and DIGIT_ lines. In the present example, it will be assumed that the accessed memory cell is coupled to the DIGIT line and increases the voltage to slightly above Vcc/2. As a result, the gate-to-source voltage of the NMOS transistor  114  is greater than for the NMOS transistor  112 . 
     Prior to activation of the RNL_ and ACT signals, the SLAT signal is activated to couple the sources of the transistors  112 ,  114  to a common node  226  through the transistors  222 ,  224 . As a result, voltage of both the DIGIT and DIGIT_ lines slightly decrease. With the greater gate-to-source voltage for the NMOS transistor  114 , the voltage of the DIGIT_ line is discharged more quickly to the common node  226  than for the DIGIT line, resulting in the PMOS transistor  102  having a greater gate-to-source voltage than for the PMOS transistor  104 . The ACT signal is then activated (typically providing Vcc, a power supply voltage), and due to the greater gate-to-source voltage of the PMOS transistor  102 , the transistor  102  begins to switch ON before the PMOS transistor  104 , further increasing the gate-to-source voltage of the NMOS transistor  114 . The RNL_ signal is activated coupling the sources of the NMOS transistors  112 ,  114  to ground, fully switching ON the transistor  114  and fully coupling the DIGIT_ line to ground. The PMOS transistor  102  is consequently fully switching ON by the grounded DIGIT_ line and fully couples the DIGIT line to Vcc, latching the DIGIT and DIGIT_ lines to respective voltages Vcc and ground. 
     As previously discussed, the transistors  222 ,  224  increase the source-to-ground resistance of the NMOS transistors  112 ,  114  to provide source degeneration and reduce the gain of the NMOS transistors  112 ,  114 . The trade-off for reducing the gain of the NMOS transistors  112 ,  114  is that the current gain is also reduced, which slows the amplification of the DIGIT and DIGIT_ lines. The slower amplification of the NMOS amplifier stage  201  allows time for the PMOS transistors  102 ,  104  to recover towards Vcc before the NMOS transistors  112 ,  114  are fully driven to ground. As a result, failure to pull-up the voltage of one of the digit lines due to transistor threshold voltage mismatch is reduced during normal operation of the sense amplifier  200 . 
       FIG. 3  illustrates a sense amplifier  300  according to another embodiment of the invention. The sense amplifier  300  is similar to the sense amplifier  200  previously described with reference to  FIG. 2 . The transistors  222 ,  224  of the sense amplifier  200 , however, have been replaced in the sense amplifier  300  with resistors  322 ,  324 . As previously discussed, the transistors  222 ,  224  increased the source-to-ground resistance of the NMOS transistors  112 ,  114  to provide source degeneration. The resistors  322 ,  324  are used to provide increased source-to-ground resistance in place of the transistors  222 ,  224 . Operation of the sense amplifier  300  is the same as for the sense amplifier  200  except that provision of an active SLAT signal is not necessary. 
       FIG. 4  illustrates a sense amplifier  400  according to another embodiment of the invention. The sense amplifier  400  is similar to the sense amplifier  200  previously described with reference to  FIG. 2 . However, additional NMOS transistors  216 ,  218  are included in the sense amplifier  400  and the common node  226  is coupled to ground. The NMOS transistors  216 ,  218  are used to enhance pull-down of the DIGIT and DIGIT_ lines to ground during sensing. The drains of the NMOS transistors  216 ,  218  are coupled to the respective drains of the NMOS transistors  112 ,  114 , and the gates of the NMOS transistors  216 ,  218  are also respectively coupled to the gates of the NMOS transistors  112 ,  114 . The sources of the NMOS transistors  216 ,  218  are coupled together and share a common node to which the RNL_ signal is coupled. 
     Operation of the sense amplifier  400  is similar to operation of the sense amplifier  200 . The increase of gate-to-source voltage of one of the NMOS transistors  112 ,  114  in response to coupling a memory cell to either the DIGIT or DIGIT_ line also increases the gate-to-source voltage of one of the NMOS transistors  216 ,  218 . With the common node  226  coupled to ground, rather than to receive the RNL_ signal, the voltage of the DIGIT and DIGIT_ lines begin to discharge to ground immediately rather than waiting for the RNL_ signal to become active. As previously discussed with reference to the sense amplifier  200 , the decreasing voltage of the DIGIT or DIGIT_ line creates a gate-to-source voltage imbalance between the PMOS transistors  102 ,  104 , with one of the two transistors switching ON before the other in response to the ACT signal becoming active. In addition to causing either of the NMOS transistors  112 ,  114  to switch ON more fully, the corresponding NMOS transistors  216 ,  218  is more fully switched ON as well. In response to the RNL_ signal becoming active, the conductive NMOS transistor  216  or  218  provides additional drive capability to pull-down the DIGIT or DIGIT_ line to ground more quickly than compared to the sense amplifier  200 . 
     In another embodiment, the sense amplifier  300  of  FIG. 3  is modified to includes additional transistors to provide greater drive capability to pull-down the DIGIT or DIGIT_ line, as previously discussed with reference to the sense amplifier  400  of  FIG. 4 . Resistors, multiple transistors, impedances sources or any other components, or combinations thereof may be used in place of the NMOS transistors  222 ,  224 , as is known in the art, to provide source degeneration and reduce the gain of the NMOS transistors  1112 ,  114 . 
     The sense amplifiers  200 ,  300 , and  400  were previously described in operation according to a particular activation sequence of signals, for example, the EQ, SLAT, ACT, and RNL_ signals. In other embodiments of the invention, the activation sequence of signals is different than that previously described. Those ordinarily skilled in the art will obtain sufficient understanding from the description provided herein to make such modifications to practice these other embodiments. The present invention is not limited to the particular sequence previously described for the previously described embodiments of the invention. 
       FIG. 5  illustrates an embodiment of a memory device  500  including at least one sense amplifier according to an embodiment of the present invention. The memory device  500  includes an address register  502  that receives row, column, and bank addresses over an address bus ADDR, with a memory controller (not shown) typically supplying the addresses. The address register  502  receives a row address and a bank address that are applied to a row address multiplexer  504  and bank control logic circuit  506 , respectively. The row address multiplexer  504  applies either the row address received from the address register  502  or a refresh row address from a refresh counter  508  to a plurality of row address latch and decoders  510 A-D. The bank control logic  506  activates the row address latch and decoder  510 A-D corresponding to either the bank address received from the address register  502  or a refresh bank address from the refresh counter  508 , and the activated row address latch and decoder latches and decodes the received row address. 
     In response to the decoded row address, the activated row address latch and decoder  510 A-D applies various signals to a corresponding memory bank  512 A-D, including a row activation signal to activate a row of memory cells corresponding to the decoded row address. Each memory bank  512 A-D includes a memory-cell array having a plurality of memory cells arranged in rows and columns. Data stored in the memory cells in the activated row are sensed and amplified by sense amplifiers  511  in the corresponding memory bank. The sense amplifiers  511  are designed according to an embodiment of the present invention. The row address multiplexer  504  applies the refresh row address from the refresh counter  508  to the decoders  510 A-D and the bank control logic circuit  506  uses the refresh bank address from the refresh counter when the memory device  500  operates in an auto-refresh or self-refresh mode of operation in response to an auto- or self-refresh command being applied to the memory device  500 , as will be appreciated by those skilled in the art. 
     A column address is applied on the ADDR bus after the row and bank addresses, and the address register  502  applies the column address to a column address counter and latch  514  which, in turn, latches the column address and applies the latched column address to a plurality of column decoders  516 A-D. The bank control logic  506  activates the column decoder  516 A-D corresponding to the received bank address, and the activated column decoder decodes the applied column address. Depending on the operating mode of the memory device  500 , the column address counter and latch  514  either directly applies the latched column address to the decoders  516 A-D, or applies a sequence of column addresses to the decoders starting at the column address provided by the address register  502 . In response to the column address from the counter and latch  514 , the activated column decoder  516 A-D applies decode and control signals to an I/O gating and data masking circuit  518  which, in turn, accesses memory cells corresponding to the decoded column address in the activated row of memory cells in the memory bank  512 A-D being accessed. 
     During data read operations, data being read from the addressed memory cells is coupled through the I/O gating and data masking circuit  518  to a read latch  520 . The I/O gating and data masking circuit  518  supplies N bits of data to the read latch  520 , which then applies two N/2 bit words to a multiplexer  522 . In the embodiment of  FIG. 3 , the circuit  518  provides 64 bits to the read latch  520  which, in turn, provides two 32 bits words to the multiplexer  522 . A data driver  524  sequentially receives the N/2 bit words from the multiplexer  522  and also receives a data strobe signal DQS from a strobe signal generator  526 . The DQS signal is used by an external circuit such as a memory controller (not shown) in latching data from the memory device  500  during read operations. The data driver  524  sequentially outputs the received N/2 bits words as a corresponding data word DQ, each data word being output in synchronism with a rising or falling edge of a CLK signal that is applied to clock the memory device  500 . The data driver  524  also outputs the data strobe signal DQS having rising and falling edges in synchronism with rising and falling edges of the CLK signal, respectively. Each data word DQ and the data strobe signal DQS collectively define a data bus DATA. 
     During data write operations, an external circuit such as a memory controller (not shown) applies N/2 bit data words DQ, the strobe signal DQS, and corresponding data masking signals DM on the data bus DATA. A data receiver  528  receives each DQ word and the associated DM signals, and applies these signals to input registers  530  that are clocked by the DQS signal. In response to a rising edge of the DQS signal, the input registers  530  latch a first N/2 bit DQ word and the associated DM signals, and in response to a falling edge of the DQS signal the input registers latch the second N/2 bit DQ word and associated DM signals. The input register  530  provides the two latched N/2 bit DQ words as an N-bit word to a write FIFO and driver  532 , which clocks the applied DQ word and DM signals into the write FIFO and driver in response to the DQS signal. The DQ word is clocked out of the write FIFO and driver  532  in response to the CLK signal, and is applied to the I/O gating and masking circuit  518 . The I/O gating and masking circuit  518  transfers the DQ word to the addressed memory cells in the accessed bank  512 A-D subject to the DM signals, which may be used to selectively mask bits or groups of bits in the DQ words (i.e., in the write data) being written to the addressed memory cells. 
     A control logic and command decoder  534  receives a plurality of command and clocking signals over a control bus CONT, typically from an external circuit such as a memory controller (not shown). The command signals include a chip select signal CS*, a write enable signal WE*, a column address strobe signal CAS*, and a row address strobe signal RAS*, while the clocking signals include a clock enable signal CKE* and complementary clock signals CLK, CLK*, with the “*” designating a signal as being active low. The command signals CS*, WE*, CAS*, and RAS* are driven to values corresponding to a particular command, such as a read, write, or auto-refresh command. In response to the clock signals CLK, CLK*, the command decoder  534  latches and decodes an applied command, and generates a sequence of clocking and control signals that control the components  502 - 532  to execute the function of the applied command. The clock enable signal CKE enables clocking of the command decoder  534  by the clock signals CLK, CLK*. The command decoder  534  latches command and address signals at positive edges of the CLK, CLK* signals (i.e., the crossing point of CLK going high and CLK* going low), while the input registers  530  and data drivers  524  transfer data into and from, respectively, the memory device  500  in response the data strobe signal DQS. The detailed operation of the control logic and command decoder  534  in generating the control and timing signals is conventional, and thus, for the sake of brevity, will not be described in more detail. Although previously described with respect to a dynamic random access memory device, embodiments of the present invention can be utilized in applications other than for a memory device where it is desirable to reduce the effects a threshold voltage mismatch when the voltage level of an input signal is amplified. 
       FIG. 6  is a block diagram of a computer system  600  including computer circuitry  602  including the memory device  500  of  FIG. 5 . Typically, the computer circuitry  602  is coupled through address, data, and control buses to the memory device  500  to provide for writing data to and reading data from the memory device. The computer circuitry  602  includes circuitry for performing various computing functions, such as executing specific software to perform specific calculations or tasks. In addition, the computer system  600  includes one or more input devices  604 , such as a keyboard or a mouse, coupled to the computer circuitry  602  to allow an operator to interface with the computer system. The computer system  600  may include one or more output devices  606  coupled to the computer circuitry  602 , such as output devices typically including a printer and a video terminal. One or more data storage devices  608  may also be coupled to the computer circuitry  602  to store data or retrieve data from external storage media (not shown). Examples of typical storage devices  608  include hard and floppy disks, tape cassettes, compact disk read-only (CD-ROMs) and compact disk read-write (CD-RW) memories, and digital video disks (DVDs). 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, many of the components described above may be implemented using either digital or analog circuitry, or a combination of both. Accordingly, the invention is not limited except as by the appended claims.