Patent Publication Number: US-9843290-B2

Title: Mixer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 14/754,886, filed by the applicant on Jun. 30, 2015, which claims the priority of Taiwanese Patent Application No. 103146075 filed on Dec. 29, 2014, and the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD 
     The disclosure relates to a mixer, and more particularly to a mixer that simultaneously achieves low power consumption and high conversion gain. 
     BACKGROUND 
     Referring to  FIG. 1 , a conventional Gilbert mixer includes a transconductance unit  12 , a mixer unit  11 , a first resistor (R 11 ) and a second resistor (R 12 ). 
     The trans conductance unit  12  receives a differential input voltage signal pair of intermediate frequency, and converts the differential input voltage signal pair into a differential input current signal pair. 
     The mixer unit  11  receives a differential oscillatory voltage signal pair, and is coupled to the trans conductance unit  12  for receiving the differential input current signal pair therefrom. The mixer unit  11  mixes the differential oscillatory voltage signal pair and the differential input current signal pair to generate a differential mixed current signal pair that includes a first mixed current signal (IRF 1 ) and a second mixed current signal (IRF 2 ) and that is of radio frequency. 
     The first resistor (R 11 ) has a first terminal that receives a supply voltage (VDD 1 ), and a second terminal that is coupled to the mixer unit  11  for receiving the first mixed current signal (IRF 1 ) therefrom and that outputs a first mixed voltage signal (VRF 1 ). 
     The second resistor (R 12 ) has a first terminal that receives the supply voltage (VDD 1 ), and a second terminal that is coupled to the mixer unit  11  for receiving the second mixed current signal (IRF 2 ) therefrom and that outputs a second mixed voltage signal (VRF 2 ). The first and second mixed voltage signals (VRF 1 , VRF 2 ) constitute a differential mixed voltage signal pair. 
     When the conventional Gilbert mixer has a relatively high conversion gain, the first and second resistors (R 11 , R 12 ) consume relatively high power. So, the conventional Gilbert mixer is unable to simultaneously achieve low power consumption and high conversion gain. 
     SUMMARY 
     Therefore, an object of the disclosure is to provide a mixer that can alleviate the drawback of the prior art. 
     According to the disclosure, the mixer includes a trans conductance unit, a gain boost unit, a mixing module and a buffer. 
     The transconductance unit receives a differential input voltage signal pair, and converts the differential input voltage signal pair into a differential input current signal pair that includes a first input current signal and a second input current signal. 
     The gain boost unit is coupled to the trans conductance unit, and generates a first auxiliary current signal that constitutes a portion of the first input current signal, and a second auxiliary current signal that constitutes a portion of the second input current signal. 
     The mixing module receives a differential oscillatory voltage signal pair, and is coupled to the trans conductance unit for receiving a remaining portion of the first input current signal and a remaining portion of the second input current signal therefrom. The mixing module mixes the remaining portions of the first and second input current signals with the differential oscillatory voltage signal pair to generate a differential mixed voltage signal pair. 
     The buffer is coupled to the mixing module for receiving the differential mixed voltage signal pair therefrom, has a pair of buffer output terminals, performs buffering on the differential mixed voltage signal pair to generate at the buffer output terminals a differential buffered voltage signal pair, and is configured to have inductance that cooperates with parasitic capacitance at the buffer output terminals to form an LC tank circuit that reaches resonance at a frequency of the differential buffered voltage signal pair to behave as an open circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiment with reference to the accompanying drawings, of which: 
         FIG. 1  is a block diagram illustrating a conventional Gilbert mixer; 
         FIGS. 2A and 2B  are circuit block diagrams illustrating an embodiment of a mixer according to the disclosure; 
         FIG. 3  is a plot illustrating conversion gain versus frequency characteristic in various conditions; 
         FIG. 4A  is a plot illustrating reflection coefficient S 22  versus frequency characteristic of the embodiment; 
         FIG. 4B  is a plot illustrating reflection coefficient S 33  versus frequency characteristic of the embodiment; 
         FIG. 5  is a plot illustrating isolation versus power characteristic of the embodiment; and 
         FIG. 6  is circuit block diagram illustrating a modification of the circuit shown in  FIG. 2B . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 2A and 2B , an embodiment of a mixer according to the disclosure includes a filter  2 , a single-ended to differential converter  3 , a mixing module  4 , a gain boost unit  5 , a transconductance unit  6 , a buffer  7  and a differential to single-ended converter  8 . 
     The filter  2  receives a differential to-be-shifted voltage signal pair (IF) of, for example, intermediate frequency, and filters the differential to-be-shifted voltage signal pair (IF) to generate a differential input voltage signal pair that includes a first input voltage signal (VIN 1 ) and a second input voltage signal (VIN 2 ).  FIG. 2A  shows an exemplary implementation of the filter  2 , but the disclosure is not limited thereto. 
     The single-ended to differential converter  3  receives a single-ended oscillatory voltage signal (LO), and converts the single-ended oscillatory voltage signal (LO) into a differential oscillatory voltage signal pair that includes a first oscillatory voltage signal (VOS 1 ) and a second oscillatory voltage signal (VOS 2 ).  FIG. 2A  shows an exemplary implementation of the single-ended to differential converter  3 , but the disclosure is not limited thereto. 
     The transconductance unit  6  is coupled to the filter  2  for receiving the differential input voltage signal pair therefrom, and converts the differential input voltage signal pair into a differential input current signal pair that includes a first input current signal (IN 1 ) and a second input current signal (IN 2 ). 
     The gain boost unit  5  is coupled to the trans conductance unit  6 , and generates a first auxiliary current signal (Ij 1 ) that constitutes a portion of the first input current signal (IN 1 ), and a second auxiliary current signal (Ij 2 ) that constitutes a portion of the second input current signal (IN 2 ). 
     The mixing module  4  is coupled to the single-ended to differential converter  3  for receiving the differential oscillatory voltage signal pair therefrom, and is coupled to the transconductance unit  6  for receiving a remaining portion of the first input current signal (IN 1 ) and a remaining portion of the second input current signal (IN 2 ) therefrom. The mixing module  4  mixes the remaining portions of the first and second input current signals (IN 1 , IN 2 ) with the differential oscillatory voltage signal pair to generate a differential mixed voltage signal pair that includes a first mixed voltage signal (VM 1 ) and a second mixed voltage signal (VM 2 ) and that is of, for example, radio frequency. 
     The buffer  7  is coupled to the mixing module  4  for receiving the differential mixed voltage signal pair therefrom, has a pair of buffer output terminals, and performs buffering on the differential mixed voltage signal pair to generate at the buffer output terminals a differential buffered voltage signal pair that includes a first buffered voltage signal (VB 1 ) and a second buffered voltage signal (VB 2 ). The buffer  7  is configured to have inductance that cooperates with parasitic capacitance at the buffer output terminals thereof to form an LC tank circuit that reaches resonance to behave as an open circuit at a frequency of the differential buffered voltage signal pair. 
     The differential to single-ended converter  8  is coupled to the buffer  7  for receiving the differential buffered voltage signal pair therefrom, and converts the differential buffered voltage signal pair into a single-ended buffered voltage signal (RF).  FIG. 2B  shows an exemplary implementation of the differential to single-ended converter  8  that includes a balun having two input terminals and an output terminal, and three capacitors respectively coupled to the input and output terminals of the balun, but the disclosure is not limited thereto. It is noted that, an input impedance seen into the balun from each of the input terminals thereof is typically approximately 50 ohms. 
     In this embodiment, the differential to-be-shifted voltage signal pair (IF) has a frequency of 0.1 GHz, the single-ended oscillatory voltage signal (LO) has a frequency of 78.9 GHz, and the single-ended buffered voltage signal (RF) has a frequency of 79 GHz. 
     In this embodiment, the gain boost unit  5  includes a first transistor (M 1 ) and a second transistor (M 2 ). The first transistor (M 1 ) has a first terminal that receives a supply voltage (VDD), a second terminal that outputs the first auxiliary current signal (Ij 1 ), and a control terminal. The second transistor (M 2 ) has a first terminal that receives the supply voltage (VDD), a second terminal that is coupled to the control terminal of the first transistor (M 1 ) and that outputs the second auxiliary current signal (Ij 2 ), and a control terminal that is coupled to the second terminal of the first transistor (M 1 ). 
     In this embodiment, the transconductance unit  6  includes a third transistor (M 3 ), a fourth transistor (M 4 ) and a current source  63 . The third transistor (M 3 ) has a first terminal that is coupled to the second terminal of the first transistor (M 1 ) and that outputs the first input current signal (IN 1 ), a second terminal, and a control terminal that is coupled to the filter  2  for receiving the first input voltage signal (VIN 1 ) therefrom. The fourth transistor (M 4 ) has a first terminal that is coupled to the second terminal of the second transistor (M 2 ) and that outputs the second input current signal (IN 2 ), a second terminal that is coupled to the second terminal of the third transistor (M 3 ), and a control terminal that is coupled to the filter  2  for receiving the second input voltage signal (VIN 2 ) therefrom. The current source  63  is coupled to the second terminal of the third transistor (M 3 ) for providing a bias current (IS) thereto, and provides a bias voltage. 
     In this embodiment, the mixing module  4  includes a mixing unit  41  and a load unit  42 . The mixing unit  41  is coupled to the single-ended to differential converter  3  for receiving the differential oscillatory voltage signal pair therefrom, and is coupled to the first terminals of the third and fourth transistors (M 3 , M 4 ) for receiving the remaining portions of the first and second input current signals (IN 1 , IN 2 ) respectively therefrom. The mixing unit  41  mixes the remaining portions of the first and second input current signals (IN 1 , IN 2 ) with the differential oscillatory voltage signal pair to generate a differential mixed current signal pair that includes a first mixed current signal (IM 1 ) and a second mixed current signal (IM 2 ). The load unit  42  is coupled to the mixing unit  41  for receiving the differential mixed current signal pair therefrom, and converts the differential mixed current signal pair into the differential mixed voltage signal pair. 
     The mixing unit  41  includes a fifth transistor (M 5 ), a sixth transistor (M 6 ), a seventh transistor (M 7 ) and an eighth transistor (M 8 ). 
     The fifth transistor (M 5 ) has a first terminal, a second terminal that is coupled to the first terminal of the third transistor (M 3 ), and a control terminal that is coupled to the single-ended to differential converter  3  for receiving the first oscillatory voltage signal (VOS 1 ) therefrom. 
     The sixth transistor (M 6 ) has a first terminal, a second terminal that is coupled to the second terminal of the fifth transistor (M 5 ), and a control terminal that is coupled to the single-ended to differential converter  3  for receiving the second oscillatory voltage signal (VOS 2 ) therefrom. The sixth transistor (M 6 ) cooperates with the fifth transistor (M 5 ) to receive the remaining portion of the first input current signal (IN 1 ) from the third transistor (M 3 ). 
     The seventh transistor (M 7 ) has a first terminal that is coupled to the first terminal of the fifth transistor (M 5 ), a second terminal that is coupled to the first terminal of the fourth transistor (M 4 ), and a control terminal that is coupled to the control terminal of the sixth transistor (M 6 ) and that receives the second oscillatory voltage signal (VOS 2 ). The seventh transistor (M 7 ) cooperates with the fifth transistor (M 5 ) to output the first mixed current signal (IM 1 ). 
     The eighth transistor (M 8 ) has a first terminal that is coupled to the first terminal of the sixth transistor (M 6 ), a second terminal that is coupled to the second terminal of the seventh transistor (M 7 ), and a control terminal that is coupled to the control terminal of the fifth transistor (M 5 ) and that receives the first oscillatory voltage signal (VOS 1 ). The eighth transistor (M 8 ) cooperates with the seventh transistor (M 7 ) to receive the remaining portion of the second input current signal (IN 2 ) from the fourth transistor (M 4 ), and cooperates with the sixth transistor (M 6 ) to output the second mixed current signal (IM 2 ). 
     The load unit  42  includes a first inductive transmission line (TL 1 ) and a second inductive transmission line (TL 2 ). 
     The first inductive transmission line (TL 1 ) has a first terminal that receives the supply voltage (VDD), and a second terminal that is coupled to the first terminal of the fifth transistor (M 5 ) for receiving the first mixed current signal (IM 1 ) therefrom and that outputs the first mixed voltage signal (VM 1 ). 
     The second inductive transmission line (TL 2 ) has a first terminal that receives the supply voltage (VDD), and a second terminal that is coupled to the first terminal of the sixth transistor (M 6 ) for receiving the second mixed current signal (IM 2 ) therefrom and that outputs the second mixed voltage signal (VM 2 ). 
     In this embodiment, the buffer  7  includes a ninth transistor (M 9 ), a tenth transistor (M 10 ), an eleventh transistor (M 11 ), a twelfth transistor (M 12 ), a third inductive transmission line (TL 3 ), a fourth inductive transmission line (TL 4 ), a first resistor (R 1 ) and a second resistor (R 2 ), and the buffer output terminals include a first buffer output terminal and a second buffer output terminal. 
     The ninth transistor (M 9 ) has a control terminal that is coupled to the second terminal of the first inductive transmission line (TL 1 ) of the load unit  42  of the mixing module  4  for receiving the first mixed voltage signal (VM 1 ) therefrom, a first terminal, and a second terminal that serves as the first buffer output terminal at which the first buffered voltage signal (VB 1 ) is outputted. 
     The tenth transistor (M 10 ) has a control terminal that is coupled to the second terminal of the second inductive transmission line (TL 2 ) of the load unit  42  of the mixing module  4  for receiving the second mixed voltage signal (VM 2 ) therefrom, a first terminal, and a second terminal that serves as the second buffer output terminal at which the second buffered voltage signal (VB 2 ) is outputted. 
     The eleventh transistor (M 11 ) has a first terminal that is coupled to the second terminal of the ninth transistor (M 9 ), a second terminal, and a control terminal that is coupled to the current source  63  for receiving the bias voltage therefrom. 
     The twelfth transistor (M 12 ) has a first terminal that is coupled to the second terminal of the tenth transistor (M 10 ), a second terminal, and a control terminal that is coupled to the control terminal of the eleventh transistor (M 11 ) and that receives the bias voltage. 
     The third inductive transmission line (TL 3 ) has a first terminal that receives the supply voltage (VDD), and a second terminal that is coupled to the first terminal of the ninth transistor (M 9 ). The third inductive transmission line (TL 3 ) has an inductance that cooperates with parasitic capacitance at the first buffer output terminal to form a first LC tank circuit that reaches resonance at a frequency of the first buffered voltage signal (VB 1 ) to behave as an open circuit. 
     The fourth inductive transmission line (TL 4 ) has a first terminal that receives the supply voltage (VDD), and a second terminal that is coupled to the first terminal of the tenth transistor (M 10 ). The fourth inductive transmission line (TL 4 ) has an inductance that cooperates with parasitic capacitance at the second buffer output terminal to form a second LC tank circuit that reaches resonance at a frequency of the second buffered voltage signal (VB 2 ) to behave as an open circuit. 
     The first resistor (R 1 ) is coupled between the second terminal of the eleventh transistor (M 11 ) and ground. The second resistor (R 2 ) is coupled between the second terminal of the twelfth transistor (M 12 ) and ground. 
     In this embodiment, each of the first and second transistors (M 1 , M 2 ) is, for example, a P-type metal oxide semiconductor field effect transistor, and each of the third to twelfth transistors (M 3 ˜M 12 ) is, for example, an N-type metal oxide semiconductor field effect transistor. In such a case, the parasitic capacitance at the first buffer output terminal refers to parasitic capacitance between source and drain terminals of each of the ninth and eleventh transistors (M 9 , M 11 ), and the parasitic capacitance at the second buffer output terminal refers to parasitic capacitance between source and drain terminals of each of the tenth and twelfth transistors (M 10 , M 12 ). A voltage gain (VG) of the buffer  7  may be derived as follows. 
                       VG   =       Z   L         1     g       m   ⁢           ⁢   9     ,   10         +     Z   L           ,   and     ⁢     
     ⁢             Z   L     =       ⁢       Z     out   ⁢           ⁢   1       ⁢             ⁢     Z     out   ⁢           ⁢   2                     =       ⁢       [       R     1   ,   2       +       (       1     sC       ds   ⁢           ⁢   11     ,   12         ⁢             ⁢     R       ds   ⁢           ⁢   11     ,   12         )     ⁢     (     1   +       g       m   ⁢           ⁢   11     ,   12       ⁢     R     1   ,   2           )         ]     ⁢                               ⁢     [       sL     3   ,   4       +       (       1     sC       ds   ⁢           ⁢   9     ,   10         ⁢             ⁢     R       ds   ⁢           ⁢   9     ,   10         )     ⁢     (     1   +       g       m   ⁢           ⁢   9     ,   10       ·     sL     3   ,   4           )         ]                 ≈       ⁢       [       R     1   ,   2       +         1     sC       ds   ⁢           ⁢   11     ,   12         ·     g       m   ⁢           ⁢   11     ,   12         ⁢     R     1   ,   2           ]     ⁢               ⁡     [       sL     3   ,   4       +       1     sC       ds   ⁢           ⁢   9     ,   10         ·     g       m   ⁢           ⁢   9     ,   10       ·     sL     3   ,   4           ]                         ⁢     (     at   ⁢           ⁢   radio   ⁢           ⁢   frequency     )                   =       ⁢       [       R     1   ,   2       +       1     s   ⁡     [       C       ds   ⁢           ⁢   11     ,   12       /     (       g       m   ⁢           ⁢   11     ,   12       ⁢     R     1   ,   2         )       ]         ·       ]     ⁢               ⁡     [           L     3   ,   4       ⁢     g       m   ⁢           ⁢   9     ,   10           C       ds   ⁢           ⁢   9     ,   10         +     sL     3   ,   4         ]           ,                   (   1   )               
where Z L  represents an output impedance seen into the buffer  7  from each of the first and second buffer output terminals, g m9,10  represents a transconductance of each of the ninth and tenth transistors (M 9 , M 10 ), g m11,12  represents a transconductance of each of the eleventh and twelfth transistors (M 11 , M 12 ), Z out1  represents an impedance seen into each of the eleventh and twelfth transistors (M 11 , M 12 ) from the corresponding one of the first and second buffer output terminals, Z out2  represents an impedance seen into each of the ninth and tenth transistors (M 9 , M 10 ) from the corresponding one of the first and second buffer output terminals, R 1,2  represents a resistance of each of the first and second resistors (R 1 , R 2 ), C ds11,12  and R ds11,12  respectively represent a parasitic capacitance and a parasitic resistance between the source and drain terminals of each of the eleventh and twelfth transistors (M 11 , M 12 ), C ds9,10  and R ds9,10  respectively represent a parasitic capacitance and a parasitic resistance between the source and drain terminals of each of the ninth and tenth transistors (M 9 , M 10 ), and L 3,4  represents an inductance of each of the third and fourth inductive transmission lines (TL 3 , TL 4 ).
 
     In this embodiment, the buffer  7  is configured to satisfy a relationship (2), and satisfy a relationship (3) at an RF operation frequency (e.g., the frequency of the differential buffered voltage signal pair, which is 79 GHz in this embodiment). 
     
       
         
           
             
               
                 
                   
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     For instance, difference between the two sides of each of the relationships (2) and (3) is less than 10% (i.e., a ratio of the left side to the right side ranges between 0.9 and 1.1). The meaning of the relationship (3) is that the effect of the capacitance C ds11,12 /(g m11,12 R 1,2 ) can be cancelled by the inductance L 3,4  at the RF operation frequency. That is, the parallel of C ds11,12 /(g m11,12 R 1,2 ) and L 3,4  behaves as an open circuit at resonance. Accordingly, the relationship (1) may be further simplified as 
                 Z   L     ≈       (       R     1   ,   2       ⁢             ⁢         L     3   ,   4       ⁢     g       m   ⁢           ⁢   9     ,   10           C       ds   ⁢           ⁢   9     ,   10           )     +         C       ds   ⁢           ⁢   9     ,   10       ⁢     L     3   ,   4       ⁢     g       m   ⁢           ⁢   11     ,   12       ⁢     R     1   ,   2             C       ds   ⁢           ⁢   11     ,   12       ⁡     (         R     1   ,   2       ⁢     C       ds   ⁢           ⁢   9     ,   10         +       L     3   ,   4       ⁢     g       m   ⁢           ⁢   9     ,   10           )             ,         
which is far larger than (1/g m9,10 ), and the voltage gain (VG) is thus approximately 1.
 
     A conversion gain (CG) of the mixer (i.e., a ratio of a difference of the differential mixed voltage signal pair to a difference of the differential input voltage signal pair) can be expressed by the following equation: 
                 C   ⁢           ⁢   G     =       2   π     ⁢       G     m   ,   LO         (       G     m   ,   LO       -     G       m   ⁢           ⁢   1     ,   2         )       ⁢     G       m   ⁢           ⁢   3     ,   4       ×     ω   RF     ×   L       ,         
where G m,LO  denotes an equivalent transconductance seen into the mixing unit  41  from the second terminal of each of the fifth and seventh transistors (M 5 , M 7 ), Gm 1,2  denotes a transconductance of each of the first and second transistors (M 1 , M 2 ), G m3,4  denotes a transconductance of each of the third and fourth transistors (M 3 , M 4 ), ω RF  denotes an angular frequency of the differential mixed voltage signal pair, and L denotes an inductance of each of the first and second inductive transmission lines (TL 1 , TL 2 ), whose equivalent impedance is typically approximately 400 ohms. It is noted that, when the differential to single-ended converter  8  is directly coupled to the mixing module  4  and receives the differential mixed voltage signal pair therefrom, the low input impedance of the balun may result in a low conversion gain of the entire mixer since an equivalent output impedance in the equation above would be derived as |jωL∥50Ω| instead of L, and would approximate to 50Ω since ωL is approximately 400Ω at the frequency of the differential buffered voltage signal pair. Accordingly, the buffer  7  prevents the conversion gain (CG) from being pulled down by the balun of the differential to single-ended converter  8 . In addition, it is known from the equation that the gain boost unit  5  can boost the conversion gain (CG), and that the conversion gain (CG) increases with increase of the trans conductance (Gm 1,2 ).
 
     Moreover, power consumption of the first and second inductive transmission lines (TL 1 , TL 2 ) and thus power consumption of the mixer can be decreased by increasing the first and second auxiliary current signals (Ij 1 , Ij 2 ). 
       FIG. 3  illustrates conversion gain versus frequency characteristic in various conditions. Because the buffer  7  can reduce the loading effect from the differential to single-ended converter  8 , it is known from  FIG. 3  that the conversion gain is higher in this embodiment than in a condition without the buffer  7 , and the conversion gain is higher in this embodiment without the buffer  7  than in the conventional Gilbert mixer. In other words, the gain boost unit  5  and the buffer  7  can enhance the conversion gain of this embodiment. 
       FIG. 4A  illustrates reflection coefficient S 22  versus frequency characteristic obtained from an input terminal of the single-ended to differential converter  3 , at which the single-ended oscillatory voltage signal (LO) is received, and  FIG. 4B  illustrates reflection coefficient S 33  versus frequency characteristic obtained from an output terminal of the differential to single-ended converter  8 , at which the single-ended buffered voltage signal (RF) is outputted. It is known from  FIGS. 4A and 4B  that at 79 GHz, the reflection coefficients S 22 , S 33  are respectively −17.5 dB and −19.5 dB. In other words, the mixer of this embodiment can achieve good energy transmission. 
       FIG. 5  illustrates relationship between isolation between the input terminal of the single-ended to differential converter  3  and the output terminal of the differential to single-ended converter  8  versus power of the single-ended oscillatory voltage signal (LO). It is known from  FIG. 5  that the isolation between the input terminal of the single-ended to differential converter  3  and the output terminal of the differential to single-ended converter  8  is good. 
       FIG. 6  shows a circuit block diagram of a mixer similar to that shown in  FIG. 2 , and differs in that the mixer of  FIG. 6  includes a buffer  7 ′ instead of the buffer  7  as shown in  FIG. 2 . The buffer  7 ′ includes a ninth transistor (M 9 ), a tenth transistor (M 10 ), an eleventh transistor (M 11 ), a twelfth transistor (M 12 ), a thirteenth transistor (M 13 ), a fourteenth transistor (M 14 ), a third inductive transmission line (TL 3 ), a fourth inductive transmission line (TL 4 ), a first resistor (R 1 ) and a second resistor (R 2 ). 
     The ninth transistor (M 9 ) has a control terminal that is coupled to the second terminal of the first inductive transmission line (TL 1 ) of the load unit  42  of the mixing module  4  for receiving the first mixed voltage signal (VM 1 ) therefrom, a first terminal, and a second terminal that outputs the first buffered voltage signal (VB 1 ). 
     The tenth transistor (M 10 ) has a control terminal that is coupled to the second terminal of the second inductive transmission line (TL 2 ) of the load unit  42  of the mixing module  4  for receiving the second mixed voltage signal (VM 2 ) therefrom, a first terminal, and a second terminal that outputs the second buffered voltage signal (VB 2 ). 
     The eleventh transistor (M 11 ) has a first terminal, a second terminal, and a control terminal that is coupled to the current source  63  for receiving the bias voltage therefrom. 
     The twelfth transistor (M 12 ) has a first terminal, a second terminal, and a control terminal that is coupled to the control terminal of the eleventh transistor (M 11 ) and that receives the bias voltage. 
     The thirteenth transistor (M 13 ) has a first terminal that is coupled to the second terminal of the ninth transistor (M 9 ), a second terminal coupled to the first terminal of the eleventh transistor (M 11 ), and a control terminal that is coupled to the current source  63  for receiving the bias voltage therefrom. 
     The fourteenth transistor (M 14 ) has a first terminal that is coupled to the second terminal of the tenth transistor (M 10 ), a second terminal coupled to the first terminal of the twelfth transistor (M 12 ), and a control terminal that is coupled to the current source  63  for receiving the bias voltage therefrom. 
     The third inductive transmission line (TL 3 ) has a first terminal that receives the supply voltage (VDD), and a second terminal that is coupled to the first terminal of the ninth transistor (M 9 ). 
     The fourth inductive transmission line (TL 4 ) has a first terminal that receives the supply voltage (VDD), and a second terminal that is coupled to the first terminal of the tenth transistor (M 10 ). 
     The first resistor (R 1 ) is coupled between the second terminal of the eleventh transistor (M 11 ) and ground. The second resistor (R 2 ) is coupled between the second terminal of the twelfth transistor (M 12 ) and ground. 
     In such modification, the cascode-type current source is used instead of the common-source current source. Since the cascode-type current source may have greater output impedance, the gain of the buffer  7 ′ is closer to 1 in comparison to the common-source current source, resulting in larger conversion gain of the entire mixer circuit. 
     In view of the above, the mixer of this embodiment can simultaneously achieve low power consumption and high conversion gain. 
     While the disclosure has been described in connection with what is considered the exemplary embodiment, it is understood that this disclosure is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.