Abstract:
An apparatus for sensing a current direction of an input signal and amplifying the sensed input signal to a logic level in use for semiconductor memory devices includes a current-direction sensing ad amplifying means for sensing a current direction of the input signal and amplifying the input signal in response to a sensing control signal to output a sensed and amplified signal, and a voltage level shift means for receiving a reference voltage from an external circuit and shifting a voltage level of the sensed and amplified signal to output a level shifted signal, thereby preventing an erroneous operation therein and improving a data access time.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor device; and, more particularly, to an apparatus for sensing a current direction of an input signal and amplifying the sensed input signal to a logic level in use for semiconductor memory devices, in which an erroneous operation can be prevented and a data access time therein is improved. 
     DESCRIPTION OF THE PRIOR ART 
     As is well known to those skilled in the art, a sense amplifier serves as an amplifying device with a high gain and a wide band, which is use to amplify signals read out from memory cells to output the amplified signals as a logic level. The typical sense amplifiers sense a voltage level of an input signal and amplify the sensed input signal to output the amplified signal as a logic level. 
     FIG. 1 is a schematic diagram illustrating a SRAM (static random access memory) having a conventional sense amplifier. 
     Referring to FIG. 1, a memory cell  100  stores data. A bit line pair transfer the data stored in the memory call  100 . The bit line pair include a bit line BL 0  and a complementary bit line/BL 0 , wherein the bit line pair are respectively held at different voltage level. For example, when the bit line BL 0  is held at a high level, the complimentary bit line/BL 0  is held at a low level. A first switching transfer MS 1 , connected between the bit line BL 0  and the memory cell  100 , is switched in response to a write word line signal WWL which is activated by a write address. A second switching transistor MS 2 , connected between the memory cell  100  and the complementary bit line/BL 0 , is switched in response to the write word signal WWL. A third switching transistor MS 3 , one of whose terminals is connected to a read bit line RBL 0 , is switched in response to a read word line signal RWL which is activated by a read address. A fourth switching transistor MS 4  receives the stored data from the memory cell  100 , one of whose terminals is connected to the other terminal of the third switching transistor MS 3  and the other terminal is connected to a ground voltage level. A first PMOS transistor MP 1  supplies a precharge voltage, i.e., a power supply voltage level, to the read bit line RBL 0  in response to a precharge signal PS. A fifth switching transistor MS 5  is switched in response to a column select signal CS to transfer a read data, wherein the read data is a signal transferred from the memory cell  100  to the read bit line RBL 0  through the third and fourth switching transistors MS 3  and MS 4 . A sense amplifier  120 , connected to the fifth switching transistor MS 5 , senses and amplifies the read data to output the amplified read data. A second PMOS transistor MP 2 , connected between the power supply voltage level VDD and an input terminal of the sense amplifier  120 , charges the input terminal of the sense amplifier  120  to the power supply voltage in response to the amplified read data. 
     A read/write operation of the SRAM having the sense amplifier will be described with conjunction to FIG.  1 . 
     At write operation, the first and second switching transistors MS 1  and MS 2  are turned on in response to the write word line address WWL, wherein the write word line signal WWL is enabled in response to the write address signal. Then, data on the bit line BL 0  and the complementary bit line/BL 0  are stored to the memory cell  100  through the turned-on first and second switching transistors MS 1  and MS 2 . At this time, the memory cell  100  is implemented with a form of inverter latch, so that the stored data can be continuously retained in the memory cell  100  until the power supply voltage is removed. 
     On the other hand, at read operation, the third switching transistor MS 3  is turned on in response to the read word line signal RWL, wherein the read word line signal RWL is enabled by a read address signal. At this time, if the data stored in the memory cell  100  is “high”, the fourth switching transistor MS 4  is turned on, so that a voltage level on the read bit line RBL 0  is pulled down from the precharge potential (power supply voltage level) to a “low” level. If the data stored in the memory cell  100  is “low”, the fourth switching transistor MS 4  is turned off, so that the read bit line RBL 0  keeps the precharge potential. Then, the voltage level on the read bit line RBL 0  is transferred to the sense amplifier  120  through the fifth switching transistor MS 5 . The sense amplifier  120  then amplifies the transferred voltage level to output the amplified voltage of a logic level as the read data. 
     As a capacity of the SRAM becomes increasing through sophisticated technology, the number of memory cells connected to the read bit line is also increased, resulting in an increase of an undesirable parasitic capacitance on the read bit line. The increase of the undesirable parasitic capacitance may delay a period of pulling up the read data to the full logic level at the read bit line. Accordingly, the sense amplifier is slow to output the read data, thereby increasing a data access time of the memory device. 
     SUMMARY OF THE INVENTION 
     It is, therefore an object of the present invention to provide an apparatus for sensing a current direction of an input signal and amplifying the sensed input signal to a logic level in use for semiconductor memory devices, in which an erroneous operation can be prevented and a data access time therein can be greatly improved. 
     It is, therefore, another object of the present invention to provide an apparatus for sensing a current direction of an input signal and amplifying the sensed input signal to a logic level in use for semiconductor memory devices, comprising; a current-direction sensing and amplifying means for a sensing a current direction of the input signal and amplifying the input signal in response to a sensing control signal to output a sensed and amplified signal; and a voltage level shift means for receiving a reference voltage from an external circuit and shifting a voltage level of the sensed and amplified signal to output a level shifted signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram illustrating SRAM having a conventional sense amplifier; 
     FIG. 2 is a block diagram illustrating a current-direction sense amplifier in accordance with the present invention; 
     FIG. 3 is a circuit diagram illustrating a current-direction sensing and amplifying unit shown in FIG. 2; 
     FIG. 4 is a circuit diagram illustrating a voltage level shift unit shown in FIG. 2; and 
     FIG. 5 is a simulated waveform of a voltage level shift unit shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a block diagram illustrating a current-direction sense amplifier according to the present invention. Referring to FIG. 2, a current-direction sensing and amplifying unit  200  senses a current direction of the input signal IN from a read bit line and amplifies the input signal IN in response to a sensing control signal SEN to output a sensed and amplified signal V 1 . A voltage level shift unit  300  receives a reference voltage V ref  from an external circuit and shifts a voltage level of the sensed and amplified signal V 1  to output a level shifted signal V 2 . 
     Since the sensed and amplified signal V 1  does not reach a full swing level, a static current may occur in an output load circuit. The voltage level shift unit  300  shifts the voltage level of the sensed and amplified signal V 1  to thereby output a level shifted signal V 2  corresponding to a full swing level. Therefore, the static current can be effectively prevented. 
     FIG. 3 shows the current-direction sensing and amplifying unit shown in FIG.  2 . 
     Referring to FIG. 3, a first NMOS transistor M 1  has a source connected to a ground voltage level GND, a gate connected to a power supply voltage level VDD and a drain receiving the input signal IN. A second NMOS transistor Ms has a source connected to the ground voltage level GND, a gate connected to the power supply voltage level VDD. A third NMOS transistor M 3  has a source connected to the drain of the first NMOS transistor M 1 . A fourth NMOS transistor M 4  has a source connected to a drain of the second NMOS transistor M 2 , a gate connected to a drain of the third NMOS transistor M 3  and a drain connected to a gate of the third NMOS transistor M 3 . A fifth NMOS transistor M 5  has a source connected to the drain of the third NMOS transistor M 3 , a gate connected to the sensing control signal SEN and a drain connected to the power supply voltage level VDD, wherein the sensed and amplified signal V 1  is outputted at an output node N 1  between the fifth NMOS transistor M 5  and the third NMOS transistor M 3 . A sixth NMOS transistor M 6  has a source connected to the drain of the fourth NMOS transistor M 4 , a gate connected to the sensing control signal SEN and a drain connected to the power supply voltage level VDD. 
     At this time, the first and second NMOS transistors M 1  and M 2  operate at a linear region, the third and fourth NMOS transistors M 3  and M 4  operate in a saturation region and a linear region, and the fifth and sixth NMOS transistors M 5  and M 6  operate in a saturation region. 
     When the read bit line is “high”, a current direction of the input signal IN is toward the current-direction sensing and amplifying unit  200  (in the direction of arrow “A”). Accordingly, the first NMOS transistor M 1  keeps a drain voltage level higher than the ground voltage level GND, so that a gate-source voltage V gs  of the third NMOS transistor M 3  is reduced. As a result, a first current I 1  flowing through the fifth and third NMOS transistors M 5  and M 3  is also reduced. Additionally, a voltage at the output node N 1  is increased to compensate the reduced first current I 1 . 
     At this time, when the voltage at the output node N 1  is increased, a voltage at a gate terminal of the fourth NMOS transistor M 4  is also increased, so that a second current I 2  flowing through the sixth and fourth NMOS transistors M 6  and M 4  is increased. Accordingly, a voltage level at a drain terminal of the fourth NMOS transistor M 4  is lowered to compensate the increased second current I 2 . At this time, since the third and fourth NMOS transistors M 3  and M 4  is in a linear region, even slight voltage change can cause the current to change very large. Therefore, voltage changes at the output node N 1  and a node N 2  accelerate the current changes, thereby completing very fast the amplification of the input signal IN. That is, when a voltage at the output node N 1  is increased, the gate-source voltage V gs  of the fourth NMOS transistor M 4  is increased more, so that the second current I 2  flows much more. Accordingly, the voltage at the output node N 1  is lowered more, so that the gate-source voltage V gs  of the third NMOS transistor M 3  is lowered much more. Therefore, the first current I 1  is decreased, resulting in increase of the voltage at the output node N 1 . 
     Consequently, even when the input signal IN changes slightly, the voltage at the output node N 1  changes very large. 
     One the other hand, when the read bit line is “low”, a current direction of the input signal IN is outward the current-direction sensing and amplifying unit  200  (in the direction of arrow “B”). Accordingly, the first NMOS transistor M 1  keeps a drain voltage level lower than the ground voltage level GND, so that a gate-source voltage V gs  of the third NMOS transistor M 3  is increased. As a result, a first current I 1  flowing through the fifth and third NMOS transistors M 5  and M 3  is also increased. Additionally, a voltage at the output node N 1  becomes decreased. 
     At this time, when the voltage at the output node N 1  is reduced, a voltage at a gate terminal of the fourth NMOS transistor M 4  is also reduced, so that a second current I 2  flowing through the sixth and fourth NMOS transistors M 6  and M 4  is decreased. Accordingly, a voltage level at a drain terminal of the fourth NMOS transistor M 4  is increased. That is, when a voltage at the output node N 1  is decreased, the gate-source voltage V gs  of the fourth NMOS transistor M 4  is decreased more, so that the second current I 2  is decreased much more. Accordingly, the voltage at the output node N 1  is increased more, so that the gate-source voltage V gs  of the third NMOS transistor M 3  is increased much more. Therefore, the first current I 1  is increased, resulting in decrease of the voltage at the output node N 1 . 
     FIG. 4 is a circuit diagram illustrating the voltage level shift unit shown in FIG. 2 according to an embodiment of the present invention. 
     Referring to FIG. 4, a bias voltage supplying unit  301  supplies a bias voltage BIAS in response to a reference voltage V ref  inputted from an external circuit. A level shifting unit  302  shifts the voltage level of the sensed and amplified signal V 1  in response to the bias voltage BIAS to output the voltage shifted signal V 2 . 
     The bias voltage supplying unit  301  includes a first PMOS transistor M 25  having a source connected to the power supply voltage VDD and a gate receiving a bias voltage BIAS, a second PMOS transistor M 26  having a source connected to a drain of the first PMOS transistor M 25  and a gate receiving the reference voltage V ref , a first NMOS transistor M 27  having a drain connected to a drain of the second PMOS transistor M 26  and a gate receiving the reference voltage V ref , a second NMOS transistor M 28  connected between the first NMOS transistor M 27  and the ground voltage level GND, whose gate terminal receives the bias voltage BIAS. The bias voltage BIAS is outputted from a third common node N 3  between the second PMOS transistor M 26  and the first NMOS transistor M 27 . At this time, the reference voltage V ref  is applied from a typical reference voltage generator, thereby keeping a constant level without an influence of voltage, temperature, processors and so on. 
     The level shifting unit  302  includes a third PMOS transistor M 21  having a source connected to the power supply voltage VDD and a gate receiving the bias voltage BIAS, a fourth PMOS transistor M 22  having a source connected to a drain of the third PMOS transistor M 21  and a gate receiving the sensed and amplified signal V 1 , a third NMOS transistor M 23  having a drain connected to the drain of the fourth PMOS transistor M 22  and a gate receiving the sensed and amplified signal V 1 , a fourth NMOS transistor M 24  connected between the third NMOS transistor M 23  and the ground voltage level GND, whose gate receives the bias voltage BIAS. The output of the level shifting unit  302 , i.e., the level shifted signal V 2  is outputted from a common node N 4  between the fourth PMOS transistor M 22  and the third NMOS transistor M 23 . 
     FIG,  5  is a simulated waveform of the sensed and amplified signal and the level shifted signal. It can be seen that the level shifted voltage V 2  reaches a full swing level with respect to the sensed and amplified signal V 1 . 
     An operation of the level shifting unit  302  will be described with reference to FIGS. 4 and 5. 
     The fourth PMOS transistor M 22  and the third NMOS transistor M 23  operate as an inverter to shift the sensed and amplified signal V 1 . The bias voltage BIAS is applied to the gates of the third PMOS transistor M 21  and the fourth NMOS transistor M 24 , wherein each transistor serves as a bias transistor. At this time, the bias voltage BIAS is very stable due to a negative feedback, so it can be kept in a constant logic threshold voltage. That is, while a typical logic threshold voltage is determined by a size of a transistor receiving an input signal, the logic threshold voltage according to the present invention is determined by the bias supplying unit  301 . Therefore, the level of the sensed and amplified signal V 1  can be shifted to a full swing level without an influence of the power supply voltage. 
     As a result, the current-direction sense amplifier according to the present invention performs the amplification operation by sensing the current direction, not the voltage level. Then, the amplified voltage level is shifted to the full swing level. Therefore, there is not the influence of the parasitic capacitance as well as the power supply voltage, preventing an error operation. Further, due to the swift amplification, the data access time can be greatly improved. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claim.