CROSS-REFERENCES TO RELATED APPLICATION 
   The present application claims priority under 35 U.S.C. §119(a) to Korean application number 10-2007-0081026, filed on Aug. 13, 2007, in the Korean Patent Office, which is incorporated by reference in its entirety as if set forth in full. 
   BACKGROUND 
   1. Technical Field 
   The embodiments described herein relate to semiconductor circuit technology and, more particularly, to a differential amplifier and an input circuit using the same. 
   2. Related Art 
   A conventional semiconductor memory device includes a signal transferring unit for receiving and transferring a signal and a signal processing unit for processing the signal transferred from the signal transferring unit according to a prescribed operation. 
   A conventional signal processing unit is also called a core circuit. A conventional core circuit integrates different kinds of elements. In fact a conventional core circuit will includes as many elements as is possible within the limits of the associated design technology and the processing capacity of the semiconductor memory device. 
   For example, a conventional core unit includes an input circuit for transferring an input signal to the core unit, and an output circuit for outputting data from the core unit. 
   The input circuit receives various signals transferred from the outside, namely, an address signal, a clock signal, and data signal and transfers them to the core circuit positioned inside the semiconductor memory device. The output circuit outputs the data, which correspond to the received address. Therefore, the input circuit, which receives the data and address signals, should perform an accurate buffering operation to ensure reliable operation. 
   A conventional input circuit includes a differential amplifier having a MOS transistor (hereinafter, a transistor). The operational characteristic of the transistor can be changed by a PVT (process, voltage, temperature) fluctuation. Also, when the input circuit is included in a mobile device, a termination operation is not performed in order to minimize operating current and conserve power; however, forgoing the termination operation can result in a voltage change. This is because the termination operation controls the voltage levels of input signals ‘IN’ and ‘VREF’ to be within a prescribed range, to cope with the fluctuation of external high and low voltages (VDD) and (VSS), ‘. 
   As shown in  FIG. 1 , when the termination operation is performed, the input signals ‘IN’ and ‘VREF’ have a voltage swing that is bounded by the external high voltage (VDD). The input signal ‘IN’ can be a clock signal and the input signal ‘VREF’ can be out of phase with the clock signal. 
   Meanwhile, as shown in  FIG. 1 , when the terminal operation is not performed, the input signals ‘IN’ and ‘VREF’ swing in an abnormal voltage range that can be bounded by an external low voltage (VSS). In such situations, the middle voltage (Vmp) of the input signals ‘IN’ and ‘VREF’ is less than ½(VDD) because of the termination operation is not performed. AS a result, the gate-source voltage (Vgs) of a transistor that receives the input signals ‘IN’ and ‘VREF’ can be low as compared with the threshold voltage (Vth) of the transistor. When the gate-source voltage (Vgs) is lower than the threshold voltage (Vth), the transistor cannot operate, and thus the input circuit operates abnormally or cannot operate at all. 
   Although the duty ratio of the input signals ‘IN’ and ‘VREF’ is constant, if the input circuit operates abnormally or cannot operate at all, then the duty ratio of an output signal of the input circuit can be distorted. As shown in  FIG. 2 , when the middle voltage (Vmp) is less than 50 percent of the external voltage (VDD), the duty ratio of the output signal of the input circuit is rapidly distorted to be less than 50 percent. If the duty ratio is distorted, the margin of a setup/hold time is reduced. Eventually, the semiconductor memory device will produce an output error. 
   SUMMARY 
   A differential amplifier capable of performing a stable output operation regardless of a voltage fluctuation of received signals and an input circuit having the same are described herein. 
   According to one aspect, a differential amplifier comprises a plurality of first switching elements configured to output first differentially amplified signals through output terminals when a voltage levels of a first input signal and a second input signal are within a first range, and a plurality of second switching elements configured to output second differentially amplified signals through the output terminals when the voltage levels of the first input signal and the second input signal are within a second range. 
   According to another aspect, an input circuit comprises a first input unit for differentially amplifying a voltage level difference between a first input signal and a second input signal to output first or second differentially amplified signals by using a plurality of switching elements configured to selectively operate when a voltage level of the first input signal and the second input signal are within a first range or when the voltage level of the first input signal and the second input signal are within a second range, and a second input unit for differentially amplifying the first or second differentially amplified signals to output third differentially amplified signals. 
   These and other features, aspects, and embodiments are described below in the section entitled “Detailed Description.” 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features and other advantages of the subject matter of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a waveform diagram illustrating a change of input signals according to a voltage fluctuation; 
       FIG. 2  is a voltage-to-duty ratio graph of a conventional input circuit; 
       FIG. 3  is a circuit diagram illustrating an example input circuit according to one embodiment; and 
       FIG. 4  is a voltage-to-duty ratio graph for the input circuit of  FIG. 3 . 
   


   DETAILED DESCRIPTION 
     FIG. 3  is a diagram illustrating an example input circuit  101  configured in accordance with one example embodiment. As shown in  FIG. 3 , the input circuit  101  can include a first input unit  100  and a second input unit  200 . 
   The first input unit  100  can include a differential amplifier. The differential amplifier can include a first resistor R 1  and a second resistor R 2 , where each resistor can have one end connected to an external power source (VDD), a first amplifier circuit  110 , a second amplifier circuit  120  and a third transistor Q 3 . 
   The first amplifier circuit  110  can be configured to amplify a level difference between a first input signal ‘IN’ and a second input signal ‘VREF’ to output a first output signal ‘OINb’ and a second output signal ‘OREFb’ when the level of a middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is higher than that of a threshold voltage (Vth) of transistors that receive the first input signal ‘IN’ and the second input signal ‘VREF’, namely transistors Q 1  and Q 2  as described below 
   The first amplifier circuit  110  can include a first resistor R 1 , a second resistor R 2 , and first and second transistors Q 1  and Q 2 . A drain of the first transistor Q 1  can be connected to the other end of the first resistor R 1  and a gate of the first transistor Q 1  can receive the first input signal ‘IN’. A drain of the second transistor Q 2  can be connected to the other end of the second resistor R 2  and a gate of the second transistor Q 2  can receive the second input signal ‘VREF’. The first output signal ‘OINb’ can be output through a common node between the first resistor R 1  and the first transistor Q 1 . The second output signal ‘OREFb’ can be output through a common node between the second resistor R 2  and the second transistor Q 2 . 
   A drain of the third transistor Q 3  can be commonly connected to sources of the first transistor Q 1  and the second transistor Q 2 . A gate of the third transistor Q 3  can receive an enable signal ‘EN’ and a source of the third transistor Q 3  can be connected to a ground terminal (VSS). Each of the first to third transistors Q 1  to Q 3  can be a NMOS transistor. 
   The second amplifier circuit  120  can be configured to amplify a level difference between the first input signal ‘IN’ and the second input signal ‘VREF’ to output the first output signal ‘OINb’ and the second output signal ‘OREFb’ in case that the level of the middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is lower than that of the threshold voltage (Vth) of the fourth and fifth transistors Q 4  and Q 5 . 
   The second amplifier circuit  120  can include fourth and fifth transistors Q 4  and Q 5 . A source of the fourth transistor Q 4  can be connected to the common node between the second resistor R 2  and the second transistor Q 2 . A gate of the fourth transistor Q 4  can receive the first input signal ‘IN’, and a drain of the fourth transistor Q 4  can be connected to the drain of the third transistor Q 3 . A source of the fifth transistor Q 5  can be connected to the common node between the first resistor R 1  and the first transistor Q 1 , a gate of the fifth transistor Q 5  can be configured to receive the second input signal ‘VREF’, and a drain of the fifth transistor Q 5  can be connected to the drain of the third transistor Q 3 . Each of the fourth and fifth transistors Q 4  and Q 5  can be a PMOS transistor. 
   The second input unit  200  can be configured to amplify a level difference between the first output signal ‘OINb’ and the second output signal ‘OREFb’ to output a differentially amplified signal ‘DIFF_OUT’. The second input unit  200  can include sixth to twelfth transistors Q 6  to Q 12 . 
   Sources of the sixth and seventh transistors Q 6  and Q 7  can be connected to the external power source (VDD). A gate and a drain of the sixth transistor Q 6  can be commonly connected to a gate of the seventh transistor Q 7 . The sixth and seventh transistors Q 6  and Q 7  can constitute a current mirror. 
   A source of the eighth transistor Q 8  can be connected to the external power source (VDD), a gate of the eighth transistor Q 8  can receive the enable signal ‘EN’, and a drain of the eighth transistor Q 8  can be connected to the drain of the sixth transistor Q 6 . 
   A source of the ninth transistor Q 9  can be connected to the external power source (VDD), a gate of the ninth transistor Q 9  can receive the enable signal ‘EN’, and a drain of the ninth transistor Q 9  can be connected to the drain of the seventh transistor Q 7 . The eighth and ninth transistors Q 8  and Q 9  can constitute a precharge circuit configured to precharge the differentially amplified signal ‘DIFF_OUT’ to a high level when the enable signal ‘EN’ is inactive, e.g., at a low level. 
   A drain of the tenth transistor Q 10  can be connected to the drain of the sixth transistor Q 6  and a gate of the tenth transistor Q 10  can receive the second output signal ‘OREFb’. 
   A drain of the eleventh transistor Q 11  can be connected to the drain of the seventh transistor Q 7  and a gate of the eleventh transistor Q 11  can receive the first output signal ‘OINb’. 
   A drain of the twelfth transistor Q 12  can be commonly connected to sources of the tenth and eleventh transistors Q 10  and Q 11 , a gate of the twelfth transistor Q 12  can receive the enable signal ‘EN’, and a source of the twelfth transistor Q 12  can be connected to the ground terminal (VSS). The differentially amplified signal ‘DIFF_OUT’ can be output through a common node between the seventh and eleventh transistors Q 7  and Q 11 . 
   A signal ‘DIFF_OUTb’ which has opposite phase to that of the differentially amplified signal ‘DIFF_OUT’ can be required in a next stage following the input circuit  101 . Thus, depending on the embodiment, an inverter IV 1  can be connected to an output terminal of the second input unit  200  in order to invert the differentially amplified signal ‘DIFF_OUT’ to the signal ‘DIFF_OUTb’. 
   The operation of the input circuit  101  will not be described in detail, starting with the situation where the level of the middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is higher than that of the threshold voltage (Vth) of the first and second transistors Q 1  and Q 2 . 
   In such a situation, the first input signal ‘IN’ and the second input signal ‘VREF’ can be separately received through different pads. The first input signal ‘IN’ can be a clock signal ‘CLK’, and the second input signal ‘VREF’ can be an inverted clock ‘CLKB’, or generally can be a signal with a phase difference relative to the first input signal ‘IN’. 
   When the enable signal ‘EN’ is deactivated, e.g., at a low level, the third transistor Q 3  and the twelfth transistor Q 12  are turned off and the eighth transistor Q 8  and the ninth transistor Q 9  are turned on. 
   Since the third transistor Q 3  and the twelfth transistor Q 12  are turned off, the current path of the first input unit  100  and the second input unit  200  is blocked so that the operation of the input circuit is stopped. Since the eighth transistor Q 8  and the ninth transistor Q 9  are turned on, the differentially amplified signal ‘DIFF_OUT’ is precharged to a high level. 
   Meanwhile, when the enable signal ‘EN’ is activated, e.g., at a high level, the third transistor Q 3  and the twelfth transistor Q 12  are turned on and the eighth transistor Q 8  and the ninth transistor Q 9  are turned off. Since the third transistor Q 3  and the twelfth transistor Q 12  are turned on, a current path through the first input unit  100  and the second input unit  200  is opened. 
   The first input signal ‘IN’ can be supplied to the gates of the first and fourth transistors Q 1  and Q 4  and the second input signal ‘VREF’ can be supplied to the gates of the second and fifth transistors Q 2  and Q 5 . 
   A current, which is in proportion to the voltage difference between a gate-source voltage (Vgs) and a threshold voltage (Vth) of each of the first and second transistors Q 1  and Q 2  of the first amplifier circuit  110  flows through the first and second transistors Q 1  and Q 2 . 
   Meanwhile, since the level of a gate-source voltage (Vgs) of each of the fourth and fifth transistors Q 4  and Q 5  of the second amplifier circuit  120  is lower than that of the threshold voltage (Vth) of each of the fourth and fifth transistors Q 4  and Q 5 , the fourth and fifth transistors Q 4  and Q 5  turn off. 
   For example, if the voltage level of the first input signal ‘IN’ is higher than that of the second input signal ‘VREF’, then the magnitude of the current that flows through the first transistor Q 1  is larger than that of the current which flows through the second transistor Q 2 . 
   Since the magnitude of the current flowing through the first transistor Q 1  is larger than that of the current flowing through the second transistor Q 2 , the voltage level of the second output signal ‘OREFb’ is higher than that of the first output signal ‘OINb’. 
   Since the voltage level of the second output signal ‘OREFb’ is higher than that of the first output signal ‘OINb’, the magnitude of the current flowing through the tenth transistor Q 10  of the second input unit  200  is larger than that of the current flowing through the eleventh transistor Q 11 . Accordingly, the level of node V 2  becomes low according to the increase of the amount of the current that flows through the tenth transistor Q 10 . Thus, the sixth and seventh transistors Q 6  and Q 7  are turned on. 
   Since the sixth and seventh transistors Q 6  and Q 7  are turned on and the amount of the current flowing through the tenth transistor Q 10  is larger than that of the current flowing through the eleventh transistor Q 11 , the differentially amplified signal ‘DIFF_OUT’ is output in a high level. 
   Next, that the situation where the level of the middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is lower than that of the threshold voltage (Vth) of the first and second transistors Q 1  and Q 2 , which receive the first input signal ‘IN’ and the second input signal ‘VREF’, will be described. 
   First, when the enable signal ‘EN’ is inactive, e.g., at a low level, the third transistor Q 3  and the twelfth transistor Q 12  are turned off and the eighth transistor Q 8  and the ninth transistor Q 9  are turned on. 
   Since the third transistor Q 3  and the twelfth transistor Q 12  are turned off, the current path through the first input unit  100  and the second input unit  200  is blocked so that the operation of the input circuit  101  is stopped. Since the eighth transistor Q 8  and the ninth transistor Q 9  are turned on, the differentially amplified signal ‘DIFF_OUT’ is precharged to a high level. 
   Meanwhile, when the enable signal ‘EN’ is activated, e.g., at a high level, the third transistor Q 3  and the twelfth transistor Q 12  are turned on and the eighth transistor Q 8  and the ninth transistor Q 9  are turned off. Since the third transistor Q 3  and the twelfth transistor Q 12  are turned on, the current path through the first input unit  100  and the second input unit  200  is opened. 
   The first input signal ‘IN’ can be supplied to the gates of the first and fourth transistors Q 1  and Q 4 , and the second input signal ‘VREF’ can be supplied to the gates of the second and fifth transistors Q 2  and Q 5 . 
   Since the level of the gate-source voltage (Vgs) of each of the first and second transistors Q 1  and Q 2  of the first amplifier circuit  110  is lower than that of each threshold voltage (Vth) of the first and second transistors Q 1  and Q 2 , the first and second transistors Q 1  and Q 2 , which are NMOS transistors in this example, cannot operate. 
   Meanwhile, since the level of the gate-source voltage (Vgs) of each of the fourth and fifth transistors Q 4  and Q 5  of the second amplifier circuit  120  is higher than that of each threshold voltage (Vth) of the fourth and fifth transistors Q 4  and Q 5 , the fourth and fifth transistors Q 4  and Q 5 , which are PMOS transistors in this example, can operate. 
   For example, if the voltage level of the second input signal ‘VREF’ is lower than that of the first input signal ‘IN’, then the magnitude of the current flowing through the fifth transistor Q 5  of the second amplifier circuit  120  is larger than that of the current flowing through the fourth transistor Q 4 . 
   when the level of the middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is higher than that of the threshold voltage (Vth) of the first and second transistors Q 1  and Q 2 , the first and second transistors Q 1  and Q 2 , which are NMOS transistors, should operate. 
   However, since the level of the middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is lower than that of the threshold voltage (Vth) of the first and second transistors Q 1  and Q 2 , the level of the gate-source voltage (Vgs) of each of the first and second transistors Q 1  and Q 2  cannot be higher than that of the threshold voltage (Vth) of each of the first and second transistors Q 1  and Q 2 , but the level of the gate-source voltage (Vgs) of each of the fourth and fifth transistors Q 4  and Q 5  can be higher than that of the threshold voltage (Vth) of each of the fourth and fifth transistors Q 4  and Q 5 . 
   Therefore, the fifth transistor Q 5  operates in place of the first transistor Q 1  and the fourth transistor Q 4  operates in place of the second transistor Q 2 . 
   Since the amount of the current flowing through the fifth transistor Q 5  is larger than that of the current flowing through the fourth transistor Q 4 , the voltage level of the second output signal ‘OREFb’ is higher than that of the first output signal ‘OINb’. 
   Since the voltage level of the second output signal ‘OREFb’ is higher than that of the first output signal ‘OINb’, the amount of the current flowing through the tenth transistor Q 10  of the second input unit  200  is larger than that of the current flowing through the eleventh transistor Q 11 . The level of node V 2  becomes low according to the increase of the amount of the current that flows through the tenth transistor Q 10 . Thus, the sixth and seventh transistors Q 6  and Q 7  are turned on. 
   Since the sixth and seventh transistors Q 6  and Q 7  are turned on and the amount of the current flowing through the tenth transistor Q 10  is larger than that of the current flowing through the eleventh transistor Q 11 , the differentially amplified signal ‘DIFF_OUT’ is outputted in a high level. 
   Accordingly, if the level of the middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is higher than that of the threshold voltage (Vth) of the first and second transistors Q 1  and Q 2 , the first and second transistors Q 1  and Q 2  of the differential amplifier operate so that the input circuit can operate. 
   But even if the level of the middle voltage (Vmp) of the first input signal ‘IN’ and the second input signal ‘VREF’ is lower than that of the threshold voltage (Vth) of the first and second transistors Q 1  and Q 2  and thus the first and second transistors Q 1  and Q 2  of the differential amplifier cannot operate, the fourth and fifth transistors Q 4  and Q 5  can substitute for the first and second transistors Q 1  and Q 2  so that the input circuit  101  can operate normally. 
   In other words, the first input unit  100  can operate normally regardless of a voltage fluctuation. Also, the second input unit  200  is suitable for a high speed operation so that the operating speed of the input circuit  101  can be increased. Besides the second input unit  200  shown in  FIG. 3 , it is possible to use any type of amplifier circuit for the second input unit. 
   Since the differential amplifier and therefore the input circuit  101  using the differential amplifier operate normally even if a change of an external voltage occurs, an accurate and stable processing of an input signal is possible. Also, a high speed operation is possible. 
   While certain embodiments have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the systems and methods described herein should not be limited based on the described embodiments. Rather, the systems and methods described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.
A differential amplifier comprises a plurality of first switching elements configured to output differentially amplified signals through output terminals when a voltage level of a first input signal and a second input signal belongs to a first range and a plurality of second switching elements configured to output the differentially amplified signals through the output terminals when the voltage level of the first input signal and the second input signal belongs to a second range.