Abstract:
Provided is a mixer including a mixing unit configured to mix high frequency data signals and local oscillation (LO) signals, generate first and second low frequency data signals, and output the first and second low frequency data signals to first and second output terminals, respectively; a common mode amplification unit coupled to the mixing unit, the common mode amplification unit configured to compare a common mode voltage of the first and second low frequency data signals and a predetermined reference voltage, the common mode amplification unit further configured to output a feedback signal at a control node based on the comparison; a first load transistor coupled to the first output terminal and the control node, the first load transistor configured to provide the first output terminal with a first load current corresponding to the feedback signal; a first calibration transistor unit connected in parallel to the first load transistor in order to calibrate an input impedance of the first output terminal; and a first current mirror unit coupled to the first calibration transistor unit, the first current mirror unit configured to discharge a first calibration current that is output from the first calibration transistor unit to prevent the first calibration current from entering the first output terminal.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2007-0044718, filed on May 8, 2007 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a mixer used for a direct conversion receiver, and more particularly, but without limitation, to a mixer for reducing a second-order intermodulation (IM2) component included in a low frequency data signal. 
         [0004]    2. Description of the Related Art 
         [0005]    Low frequency baseband data signals are up-converted into high frequency radio frequency (RF) band data signals in transmitters of communication systems. High frequency RF band data signals are down-converted into low frequency baseband data signals in communication system receivers. A heterodyne method or a direct conversion method may be used to transmit and receive data signals. The heterodyne method uses an intermediate frequency (IF) band, whereas the direct conversion method directly up-converts low frequency baseband data signals into high frequency RF band data signals and then directly down-converts high frequency RF band data signals into low frequency baseband data signals. 
         [0006]      FIG. 1  is a partial circuit diagram of a conventional direct down-conversion transceiver. Referring to  FIG. 1 , the conventional direct down-conversion transceiver includes an antenna  101 , a duplexer  102 , a transmission end Tx  103 , a reception end Rx  104 , a low noise amplifier (LNA)  105 , a surface acoustic wave (SAW) filter  106 , and a mixer  110 . 
         [0007]    When the conventional direct down-conversion transceiver serves as a transmitter, a transmission signal is transmitted through the transmission end Tx  103 , the duplexer  102 , and the antenna  101 . When the conventional direct down-conversion transceiver serves as a receiver, a signal received through the antenna  101  is input into the LNA  105  through the duplexer  102  and the reception end Rx  104 . The signal amplified by the LNA  105  is input into the mixer  110  through the SAW filter  106 . The mixer  110  mixes an input high frequency data signal radio frequency (RF) and a local oscillation (LO) signal and generates a low frequency data signal Vpn. In more detail, the mixer  110  generates the low frequency data signal Vpn having a frequency corresponding to a difference between frequencies of the high frequency data signal RF and the LO signal. 
         [0008]      FIG. 2A  is a graph illustrating heterodyne down-conversion, and  FIG. 2B  is a graph illustrating direct down-conversion. Referring to  FIGS. 2A and 2B , a horizontal axis indicates a frequency, and a vertical axis indicates signal amplitude. RFx denotes a transmission signal, RFy denotes a signal adjacent to RFx, and LO denotes an LO signal. Intermodulation is a type of signal interference that is the result of two or more signals of different frequencies being mixed together. Accordingly, the adjacent signal RFy may cause noise to occur in the transmission signal RFx. 
         [0009]    Referring to  FIG. 2A , both signals RFy and RFx are down-converted into IF band signals according to the heterodyne down-conversion. In more detail, an IF band includes a frequency f 1 -f 3  of a mixing signal of the signal RFx and the LO signal LO, and a frequency f 2 -f 3  of a mixing signal of the adjacent signal RFy and the LO signal LO. Meanwhile, a frequency f 1 -f 2  of a second-order intermodulation (IM 2 ) component caused by the intermodulation between signals RFy and RFx does not belong to the IF band, and therefore the IM2 component can be easily removed by performing band reject filtering. 
         [0010]    Referring to  FIG. 2B , both signals RFy and RFx are down-converted into baseband signals according to the direct down-conversion. In more detail, a baseband includes a frequency f 1 -f 3  of a mixing signal of the signal RFx and the LO signal LO, and a frequency f 2 -f 3  of a mixing signal of the adjacent signal RFy and the LO signal LO. However, a frequency f 1 -f 2  of the IM2 component caused by the intermodulation between signals RFy and RFx belongs to the baseband too, making it impossible to remove the IM2 component by performing band reject filtering. Therefore, a method of removing the IM2 component is additionally needed. 
         [0011]      FIG. 3  is a circuit diagram of a conventional mixer  110  including a circuit used to remove the IM2 component. Referring to  FIG. 3 , the mixer  110  is used as one of the constituents of the conventional direct down-conversion receiver illustrated in  FIG. 1 . 
         [0012]    In order to remove the IM2 component, the mixer  110  includes resistive circuits Rp, Rpc, Rn, and Rnc that are connected between a power voltage source VDD and output terminals TER 1  and TER 2 , and a mixing unit (MU) that mixes high frequency data signals RFp and RFn and LO signals LOp and LOn and outputs low frequency data signals Vp and Vn. Iim2_d denotes a differential current of the IM 2  component included in the low frequency data signals Vp and Vn. Iim2_c denotes a common current of the IM2 component included in the low frequency data signals Vp and Vn. 
         [0013]    If a value of combination resistance of the resistors Rp and Rpc is (R+ΔR), and a value of combination resistance of the resistors Rn and Rnc is (R−ΔR), the IM2 component [Vpn(fim2)] included in the low frequency data signals Vp and Vn can be approximated according to Equation 1, 
         [0000]        Vpn ( fim 2)= R*Iim 2 —   d±ΔR*Iim 2 —   c   (1) 
         [0000]    wherein, fim2 denotes a frequency of the IM2 component. 
         [0014]    Referring to Equation 1, proper control of a value ΔR can minimize the IM2 component [Vpn(fim2)] included in the low frequency data signals Vp and Vn. The value of ΔR can be controlled by changing the value of combination resistance of the resistors Rp and Rpc by varying a value of the resistor Rpc connected in parallel to the resistor Rp, or by changing the value of combination resistance of the resistors Rn and Rnc by varying a value of the resistor Rnc connected in parallel to the resistor Rn. 
         [0015]    For example, if the resistor Rp is 1 kΩ, the value of the resistor Rpc must be 99 kΩ in order that the value of combination resistance of the resistors Rp and Rpc is 0.99 kΩ. In other words, it is necessary to specify a resistor Rpc with a value more than a hundred times larger than the resistor Rp in parallel in order to reduce the combination resistance value by 1%. However, the resistor having such a large resistance value needs a large area in order to be realized in a system chip, which increases the overall area of the system chip. Therefore, an additional method is needed for high integration of the system chip. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention provides a mixer capable of reducing a second-order intermodulation (IM2) component using a limited area. 
         [0017]    The present invention also provides a mixer capable of preventing a DC offset when the IM2 component is reduced. 
         [0018]    According to an aspect of the present invention, there is provided a mixer. 
         [0019]    The mixer includes: a mixing unit configured to mix high frequency data signals and local oscillation (LO) signals, generate first and second low frequency data signals, and output the first and second low frequency data signals to first and second output terminals, respectively; a common mode amplification unit coupled to the mixing unit, the common mode amplification unit configured to compare a common mode voltage of the first and second low frequency data signals and a predetermined reference voltage, the common mode amplification unit further configured to output a feedback signal at a control node based on the comparison; a first load transistor coupled to the first output terminal and the control node, the first load transistor configured to provide the first output terminal with a first load current corresponding to the feedback signal; a first calibration transistor unit connected in parallel to the first load transistor in order to calibrate an input impedance of the first output terminal; and a first current mirror unit coupled to the first calibration transistor unit, the first current mirror unit configured to discharge a first calibration current that is output from the first calibration transistor unit to prevent the first calibration current from entering the first output terminal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above objects and advantages of the invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
           [0021]      FIG. 1  is a partial circuit diagram of a conventional direct down-conversion transceiver; 
           [0022]      FIG. 2A  is a graph illustrating heterodyne down-conversion, and  FIG. 2B  is a graph illustrating direct down-conversion; 
           [0023]      FIG. 3  is a circuit diagram of a conventional mixer including a circuit used to remove the IM2 component; 
           [0024]      FIG. 4  is a circuit diagram of a mixer capable of reducing a second-order intermodulation (IM2) component using a limited area according to an embodiment of the invention; 
           [0025]      FIG. 5  is a circuit diagram of a mixer according to an embodiment of the invention; and 
           [0026]      FIG. 6  is a circuit diagram of a mixer according to another embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]      FIG. 4  is a circuit diagram of a mixer capable of reducing a second-order intermodulation (IM2) component using a limited area according to an embodiment of the invention. Referring to  FIG. 4 , the mixer includes a mixing unit MU, a common mode amplification unit CMA, a first load transistor Tp, a first calibration transistor Tpc, a second load transistor Tn, and a second calibration transistor Tnc. 
         [0028]    The mixing unit MU mixes high frequency data signals RFp and RFn and local oscillation (LO) signals LOp and LOn and generates first and second low frequency data signals Vp and Vn. The first and second low frequency data signals Vp and Vn are output to first and second output terminals TER 1  and TER 2 , respectively. 
         [0029]    The common mode amplification unit CMA compares a common mode voltage Vcom of the first and second low frequency data signals Vp and Vn and a predetermined reference voltage Vref and outputs a feedback signal FB. The common mode amplification unit CMA includes a first common resistor R 1  connected between the first output terminal TER 1  and a common mode Nc, a second common resistor R 2  connected between the second output terminal TER 2  and the common mode Nc, and an amplifier AMP that compares the reference voltage Vref and the common mode voltage Vcom and outputs the feedback signal FB to a control node Ng. 
         [0030]    The first load transistor Tp is connected between a power voltage source VDD and the first output terminal TER 1 , and provides the first output terminal TER 1  with a first load current I 1  corresponding to the feedback signal FB. The first calibration transistor Tpc is connected in parallel to the first load transistor Tp. The second load transistor Tn is connected between the power voltage source VDD and the second output terminal TER 2 , and provides the second output terminal TER 2  with a second load current I 6  corresponding to the feedback signal FB. The second calibration transistor Tnc is connected in parallel to the second load transistor Tn. 
         [0031]    Equivalent impedances of the first load transistor Tp, the first calibration transistor Tpc, the second load transistor Tn, and the second calibration transistor Tnc are Zp, Zpc, Zn, and Znc, respectively. A value of combination impedance of impedances Zp and Zpc is (Z+AZ), and a value of combination impedance of impedances Zn and Znc is (Z-AZ). In this case, if R 1 =R 2 =Rc, the IM2 component [Vpn(fim2)] included in the first and second low frequency data signals Vp and Vn can be approximated according to Equation 2, 
         [0000]    
       
         
           
             
               
                 
                   
                     Vpn 
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         f 
                         
                           im 
                            
                           
                               
                           
                            
                           2 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           Rc 
                            
                           
                               
                           
                            
                           Z 
                         
                         
                           Rc 
                           + 
                           Z 
                         
                       
                        
                       Iim2_d 
                     
                     ± 
                     
                       
                         
                           Rc 
                            
                           
                               
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           Z 
                         
                         
                           Rc 
                           + 
                           Z 
                         
                       
                        
                       Iim2_c 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein, fim2 denotes a frequency of the IM2 component, Iim2_d denotes a differential current of the IM 2  component, and Iim2_c denotes a common current of the IM2 component (see  FIG. 3 ). 
         [0032]    Referring to Equation 2, proper control of a value AZ can minimize the IM2 component [Vpn(fim2)]. In more detail, proper selection of the equivalent impedances Zpc and Znc of the first and second calibration transistors Tpc and Tnc results in the minimization of the IM2 component [Vpn(fim2)] included in the first and second low frequency data signals Vp and Vn. 
         [0033]    Unlike the conventional embodiment shown in  FIG. 3  where the resistors Rpc and Rnc are passive elements, the mixer embodiment illustrated in  FIG. 4  uses active elements (e.g., transistors Tpc and Tnc). A system chip realized with transistors needs a relatively smaller area than a system chip realized with resistors. Embodiments of the invention thus provide a mixer capable of reducing the IM2 component that requires less area than conventional mixer circuits. 
         [0034]    As shown in  FIG. 4 , a first load current I 1  is output from the load transistor Tp, a first calibration current I 3  is output from the first calibration transistor Tpc, and a terminal current I 2  enters the first output terminal TER 1 . If the first calibration transistor Tpc is not connected to the first load transistor Tp, then I 2 =I 1 . But if the first calibration transistor Tpc is connected in parallel to the first load transistor Tp, then I 2 =I 1 +I 3 . If the first calibration current I 3  increases or decreases the current I 2 , then a DC offset may occur in the first and second low frequency data signals Vp and Vn. Likewise, a DC offset may occur if a second calibration current I 8  increases or decreases a terminal current I 7  that enters the second output terminal TER 2 . Therefore, the first and second calibration currents I 3  and I 8  must be carefully managed in order to prevent such DC offset from occurring. 
         [0035]      FIG. 5  is a circuit diagram of a mixer according to an embodiment of the invention. Referring to  FIG. 5 , the mixer includes a mixing unit MU, a common mode amplification unit CMA, a first load transistor Tp, a first calibration transistor Tpc, a first current mirror unit CMU 1 , a second load transistor Tn, a second calibration transistor Tnc, and a second current mirror unit CMU 2 . 
         [0036]    As described with reference to  FIG. 4 , the mixing unit MU mixes high frequency data signals RFp and RFn and LO signals LOp and LOn and outputs first and second low frequency data signals Vp and Vn to first and second output terminals TER 1  and TER 2 , respectively. 
         [0037]    The common mode amplification unit CMA includes a first common resistor R 1  connected between the first output terminal TER 1  and a common mode Nc, a second common resistor R 2  connected between the second output terminal TER 2  and the common mode Nc, and an amplifier AMP that compares a predetermined reference voltage Vref and a common mode voltage Vcom that is input from the common mode Nc and outputs a feedback signal FB to a control node Ng. The common mode voltage Vcom of the first and second low frequency data signals Vp and Vn is acquired by distributing voltages using the first and second common resistors R 1  and R 2 . An electric potential (e.g., a voltage level of the feedback signal FB) of the control node Ng is controlled in response to a signal (e.g., the common mode voltage Vcom) that is feedback from the first and second output terminals TER 1  and TER 2 . A first load current I 1  and a second load current I 6  increase in response to the electric potential of the control node Ng. The first and second low frequency data signals Vp and Vn increase in response to an increase and decrease of the first and second load currents I 1  and I 6 . In this regard, the common mode amplification unit CMA, the first load transistor Tp, and the second load transistor Tn are considered as a common mode feedback circuit. 
         [0038]    The first calibration transistor Tpc is connected in parallel to the first load transistor Tp between a power voltage source VDD and the first output terminal TER 1 . A first load current I 1  is output from the first load transistor Tp in response to the feedback signal FB. A first calibration current I 3  is output from the first calibration transistor Tpc in response to the feedback signal FB. The second calibration transistor Tnc is connected in parallel to the second load transistor Tn between the power voltage source VDD and the second output terminal TER 2 . The second load current I 6  is output from the second load transistor Tn in response to the feedback signal FB. The second calibration current I 8  is output from the second calibration transistor Tnc in response to the feedback signal FB. 
         [0039]    As described with reference to  FIG. 4  and Equation 2, proper selection of equivalent impedances Zpc and Znc of the first and second calibration transistors Tpc and Tnc can minimize an IM2 component included in the first and second low frequency data signals Vp and Vn. In more detail, the IM2 component included in the first and second low frequency data signals Vp and Vn can be minimized by calibrating input impedances of the first and second output terminals TER 1  and TER 2 . The input impedance of the first output terminal TER 1  is determined according to a value of composite impedance of the first load transistor Tp and the first calibration transistor Tpc that are connected in parallel. Because the first current mirror unit CMU 1  has a very large input impedance, the first current mirror unit CMU 1  does not significantly affect the input impedance of the first output terminal TER 1 . Likewise, the input impedance of the second output terminal TER 2  is determined according to a value of composite impedance of the second load transistor Tn and the second calibration transistor Tnc, and the second current mirror unit CMU 2  does not significantly affect the input impedance of the second output terminal TER 2 . 
         [0040]    The first current mirror unit CMU 1  included in the mixer of  FIG. 5  discharges the first calibration current I 3  that is output from the first calibration transistor Tpc through a separate path. If the first calibration current I 3  that is output from the first calibration transistor Tpc increases or decreases a current I 2  that enters the first output terminal TER 1 , a DC offset may occur in the first and second low frequency data signals Vp and Vn. The first current mirror unit CMU 1  of the mixer in  FIG. 5  discharges the first calibration current I 3  through the separate path in order to prevent the DC offset from occurring. In more detail, the first current mirror unit CMU 1  makes it possible to satisfy I 4 =I 3  and I 2 =I 1 . 
         [0041]    The first current mirror unit CMU 1  includes a first mirror current source Ipref, a first mirror transistor Mp, and a first discharge transistor Mpc. The first mirror current source Ipref provides the first mirror transistor Mp with a first control current Ip corresponding to the first calibration current I 3 . If a mirror current ratio of the first mirror transistor Mp and the first discharge transistor Mpc is 1:1, then Ip=I 4 . In this case, the first mirror current source Ipref provides the first mirror transistor Mp with the first control current Ip satisfying Ip=I 4 =I 3 . 
         [0042]    Likewise, the second current mirror unit CMU 2  discharges the second calibration current I 8  output from the second calibration transistor Tnc through a separate path in order to prevent the second calibration current I 8  from entering the second output terminal TER 2 . The second current mirror unit CMU 2  makes it possible to satisfy I 9 =I 8  and I 7 =I 6 , so that the second calibration current I 8  does not increase or decrease a current I 7  that enters the second output terminal TER 2 . The second current mirror unit CMU 2  includes a second mirror current source Inref, a second mirror transistor Mn, and a second discharge transistor Mnc. If a mirror current ratio of the second mirror transistor Mn and the second discharge transistor Mnc is 1:1, then the second mirror current source Inpref provides the second mirror transistor Mn with a second control current In satisfying In=I 9 =I 8 . 
         [0043]    Although the mixer illustrated in  FIG. 5  includes the calibration circuits Tpc and CMU 1  that are connected to the first output terminal TER 1  and the calibration circuits Tnc and CMU 2  that are connected to the second output terminal TER 2 , the invention is not limited thereto. In alternative embodiments, the mixer can include only one of the calibration transistors and associated CMU. For example, the mixer could include the mixing unit MU, the common mode amplification unit CMA, the first load transistor Tp, the second load transistor Tn, the first calibration transistor Tpc, and the first current mirror unit CMU 1  without the second calibration transistor Tnc and without the second current mirror unit CMU 2 . Alternatively, the mixer could include the mixing unit MU, the common mode amplification unit CMA, the first load transistor Tp, the second load transistor Tn, the second calibration transistor Tnc, and the second current mirror unit CMU 2  without the first calibration transistor Tpc and without the first current mirror unit CMU 1 . 
         [0044]      FIG. 6  is a circuit diagram of a mixer according to another embodiment of the invention. Referring to  FIG. 6 , the mixer includes a mixing unit MU, a common mode amplification unit CMA, a first load transistor Tp, a first calibration transistor unit CTU 1 , a first current mirror unit CMU 11 , a second load transistor Tn, a second calibration transistor unit CTU 2 , and a second current mirror unit CMU 21 . 
         [0045]    The first calibration transistor unit CTU 1  includes calibration transistors Tpc 1 , Tpc 2 , Tpc 3 , Tpc 4 , Tpc 5  and switches Sp 11 , Sp 21 , Sp 31 , Sp 41 , Sp 51 . The first current mirror unit CMU 11  includes mirror transistors Mpc 1 , Mpc 2 , Mpc 3 , Mpc 4 , Mpc 5 , Mp, switches Sp 12 , Sp 22 , Sp 32 , Sp 42 , Sp 52 , and current source Ipvar. The second calibration transistor unit CTU 2  includes calibration transistors Tnc 1 , Tnc 2 , Tnc 3 , Tnc 4 , Tnc 5  and switches Sn 11 , Sn 21 , Sn 31 , Sn 41 , Sn 51 . The second current mirror unit CMU 21  includes mirror transistors Mnc 1 , Mnc 2 , Mnc 3 , Mnc 4 , Mnc 5 , Mn, switches Sn 12 , Sn 22 , Sn 32 , Sn 42 , Sn 52 , and current source Invar. 
         [0046]    The mixing unit MU mixes high frequency data signals RFp and RFn and LO signals LOp and LOn and outputs first and second low frequency data signals Vp and Vn to first and second output terminals TER 1  and TER 2 , respectively. 
         [0047]    The common mode amplification unit CMA compares a predetermined reference voltage Vref and a common mode voltage Vcom of the first and second low frequency data signals Vp and Vn and outputs a feedback signal FB. The common mode amplification unit CMA comprises a first common resistor R 1  connected between the first output terminal TER 1  and a common mode Nc, a second common resistor R 2  connected between the second output terminal TER 2  and the common mode Nc, and an amplifier AMP that compares the reference voltage Vref and the common mode voltage Vcom and outputs the feedback signal FB to a control node Ng. 
         [0048]    The first load transistor Tp is connected between the power voltage source VDD and the first output terminal TER 1 , and provides the first output terminal TER 1  with a first load current corresponding to the feedback signal FB. The first load transistor Tp comprises an input terminal connected to the power voltage source VDD, an output terminal connected to the first output terminal TER 1 , and a control terminal that receives the feedback signal from the control node Ng. The second load transistor Tn is connected between the power voltage source VDD and the second output terminal TER 2 , and provides the second output terminal TER 2  with a second load current corresponding to the feedback signal FB. The second load transistor Tn comprises an input terminal connected to the power voltage source VDD, an output terminal connected to the second output terminal TER 2 , and a control terminal that receives the feedback signal from the control node Ng. 
         [0049]    The first calibration transistor unit CTU 1  that calibrates an input impedance of the first output terminal TER 1  is connected in parallel to the first load transistor Tp in response to a first calibration control signal. The first calibration control signal controls switches Sp 11 , Sp 21 , Sp 31 , Sp 41 , and Sp 51 . 
         [0050]    The first calibration transistor unit CTU 1  includes a first calibration unit, a second calibration unit, a third calibration unit, a fourth calibration unit, and a fifth calibration unit. However, the present invention is not limited thereto. Generally speaking, the first calibration transistor unit CTU 1  can include M calibration units (where M is a natural number greater than 1). 
         [0051]    The first calibration unit includes the transistor Tpc 1  and the switch Sp 11 . The second calibration unit includes the transistor Tpc 2  and the switch Sp 21 . The third calibration unit includes the transistor Tpc 3  and the switch Sp 31 . The fourth calibration unit includes the transistor Tpc 4  and the switch Sp 41 . The fifth calibration unit includes the transistor Tpc 5  and the switch Sp 51 . 
         [0052]    One terminal of an mth calibration unit switch (m is a natural number between 1 and 5) included in the first calibration transistor unit CTU 1  is connected to the first output terminal TER 1 . An mth calibration unit transistor included in the first calibration transistor unit CTU 1  includes an input terminal connected to the power voltage source VDD, an output terminal connected to another terminal of the mth calibration unit switch, and a control terminal that receives the feedback signal FB from a control node Ng. 
         [0053]    If the mth calibration unit switch is turned on in response to the first calibration control signal, the mth calibration unit transistor is connected in parallel to the first load transistor Tp. The first calibration control signal controls the number of the first through fifth calibration unit transistors Tpc 1 , Tpc 2 , Tpc 3 , Tpc 4 , and Tpc 5  that are connected in parallel to the first load transistor Tp, thereby calibrating the input impedance of the first output terminal TER 1 . As described above, the calibration of the input impedance of the first output terminal TER 1  can result in the minimization of an IM2 component included in the first and second low frequency data signals Vp and Vn. The input impedance of the first output terminal TER 1  has a value of composite impedance of the first load transistor Tp and the first calibration transistor unit CTU 1  that is connected in parallel to the first load transistor Tp. Because the first current mirror unit CMU 11  has a very large input impedance, the first current mirror unit CMU 11  does not affect the input impedance of the first output terminal TER 1 . 
         [0054]    The first current mirror unit CMU 11  prevents the first calibration current from entering the first output terminal TER 1  by discharging the first calibration current through a separate path in response to a first discharge control signal. The first discharge control signal controls switches Sp 12 , Sp 22 , Sp 32 , Sp 42 , and Sp 51 . Because the calibration current does not increase or decrease a current that enters the first output terminal TER 1 , a DC offset is prevented in the first and second low frequency data signals Vp and Vn. 
         [0055]    The first current mirror unit CMU 11  includes current source Ipvar, a first mirror transistor Mp, a first discharge unit, a second discharge unit, a third discharge unit, a fourth discharge unit, and a fifth discharge unit. However, the present invention is not limited thereto. Generally speaking, the first current mirror unit CMU 11  can include the source Ipvar, and M discharge units (where M is a natural number greater than 1). 
         [0056]    The first discharge unit includes the switch Sp 12  and the transistor Mpc 1 . The second discharge unit includes the switch Sp 22  and the transistor Mpc 2 . The third discharge unit includes the switch Sp 32  and the transistor Mpc 3 . The fourth discharge unit includes the switch Sp 42  and the transistor Mpc 4 . The fifth discharge unit includes the switch Sp 52  and the transistor Mpc 5 . 
         [0057]    The first mirror current source Ipvar provides the first mirror transistor Mp with the first control current Ip corresponding to the first calibration current that is output from the first calibration transistor unit CTU 1 . The first mirror current source Ipvar is a variable current source that provides the first mirror transistor Mp with the first control current Ip corresponding to the number of the discharge unit switches Sp 12  through Sp 52  that are turned on. In more detail, the higher the first calibration current output from the first calibration transistor unit CTU 1 , the higher the first control current Ip becomes. The lower the first calibration current output from the first calibration transistor unit CTU 1 , the lower the first control current Ip becomes. The first mirror transistor Mp includes an input terminal that receives the first control current Ip, a control terminal connected to the input terminal, and an output terminal that discharges the first control current Ip. 
         [0058]    One terminal of an mth discharge unit switch (m is a natural number between 1 and 5) included in the first current mirror unit CMU 11  is connected to the first output terminal TER 1 . An mth discharge unit transistor included in the first current mirror unit CMU 11  includes an input terminal connected to another terminal of the mth discharge unit switch, a control terminal connected to the control terminal of the first mirror transistor Mp, and an output terminal connected to a ground voltage. The mth discharge unit transistor included in the first current mirror unit CMU 11  discharges the first calibration current that is output from the first calibration transistor unit CTU 1  when the mth discharge unit switch is turned on in response to the first discharge control signal. The first discharge control signal controls switches Sp 12  through Sp 52  to prevent the first calibration current from entering the first output terminal TER 1 . 
         [0059]    The second load transistor Tn, the second calibration transistor unit CTU 2 , and the second current mirror unit CMU 21  will now be described in brief. 
         [0060]    The second load transistor Tn is connected between the power voltage source VDD and the second output terminal TER 2 , and provides the second output terminal TER 2  with a second load current corresponding to the feedback signal FB. 
         [0061]    The second calibration transistor unit CTU 2  is connected in parallel to the second load transistor Tn in response to a second calibration control signal in order to calibrate input impedance of the second output terminal TER 2 . The second calibration control signal controls switches Sn 11 , Sn 21 , Sn 31 , Sn 41 , and Sn 51 . The input impedance of the second output terminal TER 2  has a value of composite impedance of the second load transistor Tn and the second calibration transistor unit CTU 2  that is connected in parallel thereto. The calibration of the input impedance of the second output terminal TER 2  can result in the minimization of the IM 2  component included in the first and second low frequency data signals Vp and Vn. 
         [0062]    Generally speaking, the second calibration transistor unit CTU 2  can include N calibration units (N is a natural number greater than 1). In the illustrated embodiment, an nth calibration unit (n is a natural number between 1 and 5) included in the second calibration transistor unit CTU 2  includes an nth calibration unit transistor Tnc 1 , Tnc 2 , Tnc 3 , Tnc 4 , or Tnc 5  and an nth calibration unit switch Sn 11 , Sn 21 , Sn 31 , Sn 41 , or Sn 51 . The nth calibration unit transistor Tnc 1 , Tnc 2 , Tnc 3 , Tnc 4 , or Tnc 5  included in the second calibration transistor unit CTU 2  is connected in parallel to the second load transistor Tn when the nth calibration unit switch Sn 11 , Sn 21 , Sn 31 , Sn 41 , or Sn 51  is turned on in response to the second calibration control signal. 
         [0063]    The second current mirror unit CMU 21  prevents the second calibration current from entering the second output terminal TER 2  by discharging the second calibration current through a separate path in response to a second discharge control signal. Accordingly, the second calibration current does not increase or decrease a current that enters the second output terminal TER 2 , and a DC offset is prevented in the first and second low frequency data signals Vp and Vn. 
         [0064]    The second current mirror unit CMU 21  includes a second mirror current source Invar, a second mirror transistor Mn, and n discharge units. An nth discharge unit (n is a natural number between 1 and 5) included in the second current mirror unit CMU 21  includes an nth discharge unit switch and an nth discharge unit transistor. The nth discharge unit transistor included in the second current mirror unit CMU 21  discharges the second calibration current that is output from the second calibration transistor unit CTU 2  when the nth discharge unit switch is turned on in response to the second discharge control signal. The second discharge control signal controls the switches Sn 12  through Sn 52  to prevent the second calibration current from entering the second output terminal TER 2 . The second mirror current source Invar is a variable current source that provides the second mirror transistor Mn with the second control current In corresponding to the number of switches Sn 12  through Sn 52  that are turned on. 
         [0065]    Although the mixer of the embodiment illustrated in  FIG. 6  includes the first calibration transistor unit CTU 1  and the first current mirror unit CMU 11  connected to the first output terminal TER 1 , and the second calibration transistor unit CTU 2  and the second current mirror unit CMU 21  connected to the second output terminal TER 2 , the present invention is not limited thereto. For example, in an alternative embodiment, the mixer could include the mixing unit MU, the common mode amplification unit CMA, the first load transistor Tp, the second load transistor Tn, the first calibration transistor unit CTU 1  and the first current mirror unit CMU 11  without the second calibration transistor unit CTU 2  and without the second current mirror unit CMU 21 . Alternatively, the mixer could include the mixing unit MU, the common mode amplification unit CMA, the first load transistor Tp, the second load transistor Tn, the second calibration transistor unit CTU 2  and the second current mirror unit CMU 21  without the first calibration transistor unit CTU 1  and without the first current mirror unit CMU 11 . 
         [0066]    Embodiments of the invention thus provide a mixer that can be used for a direct conversion receiver. The disclosed mixers can reduce an IM2 signal component using a limited circuit area. Furthermore, embodiments of the invention can prevent a DC offset from occurring when the IM2 component is reduced. 
         [0067]    While the invention has been particularly shown and described with reference to an exemplary embodiment thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.