Abstract:
A semiconductor memory device includes a memory cell array and a differential amplifying and latching circuit for latching and outputting each of signal pairs output from the memory cell array in case of a first latency operation, and for amplifying a voltage difference of each of the signal pairs output from the memory cell array in case of a second latency operation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor memory device, and more particularly, to a semiconductor memory device and data read method thereof which can stably output a data read from a memory cell array.  
           [0003]    2. Description of Related Art  
           [0004]    A conventional semiconductor memory device transmits data read from a memory cell array to common data I/O lines, and outputs the data transmitted to the common data I/O lines via a current-voltage converter, a differential amplifier (or latch), a data output buffer, and a data output driver.  
           [0005]    In greater detail, the differential amplifier is arranged at an output stage of the current-voltage converter to amplify output data of the current-voltage converter and to transmit it to the data output buffer. Alternatively, a latch latches output data of the current-voltage converter and transmits it to the data output buffer.  
           [0006]    [0006]FIG. 1 is a block diagram illustrating a conventional semiconductor memory device. The semiconductor memory device of FIG. 1 includes a memory cell array  10 , current-voltage converters  20 - 1  to  20 -n, differential amplifiers  22 - 1  to  22 -n, data output buffers  24 - 1  to  24 -n, and data output drivers  26 - 1  to  26 -n.  
           [0007]    In particular, in FIG. 1, IVC denotes the current-voltage converters, DA denotes the differential amplifiers, DOB denotes the data output buffers, and DOD denotes the data output drivers. LIO 11 /B to LIO 1 n/B, . . . , LIOm 1 /B to LIOmn/B denote local data I/O line pairs, and MIO 1 /B to MIOn/B denote main data I/O line pairs.  
           [0008]    Functions of the components of FIG. 1 are described focusing on a read operation. Data stored in the memory cell array  10  is transferred to the local data I/O line pairs LIO 11 /B to LIO 1 n/B, . . . , LIOm 1 /B to LIOmn/B and to the main data I/O data line pairs MIO 1 /B to MIOn/B, in sequence. The current-voltage converters  20 - 1  to  20 -n convert a current difference of the data pairs transferred to the main data I/O line pairs MIO 1 /B to MIOn/B, respectively, into a voltage difference to generate data X. The data output buffers  24 - 1  to  24 -n buffer the data Z output from the differential amplifiers  22 - 1  to  22 -n, respectively. The data output drivers  26 - 1  to  26 -n drive the data output from the data output buffers  24 - 1  to  24 -n to output data D 1  to Dn, respectively.  
           [0009]    [0009]FIGS. 2A to  2 C are timing diagrams illustrating operation of the current-voltage converter and the differential amplifier. FIG. 2A shows the timing diagram during normal operation, FIG. 2B shows the timing diagram during a high-frequency operation, and FIG. 2C shows the timing diagram according to a process variation.  
           [0010]    In FIGS. 2A to  2 C, CLK denotes a clock signal, CMD denotes a command signal, and a hatched portion denotes an invalid data period.  
           [0011]    Referring to FIG. 2A, data CSA 1  to CSA 4  are output from the current-voltage voltage converters  20 - 1  to  20 -n in sequence. When a signal Y is input so as to enable the differential amplifiers  22 - 1  to  22 -n, the differential amplifiers  22 - 1  to  22 -n sequentially receive the data CSA 1  to CSA 4  and sequentially generate the data DO 1  to DO 4  in response to the signal Y. Period “t1” represents a time period from a time point that the clock signal CLK is generated when the read command is applied to a time point that the first data CSA 1  begins to be output through the current-voltage converters  20 - 1  to  20 -n. Period “t2” represents a time period from a time point that the clock signal CLK is generated when the read command is applied to a time point that the signal Y is generated. A time period “t3” represents an enable period of the signal Y.  
           [0012]    Referring to FIG. 2B, a cycle that the clock signal CLK is generated becomes faster, and the data CSA 1  to CSA 4  are sequentially output from the current-voltage converters  20 - 1  to  20 -n in response to the clock signal CLK. The differential amplifiers  22 - 1  to  22 -n receive the.data CSA 1  to CSA 4  and generate data DO 1  to DO 4  in response to the signal Y. At this point, when the data CSA 2  is input to the differential amplifiers  22 - 1  to  22 -n during the enable period t 3  of the signal Y, the differential amplifiers  22 - 1  to  22 -n output not the data DO 1  but the next data DO 2 . This is because when the data is transited during the enable period t 3  of the differential amplifiers  22 - 1  to  22 -n, the output data of the differential amplifiers  22 - 1 , to  22 -n is changed. Accordingly, since the data DO 1  cannot be output in case of FIG. 2B, a data read error occurs. That is, in FIG. 2B, the data DO 2  to DO 4  are cut partially, but the data DO 2  to DO 4  are connected to be output by the data output buffers.  
           [0013]    Referring to FIG. 2C, due to a process variation, the data CSA 1  to CSA 4  output from the current-voltage converters  20 - 1  to  20 -n are delayed by a time period t 4 . Even though the data CSA 1  to CSA 4  are delayed by the time period t 4 , when the data CSA 1  to CSA 4  are input within the enable period t 3  of the signal Y, the differential amplifiers  22 - 1  to  22 -n can output the data DO 1  to DO 4  stably. In FIG. 2C, the data DO 1  to DO 4  are cut partially, but the data DO 1  to DO 4  are connected to be output by the data output buffers. Therefore, the data read error does not occur.  
           [0014]    When a data read path of the semiconductor memory device is configured by the current-voltage converter and the differential amplifier, a data read error occurs during the high-frequency operation but the data read error resulting from the process variation does not occur.  
           [0015]    [0015]FIG. 3 is a block diagram illustrating a configuration of a data read path of another conventional semiconductor memory device. The semiconductor memory device of FIG. 3 includes a memory cell array  10 , current-voltage converters  20 - 1  to  20 -n, latches  28 - 1  to  28 -n, data output buffers  24 - 1  to  24 -n, and data output drivers  26 - 1  to  26 -n.  
           [0016]    In FIG. 3, LA denotes the latch. Like references of FIGS. 1 and 3 denote like parts. The latch LA latches and outputs output data of the current-voltage converters  20 - 1  to  20 -n in response to the signal Y. The remaining components of FIG. 3 except the latch LA can be understood with reference to the description of FIG. 1, and thus their description is omitted to avoid a redundancy.  
           [0017]    [0017]FIGS. 4A to  4 C are timing diagrams illustrating operation of the current-voltage converter and the latch. FIG. 4A shows the timing diagram during normal operation, FIG. 4B shows the timing diagram during a high-frequency operation, and FIG. 4C shows the timing diagram according to a process variation.  
           [0018]    In FIGS. 4A to  4 C, CLK denotes a clock signal, CMD denotes a command signal, and a hatched portion denotes an invalid data period.  
           [0019]    Referring to FIG. 4A, data CSA 1  to CSA 4  are output from the current-voltage converters  20 - 1  to  20 -n in sequence. The latches  28 - 1  to  28 -n sequentially receive the data CSA 1  to CSA 4  and sequentially generate the data DO 1  to DO 4  in response to the signal Y. Period “t1” represents a time period from a time point that the clock signal CLK is generated when the read command is applied to a time point that the first data CSA 1  begins to be output through the current-voltage converters  20 - 1  to  20 -n. Period “t2” represents a time period from a time point that the clock signal CLK is generated when the read command is applied to a time point that the signal Y is generated. Period “t3” represents an enable period of the signal Y.  
           [0020]    Referring to FIG. 4B, a generation cycle of the clock signal CLK becomes faster, and the data CSA 1  to CSA 4  are sequentially output from the current-voltage converters  20 - 1  to  20 -n in response to the clock signal CLK. The latches  28 - 1  to  28 -n receive and latch the data CSA 1  to CSA 4  and generate the data DO 1  to DO 4  at a rising edge of the signal Y.  
           [0021]    The latches  28 - 1  to  28 -n maintain the signals latched at a rising edge of the signal Y “as is” even though a state of the data output from the current-voltage converters  20 - 1  to  20 -n is varied during the enable period t 3  of the signal Y. Accordingly, the data read error does not occur.  
           [0022]    Referring to FIG. 4C, due to a process variation, the data CSA 1  to CSA 4  output from the current-voltage converters  20 - 1  to  20 -n are delayed by a time period t 4 . The latches  28 - 1  to  28 -n latch the invalid data at a rising edge of the signal Y and thus cannot output the data DO 1 . The latches  28 - 1  to  28 -n latch the data CSA 1  to CSA 4  and output the data DO 2  to DO 4  in response to the signal Y. In this case, the data DO 1  cannot be output, leading to a data read error.  
           [0023]    As described above, when the latches  28 - 1  to  28 -n are arranged at output stages of the current-voltage converters  20 - 1  to  20 -n, the data can be output stably during the high-frequency operation, but when the process variation occurs, the invalid data is output.  
           [0024]    When the conventional semiconductor memory device is configured to include the current-voltage converter and the differential amplifier in the data read path, the data read error occurs during high-frequency operation, whereas the data can be output stably when the process is varied. On the other hand, when the conventional semiconductor memory device is configured to include the current-voltage converter and the latch in the data read path, the data can be output stably during high-frequency operation, whereas the data read error occurs when the process is varied.  
           [0025]    CAS latency is defined as the period of time in clock cycles between when a read command is applied to a semiconductor memory device and when valid data are applied at the outputs of the device, wherein the number of clock cycles is an integer. For example, a CAS latency of 2 means that valid data are output from the device two clock cycles after the read command is applied.  
           [0026]    When the CAS latency is 2, the semiconductor memory device requires a frequency characteristic lower than when the CAS latency is 3. When the CAS latency is 3, the semiconductor memory device requires a time tAA, i.e., the data read speed not higher and a frequency characteristic higher than when the CAS latency is 2.  
           [0027]    Accordingly, the data read error can be reduced by outputting the output data of the current-voltage converter through the latch in case of the CAS latency operation that is relatively high in need for a frequency characteristic during a read operation, and by outputting the output data of the current-voltage converter through the differential amplifier in case of the CAS latency operation that is relatively low in need for a frequency characteristic during a read operation.  
         SUMMARY OF THE INVENTION  
         [0028]    It is an object of the present invention to provide a semiconductor memory device which can output data stably by differentiating data read paths in a read operation that have a high need for frequency performance characteristics from those in a read operation that is relatively low in need for a frequency performance characteristic during the read operation.  
           [0029]    It is another object of the present invention to provide a data read method which can output data in a stable fashion.  
           [0030]    The present invention is directed to a semiconductor memory device. The device includes a memory cell array and a differential amplifying and latching circuit for latching and outputting each of signal pairs output from the memory cell array in case of a first latency operation, and for amplifying a voltage difference of each of the signal pairs output from the memory cell array in case of a second latency operation.  
           [0031]    The present invention further provides a semiconductor memory device which includes a memory cell array, a latch means for latching and outputting each of signal pairs output from the memory cell array in case of a first latency operation and a differential amplifying circuit for amplifying and outputting each of the signal pairs output from the memory cell array in case of a second latency operation.  
           [0032]    The present invention further provides a semiconductor memory device that includes a memory cell array, a first amplifying circuit for amplifying and outputting signals output from the memory cell array in case of a first latency operation, and a second amplifying circuit for amplifying and outputting the signals output from the memory cell array in case of a second latency operation.  
           [0033]    The present invention further provides a method of reading data in a semiconductor memory device. In accordance with the method, data is read from a memory cell array. In the case of a first latency operation, data output from the memory cell array is latched and output. In the case of a second latency operation, the data output from the memory cell array is amplified and output. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0035]    [0035]FIG. 1 is a block diagram illustrating a conventional semiconductor memory device.  
         [0036]    [0036]FIGS. 2A to  2 C are timing diagrams illustrating operation of a current-voltage converter and a differential amplifier of FIG. 1.  
         [0037]    [0037]FIG. 3 is a block diagram illustrating another conventional semiconductor memory device.  
         [0038]    [0038]FIGS. 4A to  4 C are timing diagrams illustrating operation of a current-voltage converter and a latch of FIG. 3.  
         [0039]    [0039]FIG. 5 is a block diagram illustrating a semiconductor memory device according to a first preferred embodiment of the present invention.  
         [0040]    [0040]FIG. 6 is a circuit diagram illustrating a differential amplifier and latch of FIG. 5.  
         [0041]    [0041]FIG. 7A is a timing diagram illustrating operation of a current-voltage converter and the differential amplifier and latch of FIG. 5, wherein a CAS latency is 2, and the differential amplifier and latch operates as the differential amplifier.  
         [0042]    [0042]FIG. 7B is a timing diagram illustrating operation of the current-voltage converter and the differential amplifier and latch of FIG. 5, wherein the CAS latency is 3, and the differential amplifier and latch operates as the latch.  
         [0043]    [0043]FIG. 8 is a block diagram illustrating a semiconductor memory device according to a second preferred embodiment of the present invention.  
         [0044]    [0044]FIG. 9 is a block diagram illustrating a semiconductor memory device according to a third preferred embodiment of the present invention.  
         [0045]    [0045]FIG. 10 is a circuit diagram illustrating the differential amplifier and latch of FIG. 9.  
         [0046]    [0046]FIG. 11 is a circuit diagram illustrating a differential amplifier of FIG. 10.  
         [0047]    [0047]FIG. 12 is a circuit diagram illustrating a latch of FIG. 10. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0048]    [0048]FIG. 5 is a block diagram illustrating a semiconductor memory device according to a first preferred embodiment of the present invention. The semiconductor memory device of FIG. 5 includes a differential amplifier and latches  30 - 1  to  30 -n in contrast to the differential amplifiers  22 - 1  to  22 -n in the prior art device of FIG. 1. Like reference numerals of FIGS. 1 and 5 denote like parts.  
         [0049]    The differential amplifier and latches  30 - 1  to  30 -n output data output from the current-voltage converters  20 - 1  to  20 -n through the differential amplifiers in response to the signal Y when the CAS latency is 2, and output data output from the current-voltage converters  20 - 1  to  20 -n through the latches in response to the signal Y when the CAS latency is 3. That is, the differential amplifier and latches  30 - 1  to  30 -n operate as the differential amplifier in response to the signals CL 2  and Y, and operate as the latch in response to the signals CL 3  and Y. It should be noted that the semiconductor memory device of FIG. 5 includes the current-voltage converters  20 - 1  to  20 -n, but does not always require the current-voltage converters  20 - 1  to  20 -n.  
         [0050]    The remaining components except the differential amplifier and latches  30 - 1  to  30 -n can be understood with reference to a description of FIG. 1, and their description is omitted to avoid redundancy.  
         [0051]    [0051]FIG. 6 is a circuit diagram illustrating one embodiment of the differential amplifier and latch of FIG. 5. The differential amplifier and latch of FIG. 6 includes a PMOS latch  40  having PMOS transistors P 1  and P 2 , a pre-charge circuit  42  having PMOS transistors P 3  to P 5 , a NMOS latch  44  having NMOS transistors N 7  and N 8 , NMOS transistors N 1  to N 6  and N 9  to N 11 , and a PMOS transistor P 6 .  
         [0052]    When the CAS latency is set to 2, the signal CL 2  has a logic “high” level. As a result, the NMOS transistors N 1 , N 4 , N 5 , and N 6  are turned on, whereupon the NMOS transistors N 7  and N 8  are turned off. Also, the signal CL 3  has a logic “low” level. So, the NMOS transistors N 2  and N 3  are turned off. Accordingly, in this case, the differential amplifier and latch is configured by the PMOS latch  40 . When the signal Y has a logic “low” level, the PMOS transistors P 3  to P 5  are turned on so that nodes A and B are pre-charged, and the PMOS transistor P 6  is turned on so that nodes C and D are pre-charged. In this state, when the signal Y is transitioned to a logic “high” level, the NMOS transistor N 11  is turned on, the PMOS transistor P 6  is turned off, and the pre-charge circuit  42  is disabled, whereupon operation of the differential amplifier and latch is enabled. At this time, when an input signal IN having a logic “high” level and an inverted input signal INB having a logic “low” level are applied from the current-voltage converter, the node D goes to a logic “low” level, and the node C goes to a logic “high” level. As a result, the node A is transitioned from a pre-charge level to a logic “high” level, and the node B is transitioned from a pre-charge level to a logic “low” level. Accordingly, output data OUT having a logic “high” level is generated, and inverted output data OUTB having a logic “low” level is generated. Signals of the nodes A and B are amplified by the PMOS latch  40 . However, when the input data IN and the inverted input data INB are transitioned to a logic “low” level and a logic “high” level during an enable period of the signal Y, respectively, the output data OUT and the inverted output data OUTB are transitioned to a logic “low” level and a logic “high” level, respectively. That is, the differential amplifier and latch DA &amp; LA operates as a differential amplifier.  
         [0053]    When the CAS latency is set to 3, the signal CL 3  has a logic “high” level. As a result, the NMOS transistors N 2  and N 3  are turned on. At this time, the signal CL 2  has a logic “low” level, and so the NMOS transistors N 1 , N 4 , N 5 , and N 6  are turned off. Accordingly, in this case, the differential amplifier and latch is configured by the PMOS latch  40  and the NMOS latch  44 . When the signal Y is transitioned to a logic “high” level, the NMOS transistor N 11  is turned on, the PMOS transistor P 6  is turned off, and operation of the pre-charge circuit  42  is disabled, whereupon operation of the differential amplifier and latch DA &amp; LA is enabled. At this time, when the input data IN having a logic “high” level and the inverted input data INB having a logic “low” level are applied from the current-voltage converter, the node D goes to a logic “low” level, and the node C goes to a logic “high” level. As a result, the node A is transitioned to a logic “high” level, and the node B is transitioned to a logic “low” level. Accordingly, the output data OUT is transitioned to a logic “high” level, and the inverted output data OUTB is transitioned to a logic “low” level. Signals of the nodes A and B are latched by the PMOS latch  40  and the NMOS latch  44 . Thereafter, even though the input data IN and the inverted input data INB are transitioned to a logic “low” level and a logic “high” level within the enable period of the signal Y, respectively, so that signal levels of the nodes C and D are transitioned, since a current path cannot be formed between the node A and the node C, and the node B and the node D by the NMOS latch  44 , the output data OUT and the inverted output data OUTB are maintained to a latched level. That is, the differential amplifier and latches DA &amp; LA operates as the latch.  
         [0054]    [0054]FIG. 7A is a timing diagram illustrating operation of the current-voltage converter and the differential amplifier and latch of FIG. 5, wherein the CAS latency is 2, and the differential amplifier and latch operates as the differential amplifier. FIG. 7B is a timing diagram illustrating operation of the current-voltage converter and the differential amplifier and latch of FIG. 5, wherein the CAS latency is 3, and the differential amplifier and latch operates as the latch.  
         [0055]    In FIGS. 7A and 7B, CLK denotes a clock signal, CMD denotes a command signal, and a hatched portion denotes an invalid data period.  
         [0056]    Referring to FIG. 7A, since the differential amplifier and latch operates as the differential amplifier, even though a time period t 5  of from a rising edge of the clock signal CLK to an enable time point of the signal Y is reduced when the read command is applied, the data can be output in a stable fashion. Also, since a time tAA 2  is reduced as an enable time point of the signal Y becomes faster, the data read speed also becomes faster.  
         [0057]    Referring to FIG. 7B, since the differential amplifier and latch operates as the latch, even though a cycle of the clock signal CLK becomes shortened, the data can be output stably. That is, even though a next data is input within the enable time t 3  of the signal Y, since the previous data is latched and outputted, the data read error does not occur.  
         [0058]    Accordingly, in case that the CAS latency is 2, since the differential amplifier and latch operates as the differential amplifier, and the enable time point of the signal Y becomes faster, the data read speed is improved. Also, in the case in which the CAS latency is 3, the differential amplifier and latch operates as the latch, and thus the data read error does not occur.  
         [0059]    [0059]FIG. 8 is a block diagram illustrating a semiconductor memory device according to a second preferred embodiment of the present invention. The semiconductor memory device of FIG. 8 additionally includes differential amplifiers  32 - 1  to  32 -n between the current-voltage converters  20 - 1  and  20 -n and the differential amplifier and latches  30 - 1  to  30 -n, in contrast to the configuration of FIG. 5. Like reference numerals of FIGS. 5 and 8 denote like parts.  
         [0060]    In the semiconductor memory device of FIG. 8, since a voltage difference of data output from the current-voltage converters  20 - 1  to  20 -n is relatively small, the output data of the current-voltage converters  20 - 1  to  20 -n are amplified by the differential amplifiers  32 - 1  to  32 -n and then are applied to the differential amplifier and latches  30 - 1  to  30 -n, thereby performing a stable data read operation.  
         [0061]    [0061]FIG. 9 is a block diagram illustrating a semiconductor memory device according to a third preferred embodiment of the present invention. The semiconductor memory device of FIG. 9 includes differential amplifier and latches  50 - 1  to  50 -n arranged between the current-voltage converters  20 - 1  to  20 -n and the data output buffers  24 - 1  to  24 -n, the differential amplifier and latches  50 - 1  to  50 -n including differential amplifiers  52 - 1  to  52 -n and latches  54 - 1  to  54 -n, respectively. Like reference numerals of FIGS. 5 and 9 denote like parts.  
         [0062]    In FIG. 9, the differential amplifiers  52 - 1  to  52 -n amplify and output the output data of the current-voltage converters  20 - 1  to  20 -n in response to the signal Y when the CAS latency is 2. The latches  54 - 1  to  54 -n latch and output the output data of the current-voltage converters  20 - 1  to  20 -n at a rising edge of the signal Y when the CAS latency is 3. That is, the differential amplifier and latch of the semiconductor memory device of FIG. 9 includes the differential amplifier and the latch which are formed not integrally but separately. The semiconductor memory device of FIG. 9 does not always require the current-voltage converters  20 - 1  to  20 -n.  
         [0063]    [0063]FIG. 10 is a circuit diagram illustrating one embodiment of the differential amplifier and latch of FIG. 9. A differential amplifying circuit  52  includes an inverter I 1  and CMOS transmission gates C 1  and C 2  in addition to a differential amplifier  60 . A latching circuit  54  includes an inverter  12  and CMOS transmission gates C 3  and C 4  in addition to a latch  62 .  
         [0064]    When the CAS latency is set to 2, the signal CL 2  has a logic “high” level, and the signal CL 3  has a logic “low” level. The CMOS transmission gates C 1  and C 2  are turned on so that data CSA and CSAB output from the current-voltage converters are transferred. At this moment, when the data output through the CMOS transmission gate C 1  is a data IN/INB, the differential amplifier  60  amplifies a voltage difference of the data IN/INB to generate the data OUT/OUTB in response to the signal Y. The CMOS transmission gate C 2  transmits the data OUT/OUTB. At this time, the data output through the CMOS transmission gate C 2  is a data DO/DOB.  
         [0065]    When the CAS latency is set to 3, the signal CL 3  has a logic “high” level, and the signal CL 2  has a logic “low” level. The CMOS transmission gates C 3  and C 4  are turned on so that data CSA and CSAB output from the current-voltage converters are transferred. At this moment, when the signal output through the CMOS transmission gate C 3  is data IN/INB, the latch  62  latches the data IN/INB to generate the data OUT/OUTB in response to the signal Y. An enable time point of the signal Y generated at this time becomes slower than an enable time point of the signal Y generated when the CAS latency is 2. The CMOS transmission gate C 4  transmits the data OUT/OUTB. At this time, the data output through the CMOS transmission gate C 4  is data DO/DOB. Even though not shown in FIG. 10, another differential amplifier can additionally be arranged at a front stage of the differential amplifier  60 .  
         [0066]    [0066]FIG. 11 is a circuit diagram illustrating one embodiment of the differential amplifier of FIG. 10. The differential amplifier of FIG. 11 includes the PMOS transistors P 1  to P 5  and the NMOS transistors N 9  to N 11  of the differential amplifier and latch of FIG. 6.  
         [0067]    Operation of the differential amplifier of FIG. 11 is described below. When the signal Y has a logic “low” level, the PMOS transistors P 3  to P 5  are turned on, so that the nodes A and B are pre-charged. When the signal Y is transitioned to a logic “high” level, the PMOS transistors P 3  to P 5  are turned off, and the NMOS transistor N 11  is turned on, whereupon an operation of the differential amplifier is enabled. In this state, when the data IN having a logic “high” level and the data INB having a logic “low” level are applied, the NMOS transistor N 10  is turned on, and the NMOS transistor N 9  is turned off, whereupon the nodes A and B are transitioned to a logic “high” level and a logic “low” level, respectively. The PMOS latch  40  amplifies signals of the nodes A and B. Accordingly, the output data OUT and the inverted output data OUTB are transitioned to a logic “high” level and a logic “low” level. However, when the signal having a logic “low” level and the signal having a logic “high” level are generated during the signal Y having a logic “high” level, the nodes A and B are transitioned to a logic “low” level and a logic “high” level, respectively. That is, when levels of the data IN and INB are varied in state that the signal Y remains low, states of the output data OUT and the inverted output data OUTB are varied.  
         [0068]    [0068]FIG. 12 is a circuit diagram illustrating one embodiment of the latch of FIG. 10. The latch of FIG. 12 includes the PMOS transistors P 1  to P 5  and the NMOS transistors N 2 , N 3 , and N 9  to N 11  of the differential amplifier and latch of FIG. 6.  
         [0069]    Operation of the latch of FIG. 12 is described below. When the signal Y is transitioned to a logic “high” level, the PMOS transistors P 3  to P 5  are turned off, and the NMOS transistor N 11  is turned on, whereupon an operation of the latch is enabled. In this state, when the data IN having a logic “high” level and the data INB having a logic “low” level are applied, the NMOS transistor N 10  is turned on, and the NMOS transistor N 9  is turned off, whereupon the nodes A and B are transitioned to a logic “high” level and a logic “low” level, respectively. The PMOS latch  40  and the NMOS latch  44  latch the nodes B and A, respectively. Accordingly, the output data OUT and the inverted output data OUTB are transitioned to a logic “high” level and a logic “low” level. However, even though the data having a logic “low” level and the data having a logic “high” level are generated while the signal Y is at a logic “high” level, the data cannot be transferred to the nodes A and B through the NMOS latch  44 , and thus the nodes A and B maintain a latched level. That is, levels of the output data OUT and the inverted output data OUTB are not varied and maintain a first-latched level.  
         [0070]    An enable time point when the differential amplifier and latch is operated as the differential amplifier of FIG. 11 becomes faster that that when operated as the latch of FIG. 12.  
         [0071]    The differential amplifier of FIG. 11 and the latch of FIG. 12 are configured using the differential amplifier and latch of FIG. 6 but can be configured using the conventional differential amplifier and the conventional latch, respectively.  
         [0072]    As described herein, the semiconductor memory device outputs the output signal of the current-voltage converter through the latch in the case in which the CAS latency operation is relatively high in need for a frequency characteristic. Also, in case in which the CAS latency operation is relatively low in need for a frequency characteristic, the semiconductor memory device makes the enable time point of the signal Y faster than in case in which the CAS latency operation is relatively high in need for a frequency characteristic and outputs the output data of the current-voltage converter through the differential amplifier. As a result, the data read error can be reduced.  
         [0073]    While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.