Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 10/328,796, filed Dec. 24, 2002, entitled “Input Buffer of Differential Amplification Type in Semiconductor Device,” now U.S. Pat. No. 6,812,743, which is hereby incorporated by reference. 

   TECHNICAL FIELD 
   The present disclosure relates to a semiconductor memory device, and more particularly, to an input buffer having a differential amplifier in the semiconductor memory device. 
   BACKGROUND 
   A semiconductor device is fabricated by semiconductor technologies such as a silicon wafer forming technology, and a logic design technology and so on. A final product through a semiconductor fabricating process is a chip of a plastic package type. The chip may be fabricated with different logics and functions according to a purpose for use thereof. Most of important semiconductor chips configuring an application system are assembled to a printed circuit board (PCB) and several driving voltages for driving the chips are supplied thereto. 
   All semiconductor devices including semiconductor memory operate according to input/output signals having specific purposes. Namely, operations of the semiconductor device are carried out in response to a combination signal of input/output signals. An output signal of a certain semiconductor device may be used as an input signal of another semiconductor device in the same system. 
   An input buffer is a part buffering a signal applied from an external circuit and providing the buffered signal into the semiconductor device and there is a static input buffer as the simplest type. The static input buffer consists of a PMOS transistor and an NMOS transistor connected in series between a power supply voltage and a ground voltage. The static input buffer has an advantage that the configuration is very simple, however, tolerance against a noise is weak, and, an input signal having large amplitude is required. Namely, the large amplitude between a logic high level and a logic low level of the input signal is required. Therefore, the static input buffer is not proper to be applied to a device, which the amplitude of input signals is small or a high operation frequency is required. Accordingly, an input buffer of a differential amplification type, which has a high tolerance against a noise and is easy to be applied to a high frequency, is suggested. 
     FIG. 1  is a schematic diagram illustrating a conventional input buffer of a differential amplification type. In particular, the input buffer  100  of the differential amplification type includes a differential amplification unit  11  and an internal buffer  12 . The differential amplification unit  11  compares an input signal applied to a positive terminal (+) thereof with a reference voltage signal applied to a negative terminal (−) thereof. The internal buffer  12  buffers an output from the differential amplification unit  11  and issues an internal signal INT. 
     FIG. 2  is a circuit diagram illustrating the differential amplification unit  11  in  FIG. 1 . As shown, the differential amplification unit  11  includes a differential amplifier  11 A and an inverter  11 B. The differential amplifier  11 A includes a first NMOS transistor MN 1 , a second NMOS transistor MN 2 , a first PMOS transistor MP 1 , and a second PMOS transistor MP 2  to form a current mirror. The reference voltage Vref is applied to a gate of the first NMOS transistor MN 1 , and the input signal IN is applied to a gate of the second NMOS transistor MN 2 . The differential amplifier  11 A further includes a third NMOS transistor MN 3 , which is coupled between ground and the first and second transistors MN 1  and MN 2 . A buffer enable signal is applied to the gate of the transistor MN 3  for a bias. Third and fourth PMOS transistors MP 3  and MP 4  forming a current mirror are coupled to the first and second transistors MP 1  and MP 2 , respectively, in parallel. The buffer enable signal buffer_en is applied to the gates of the third and fourth PMOS transistors MP 3  and MP 4 . 
   The inverter  11 B includes a pull-up PMOS transistor MP 5  and a pull-down NMOS transistor MN 4 . An output signal of the differential amplifier  11 A is commonly applied to the gates of the pull-up PMOS transistor MP 5  and a pull-down transistor MN 4 . 
     FIG. 3  is a circuit diagram illustrating the internal buffer  12  in  FIG. 1 . The internal buffer  12  includes a first inverter INV 1  and a second inverter INV 2  coupled in series. The number of the inverters in the internal buffer  12  may be adjusted. 
   For example, an operation of the input buffer of the differential amplification type will be described by referring to  FIGS. 2 and 3 . When the input signal IN is applied to an input terminal of the differential amplifier  11 A, the differential amplifier  11 A will be operated according to states whether the voltage level of the input signal IN is higher or lower than that of the reference voltage signal Vref because since the voltage of the input signal IN is higher or lower than the reference voltage signal Vref. The reference voltage signal Vref is a static voltage, which always maintains a constant voltage level. The reference voltage signal Vref may be provided through a specific input pin from an external circuit or may be generated in the semiconductor device. 
   A constant current I 1  flows at the first NMOS transistor MN 1  receiving the reference voltage signal Vref. A current flowing at the second NMOS transistor MN 2 , which is symmetrically configured with the first NMOS transistor MN 1 , is determined by the voltage level of the input signal IN. In particular, the differential amplifier  11 A determines a voltage level of an output node N 1  according to a comparison result between the currents I 1  and I 2 . 
   If the current I 2  is relatively higher than the current I 1 , the voltage level of the output node N 1  decreases. Finally, a signal of a logic high level is outputted via the inverter  11 B. In this case, the input buffer  100  issues a signal of a logic high level. On the other hand, if the current I 2  is relatively lower than the current I 1 , an output of the inverter  11 B becomes a signal having a logic low level. In this case, the input buffer issues a signal of a logic low level. The buffer enable signal buffer_en is disabled to a logic low level at a power-down mode or a self-refresh mode so that the input buffer is inactivated to a precharge state and, in other modes, the buffer enable signal buffer_en is adjusted to a logic high level so that the input buffer is activated. 
   The input buffer  100  basically plays a role of an interface for transmitting signals between an internal circuit and an external circuit of a semiconductor device. Therefore, an input buffer configured to rapidly transmit signal variation at an input terminal is required. The semiconductor device performing an amplification operation with high gain is suggested to rapidly transmit signals. 
   Because the reference voltage signal Vref having a constant potential level is always supplied to the gate of the first NMOS transistor MN 1  in the input buffer  100  of the differential amplification type illustrated in  FIGS. 1 to 3 , the current I 1  flowing at the first NMOS transistor MN 1  is fixed. Therefore, high amplitude of the input signal IN is required for a rapid operation. Generally, however, small amplitude of an input signal is required to improve signal integration and rapidly vary a voltage level of a signal in a system operated with a high speed. 
   SUMMARY OF THE DISCLOSURE 
   An input buffer of a differential amplification type and a method for improving a signal transmission speed without increase of amplitude of an input signal are described herein. An input buffer for buffering an input signal to provide an internal signal in a semiconductor device includes: a reference voltage adjusting unit configured to receive a reference voltage and to output a first reference signal having a potential level higher than that of the reference voltage and a second reference signal having a potential level lower than that of the reference voltage; a multiplexing unit configured to selectively output one of the first and second reference signals as a newly selected reference signal in response to the internal signal; a differential amplification unit configured to amplify the input signal by comparing the input signal with the newly selected reference signal outputted to generate an amplified input signal; and an internal buffer configured to output the internal signal by buffering the amplified input signal. 
   Alternatively, an input buffer of a differential amplification type for buffering an input signal to provide an internal signal in a semiconductor device includes: a multiplexing unit configured to selectively output a first reference signal having a higher potential level than that of a reference voltage or a second reference signal having a lower potential level than that of the reference voltage in response to the internal signal; a differential amplification unit configured to amplify an input signal by comparing the reference signal selectively outputted from the multiplexing means with the input signal; and an internal buffer configured to output the internal signal by buffering the amplified input buffer. 
   Further, a method for outputting an internal signal in an input buffer of a differential amplification type for use in a semiconductor device comprises the steps of: selectively outputting a first reference signal having a potential level higher than that of a reference voltage or a second reference signal having a potential level higher than that of the reference voltage in response to the internal signal; amplifying an input signal by detecting a phase of the input signal by comparing the input signal with the selectively outputted reference signal; and buffering and outputting a amplified signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The disclosure will be described in terms of several embodiments to illustrate its broad teachings. Reference is also made to the attached drawings. 
       FIG. 1  is a schematic diagram illustrating a conventional input buffer of a differential amplification type; 
       FIG. 2  is a circuit diagram illustrating the differential amplification unit  11  in  FIG. 1 ; 
       FIG. 3  is a circuit diagram illustrating the internal buffer  12  in  FIG. 1 ; 
       FIG. 4  is a block diagram illustrating an input buffer of a differential amplification type; 
       FIG. 5  is a circuit diagram illustrating the reference voltage adjusting unit  23  in  FIG. 4 ; and 
       FIG. 6  is a circuit diagram illustrating the multiplexer  24  in  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   The present disclosure will be described in detail with reference to the accompanying drawings. In particular, an input buffer of a semiconductor memory device capable of improving a signal transmission speed without increase of voltage level variation for an input signal is described herein. 
   Referring to  FIG. 4 , the input buffer  200  includes a differential amplification unit  21 , an internal buffer  22 , a reference voltage adjusting unit  23  and a multiplexer  24 . The reference voltage adjusting unit  23  receives a reference voltage signal Vref and generates a first reference voltage signal Vref+ and a second reference voltage signal Vref−. The first reference voltage signal Vref+ has a higher voltage level than that of the reference voltage Vref whereas the second reference voltage signal Vref− has a lower voltage level than that of the reference voltage Vref. 
   The multiplexer  24  selectively outputs the first reference signal Vref+ or the second reference signal Vref− in response to an internal signal INT, which is an output signal of the input buffer  22 . The selectively outputted first or second reference signal Vref+ or Vref− is applied to the negative terminal (−) of the differential amplification unit  21 , and an input signal IN is applied to a positive terminal (+) of the differential amplification unit  23 . 
   When the input signal IN of a logic high level is supplied to the input buffer  200 , outputs of the differential amplification unit  21  and the internal buffer  22  become a signal of a logic high level. The internal signal INT is the output of the internal buffer  22 , which is used as a selection signal of the multiplexer  24 . 
   If the output signal of the internal buffer  22  having a logic high voltage level is applied to the multiplexer  24 , the multiplexer  24  outputs the first reference voltage signal Vref+ from the outputs of the reference voltage adjusting unit  23 . On the other hand, if the output signal of the internal buffer  22  having a logic low voltage level is applied to the multiplexer  24 , the second reference voltage signal Vref− is outputted. 
   A voltage level of the input signal IN applied to the positive terminal (+) of the differential amplification unit  21  may become an opposite voltage level. If the reference voltage signal is adjusted in advance, the voltage level difference between the input signal and the reference signal (Vref+ or Vref−) is increased when the voltage level of the input signal IN is adjusted to an opposite voltage level and then applied to the positive terminal (+) of the differential amplification unit  21  so that a voltage level of the output signal of the input buffer may be rapidly adjusted to the opposite voltage level. If the input signal of a logic high level is applied to the differential amplification unit  21 , the first reference signal Vref+ is selected as the reference signal. Therefore, if the voltage level of the input signal is adjusted to a logic low voltage level, the difference between the first reference signal Vref+ and the input signal increases. 
   Referring to  FIG. 5 , the reference voltage adjusting unit  23  includes a first reference voltage adjusting unit  23 A and a second reference voltage adjusting unit  23 B. The first reference voltage adjusting unit  23 A includes a first resistance R 1 , a first NMOS transistor T 1 , a second NMOS transistor T 2 , and a second resistance R 2  connected in series between a power supply voltage VDD and a ground GND. Two NMOS transistors T 1  and T 2  are diode-coupled, and the reference voltage Vref is applied to a junction of the two NMOS transistors T 1  and T 2 . Two resistances R 1  and R 2  are used for a current source flowing through the first and second NMOS transistors T 1  and T 2 . A first voltage Vref+Vt, which is higher than the reference voltage Vref by a threshold voltage Vt of the first NMOS transistor T 1 , is outputted from a junction of the first resistance R 1  and the first NMOS transistor T 1 . A second voltage Vref−Vt, which is lower than the reference voltage Vref by a threshold voltage Vt of the second NMOS transistor T 2 , is outputted from a junction of the second NMOS transistor T 2  and the second resistance R 2 . 
   The second reference voltage adjusting unit  23 B includes third and fourth resistances R 3  and R 4  coupled in series between the first voltage Vref+Vt and the reference voltage Vref, and fifth and sixth resistances R 5  and R 6  coupled in series between the reference voltage Vref and the second voltage Vref−Vt. The third and fourth resistances R 3  and R 4  may operate as a voltage divider. The first reference signal, which is outputted from a node between the third and fourth resistances R 3  and R 4 , is determined by the following equation.
 
 Vref+=Vref+[ ( Vref+Vt )− Vref]*R 4/( R 3+ R 4)→  (1)
 
   The fifth and sixth resistances R 5  and R 6  are also operated as a voltage divider. The second reference signal Vref−, which is outputted from a junction of the fifth and sixth resistances R 5  and R 6 , is determined by the following equation.
 
 Vref−= ( Vref−Vt )+[ Vref− ( Vref−Vt )]* R 6/( R 5+ R 6)→  (2)
 
   Referring to the above equations (1) and (2), the levels of the first and second reference signals Vref+ and Vref− are determined by a predetermined resistance ratio. If a resistance value of the third resistance R 3  is identical to that of the fourth resistance R 4 , the equation (1) becomes the following equation.
 
 Vref+=Vref+Vt/ 2 →  (3)
 
   Further, if a resistance value of the fifth resistance R 5  is identical to that of the sixth R 6 , the equation (2) becomes the following equation.
 
 Vref−= ( Vref−Vt )+ Vt/ 2(  (4)
 
   Even if there is a plurality of input buffers in one semiconductor device, it is not necessary to set the reference voltage adjusting unit  23  per each input buffer. That is, one reference voltage adjusting unit  23  may be configured to one semiconductor device. On the other hand, the number of the multiplexer  24  is determined according to the number of the input buffers. 
   As shown in  FIG. 6 , the multiplexer  24  includes a first transmission gate TG 1  configured to output the first reference signal Vref+ in response to the internal signal INT, a second transmission gate TG 2  configured to output the second reference signal Vref− in response to the internal signal INT, and a third inverter INV 3  configured to generate an inverted signal of the internal signal INT. Herein, the first and second transmission gates TG 1  and TG 2  are configured with an opposite type so that one of reference voltage signals Vref+ and Vref− is selectively outputted. 
   If the internal signal INT is a logic low level, the second transmission gate TG 2  is turned on and the first transmission gate TG 1  is turned off so that the second reference signal Vref− having a voltage level lower than that of the reference voltage Vref is outputted. On the contrary, if the internal signal INT is a logic high level, the first transmission gate TG 1  is turned on and the second transmission gate TG 2  is turned off so that the first reference signal Vref+ having a voltage level higher than that of the reference voltage Vref is outputted. Even if the differential amplifier of an NMOS bias type is used, a differential amplifier of a PMOS bias type may be applied. 
   To generate the reference voltages Vref+ and Vref−, an example is introduced according to the equations (1) and (2). However, any reference voltage adjusting unit configured to generate the reference voltage signals, which have a higher or lower voltage level than that of the reference voltage Vref, may be applied to the input buffer  200 . 
   Accordingly, when the input signal of a logic high level is applied to the input buffer, the first reference voltage signal Vref+ is selected by the internal signal as the reference voltage of the differential amplifier. Therefore, when the voltage level of the input signal is adjusted to a logic low level because the voltage level difference between the input signal and the first reference voltage signal Vref+ is high, a signal transmission speed effectively is improved in the input buffer so that the input buffer may be applied to the high frequency operation of the semiconductor device. 
   Many changes and modifications to the embodiments described herein could be made. The scope of some changes is discussed above. The scope of others will become apparent from the appended claims.

Technology Category: 5