Abstract:
An input buffer of a semiconductor memory device includes a first differential amplifying portion including a first MOS transistor for receiving a first external input signal and a second MOS transistor for receiving a second external input signal. The voltage difference between the first and second external input signals is amplified and output as a first intermediate output voltage. A second differential amplifying portion includes a third MOS transistor for receiving the first external input signal and a fourth MOS transistor for receiving the second external input signal. The voltage difference between the first and second external input signals are amplified and output as a second intermediate output voltage. The first intermediate output of the first amplifying portion is combined with the second intermediate output of the second amplifying portion and the combined result is output as an output signal. The input buffer is less susceptible to fluctuations in ground and supply voltage levels due to noise, and the set-up time and hold time margins of the output signal are improved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device, and more particularly, to an input buffer for a semiconductor memory device. 
     2. Description of the Related Art 
     Semiconductor memory devices commonly include input buffers for converting the voltage level of a signal input from an external circuit to a voltage level suitable for an internal circuit. The input buffer operates to correctly detect the voltage level of the external signal to allow the semiconductor memory device to operate within normal parameters. 
     FIG. 1 is a circuit diagram of an N-type input buffer  101  of a conventional semiconductor memory device. Referring to FIG. 1, a conventional N-type semiconductor memory device  101  includes an NMOS transistor  111  for receiving external data IN, an NMOS transistor  112  for receiving a reference voltage Vref, a current mirror  131  constituted of PMOS transistors  121  and  122 , a PMOS transistor  123  for providing a supply voltage Vdd to the current mirror  131  in response to an external control signal PBPUB, and an inverter  141  for inverting data from a node N 1  and for outputting output data OUT of the N-type input buffer  101 . 
     In the case where the external input data IN is logic high in the N-type input buffer  101 , assuming that there is noise present in a ground voltage Vss, it takes longer for the data output from the node N 1  to transition from logic high to logic low due to the noise. Therefore, the length of time, or “skew”, for the data output from the node N 1  to transition from logic high to logic low, i.e. “high-voltage skew”, becomes larger. Accordingly, the set-up time and hold time margins of the data OUT output of the N-type input buffer  101  are reduced. 
     FIG. 2 is a circuit diagram of a P-type input buffer of a conventional semiconductor memory device. Referring to FIG. 2, a conventional P-type input buffer  201  includes a PMOS transistor  211  for receiving external data, a PMOS transistor  212  for receiving a reference voltage, a current mirror  231  constituted of NMOS transistors  221  and  222 , a PMOS transistor  213  for providing a supply voltage Vdd to the PMOS transistors  211  and  212  in response to the external control signal PBPUB, and an inverter  241  for inverting data from a node N 2  and for outputting the output data OUT of the P-type input buffer  201 . 
     In the case where the external data IN is logic low in the P-type input buffer  201 , assuming the presence of noise in the supply voltage Vdd, it takes longer for the data output from the node N 2  to transition from logic low to logic high due to the noise. Therefore, the skew time for the data output from the node N 2  to transition from logic low to logic high, i.e. “low-voltage skew”, becomes larger. Accordingly, the set-up time and hold time margins of the data OUT output of the P-type input buffer  201  are reduced. 
     As mentioned above, according to the conventional technology, since the high-voltage skew or low-voltage skew of the data OUT output from the input buffers  101  and  201  is relatively larger, the set-up time and hold time margins of the data OUT are reduced. Furthermore, it is increasingly difficult to reduce the skew of the data OUT as the trend toward ever-lower supply voltages Vdd continues. 
     SUMMARY OF THE INVENTION 
     To address the above-mentioned limitations, it is an object of the present invention to provide an input buffer for a semiconductor memory device by which it is possible to reduce the skew of output data. 
     It is another object of the present invention to provide an input buffer for a semiconductor memory device by which it is possible to reduce the skew of output data in a configuration that is amenable to use with semiconductor circuits of ever-lowering supply voltages. 
     Accordingly, to achieve the above objects, there is provided an input buffer for a semiconductor memory device. The input buffer includes a first differential amplifying portion including a first MOS transistor for receiving a first external input signal and a second MOS transistor for receiving a second external input signal. The voltage difference between the first and second external input signals is amplified and output as a first intermediate output voltage. A second differential amplifying portion includes a third MOS transistor for receiving the first external input signal and a fourth MOS transistor for receiving the second external input signal. The voltage difference between the first and second external input signals are amplified and output as a second intermediate output voltage. The first intermediate output of the first amplifying portion is combined with the second intermediate output of the second amplifying portion and the combined result is output as an output signal. 
     In a preferred embodiment, the first and second MOS transistors comprise NMOS transistors and the third and fourth MOS transistors comprise PMOS transistors. 
     The first differential amplifying portion preferably further comprises a first current mirror activated by the output of the second MOS transistor, for providing a supply voltage to the first and second MOS transistors. The first current mirror is preferably comprised of a plurality of PMOS transistors. 
     The second differential amplifying portion preferably further comprises a second current mirror activated by the output of the fourth MOS transistor, for providing a ground voltage to the third and fourth MOS transistors. The second current mirror is preferably comprised of a plurality of NMOS transistors. 
     Either the first external signal or the second external signal may comprise a reference voltage. 
     The input buffer of the present invention is less susceptible to fluctuations in ground and supply voltage levels due to noise, and the set-up time and hold time margins of the output signal are improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of preferred embodiments 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. 
     FIG. 1 is a circuit diagram of an N-type input buffer for a conventional semiconductor memory device. 
     FIG. 2 is a circuit diagram of a P-type input buffer for a conventional semiconductor memory device. 
     FIG. 3 is a circuit diagram of an input buffer of a semiconductor memory device according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An input buffer of a semiconductor memory device according to a preferred embodiment of the present invention will be described with reference to FIG.  3 . An input buffer  301  includes a first differential amplifier  311  and a second differential amplifier  312 . The first and second differential amplifiers  311  and  312  each receive first and second external input signals Vin 1  and Vin 2  and generate intermediate output signals Vout 1  and Vout 2 , respectively. The output signal Vout of the input buffer  301  is obtained by combining, or summing, the output signals Vout 1  and Vout 2 . 
     The first differential amplifier  311  includes a first current mirror  341  and first and second NMOS transistors  321  and  322 . The first NMOS transistor  321  is activated or deactivated (turned on or off) by the first external signal Vin 1 . Namely, for example when the first external signal Vin 1  is logic high, the first NMOS transistor  321  is turned on and lowers the voltage of node N 3  to the level of the ground voltage Vss. When the first external signal Vin 1  is logic low, the first NMOS transistor  321  is turned off. The second NMOS transistor  322  is turned on or off by the second external signal Vin 2 . When the voltage level of the second external signal Vin 2 , which is input to the second NMOS transistor  322 , is higher than that of the first external signal Vin 1 , the second NMOS transistor  322  conducts more current than the first NMOS transistor  321 . Accordingly, the voltage of node N 4  is lowered to the ground voltage level. When the voltage level of the second external signal Vin 2  is lower than that of the first external signal Vin 1 , the second NMOS transistor  322  is turned off. Accordingly, the voltage of node N 4  is placed in a floating state. 
     The first current mirror  341  includes PMOS transistors  333  and  334  and is connected to the first and second NMOS transistors  321  and  322  as shown. The first current mirror  341  is turned on or off by the voltage level of node N 4 . Namely, when the second NMOS transistor  322  is turned on and the voltage of node N 4  is lowered to the level of the ground voltage Vss, the PMOS transistors  333  and  334  are turned on and apply the supply voltage Vdd to the node N 3 . When the second NMOS transistor  322  is turned off, the node N 4  is floated and therefore is in a state of high impedance. Accordingly, the PMOS transistors  333  and  334  are turned off, and thus the supply voltage Vdd is not applied to node N 3 . 
     The operation of the first differential amplifier  311  will now be described. When the voltage of the first external input signal Vin 1  is higher than that of the second external input signal Vin 2 , the first NMOS transistor  321  conducts more current than the second NMOS transistor  322 . Accordingly, the voltage of node N 3  is lowered to the level of the ground voltage Vss. However, since the voltage of node N 4  is much higher than the ground voltage Vss, the PMOS transistors  333  and  334  are turned off. Therefore, the output signal Vout 1  becomes logic low. When the voltage of the second external signal Vin 2  is higher than that of the first external signal Vin 1 , the second NMOS transistor  322  conducts more current than the first NMOS transistor  321 . Accordingly, the voltage of node N 4  is lowered to the level of the ground voltage Vss and the voltage of node N 3  is much higher than that of the ground voltage Vss. Therefore, the PMOS transistors  333  and  334  arc turned on. Accordingly, since the voltage of the node N 3  is increased to the level of the supply voltage Vdd, the output signal Vout 1  becomes logic high. 
     The response of the first differential amplifier  311  is greatly affected by noise present in the ground voltage Vss. However, it is only slightly affected by noise present in the supply voltage Vdd. 
     The second differential amplifier  312  includes a second current mirror  342  and first and second PMOS transistors  331  and  332 . The first PMOS transistor  331  is turned on or off by the first external signal Vin 1 . Namely, when the first external signal Vin 1  is logic low, the first PMOS transistor  331  is turned on, thus increasing the voltage of node N 5  to the level of the supply voltage Vdd. When the first external signal Vin 1  is logic high, the first PMOS transistor  331  is turned off. The second PMOS transistor  332  is turned on or off by the second external signal Vin 2 . Namely, when the second external signal Vin 2  is logic low, the second PMOS transistor  332  is turned on, thus increasing the voltage level of node N 6  to the level of the supply voltage Vdd. When the second external signal Vin 2  is logic high, the second PMOS transistor is turned off. 
     The second current mirror  342  includes NMOS transistors  323  and  324  and is connected to the first and second PMOS transistors  331  and  332 . The second current mirror  342  is turned on or off by the voltage level of node N 6 . Namely, when the second PMOS transistor  332  is turned on and the voltage level of node N 6  is increased to the level of the supply voltage Vdd, the NMOS transistors  323  and  324  are turned on, thus lowering the voltage of node N 5  to the level of the ground voltage Vss. When the second PMOS transistor  332  is turned off, the node N 6  is floated and is in the state of high impedance. Accordingly, the NMOS transistors  323  and  324  are turned off, and thus the ground voltage level Vss is not applied to node N 5 . 
     The operation of the second differential amplifier  312  will now be described. When the voltage of the first external signal Vin 1  is higher than that of the second external signal Vin 2 , the second PMOS transistor  332  conducts more current than the first PMOS transistor  331 . Thus, the voltage of node N 6  is lowered to the level of the ground voltage Vss. Accordingly, the intermediate output signal Vout 2  becomes logic low. When the voltage of the second external signal Vin 2  is higher than that of the first external signal Vin 1 , the first PMOS transistor  331  conducts more current than the second PMOS transistor  332 . Thus, since node N 6  is floated and is in the state of high impedance, the NMOS transistors  323  and  324  are turned off. Since the supply voltage Vdd is applied to node N 5  in this state, the voltage of the node N 5  is increased to the level of the supply voltage Vdd. Accordingly, the intermediate output signal Vout 2  becomes logic high. 
     The response of the second differential amplifier  312  is greatly affected by noise present in the supply voltage Vdd, however, it is only slightly affected by noise present in the ground voltage Vss. 
     The overall operation of the input buffer  301  will now be described with reference to FIG.  3 . When the voltage of the first external input signal Vin 1  is higher than that of the second external input signal Vin 2 , the first NMOS transistor  321  and the second PMOS transistor  332  are turned on. Then, since the voltage of node N 3  is lowered to the level of the ground voltage Vss, the intermediate output signal Vout 1  becomes logic low and the voltage of node N 6  is increased to the level of the supply voltage Vdd. When the voltage of node N 6  is increased to the level of the supply voltage Vdd, the NMOS transistors  323  and  324  are turned on. Therefore, the voltage of node N 5  is lowered to the level of the ground voltage Vss. Accordingly, the intermediate output signal Vout 2  becomes logic low. Therefore, the generated output signal Vout is logic low, since both of the output signals Vout 1  and Vout 2  are logic low. 
     When the voltage of the second external input signal Vin 2  is higher than that of the first external input signal Vin 1 , the second NMOS transistor  322  and the first PMOS transistor  331  are turned on. Then, since the voltage of node N 5  is increased to the level of the supply voltage Vdd, the second intermediate output signal Vout 2  becomes logic high and the voltage of node N 4  is lowered to the level of the ground voltage Vss. When the voltage of node N 4  is lowered to the level of the ground voltage Vss, the PMOS transistors  333  and  334  are turned on. Therefore, the voltage of node N 3  is increased to the level of the supply voltage Vdd. Accordingly, the first intermediate output signal Vout 1  becomes logic high. Therefore, the generated output signal Vout is logic high since both of the intermediate output signals Vout 1  and Vout 2  are logic high. 
     In alternative embodiments, either the first external input signal Vin 1  or the second external input signal Vin 2  can be replaced by the reference voltage. 
     When noise is present in the supply voltage Vdd, the voltages Vgs between the gates and sources of the first and second PMOS transistors  331  and  332  change in the second differential amplifier  312 , and the drain currents of the first and second PMOS transistors  331  and  332  change. Since only the voltages Vds between the drains and sources of the second NMOS transistor  322  and the PMOS transistor  333  change, and the voltage Vgs between the gate and source of the first NMOS transistor  321  does not change in the first differential amplifier  311 , the drain currents of the first and second PMOS transistors  331  and  332  of the second differential amplifier do not change. Therefore, the variation of the output signal Vout as a result of noise is reduced to half that compared to the case where only the second differential amplifier  312  is employed. Namely, the change in voltage level of the output signal Vout is slight although noise is present in the supply voltage Vdd. 
     When noise is present in the ground voltage Vss, in the first differential amplifier  311 , since the gate-source voltages Vgs of the first and second NMOS transistors  321  and  322  change, the drain currents of the first and second NMOS transistors  321  and  322  change. In the second differential amplifier  312 , since only the drain-source voltages Vds of the second PMOS transistor  332  and the NMOS transistor  323  change and the gate-source voltage of the first PMOS transistor  331  does not change, the drain currents of the first and second NMOS transistors  321  and  322  do not change. Therefore, the change of the output signal Vout is reduced to half that compared to the case where only the first differential amplifier  311  is used. Namely, the change of the output signal Vout is slight although the noise is generated in the ground voltage Vss. 
     According to the input buffer  301  shown in FIG. 3, when noise is present in the ground voltage Vss, the high-voltage skew of the first differential amplifier  311  is reduced, which is compensated for by the second differential amplifier  312 . When noise is generated in the supply voltage Vdd, the low-voltage skew of the second differential amplifier  312  is reduced, which is compensated for by the first differential amplifier  311 . Therefore, since the high-voltage skew and the low-voltage skew of the output signal Vout output from the input buffer  301  are improved, the set-up time and hold time margins of the output signal Vout are improved. In particular, although the supply voltage Vdd applied to the input buffer  301  is low, the set-up time and hold time margins of the output signal Vout output from the input buffer  301  are to slightly affected and are improved. 
     As mentioned above, according to the present invention, since the high-voltage skew and the low-voltage skew of the output signal Vout are reduced significantly, although noise is generated in the supply voltage Vdd and the ground voltage Vss, the set-up time and hold time margins of the output signal Vout are improved. In particular, the high-voltage skew and the low-voltage skew of the output signal Vout are reduced significantly. These advantages are realized even as supply voltages are lowered according to current trends in semiconductor device technology. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.