Abstract:
A current-reusing bleeding mixer capable of providing a higher conversion gain, linearity and lower noise figure employing a field-effect transistor includes a first to a fourth transistor and a first and a second load element. The first transistor amplifies a radio frequency (RF) signal. The second and the third transistor, each connected to the first transistor, receive a balanced local oscillator (LO) signal to mix it with the RF signal. The first and the second load element are connected between a supply voltage source and the second transistor and between the supply voltage source and the third transistor, respectively. The fourth transistor, connected between the supply voltage source and the first transistor, amplifies the RF signal and bleeds a current from the supply voltage source.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a mixer for converting a frequency; and, in particular, to a current-reusing bleeding mixer capable of providing a higher conversion gain and linearity, and a lower noise figure employing field-effect transistors. 
     DESCRIPTION OF THE PRIOR ART 
     In recent years, RF IC technologies have evolved ever rapidly together with the exploding consumer wireless communications infrastructure. Ever since the invention of the superheterodyne concept by Armstrong in 1918, mixers have been of critical importance in determining the overall performance of radio receivers, virtually all of which require at least one mixer. Of components comprising a heterodyne radio receiver, a down-conversion mixer is probably the most important block that influences the performance. A down-conversion mixer performance is a dominant factor in a system noise figure and linearity, and determines performance requirements of its adjacent blocks, especially those of a low-noise amplifier. 
     Mixers can generally be categorized into passive and active mixers. Passive mixers, such as diode mixers and passive field effect transistor (FET) mixers, have no conversion gain. On the other hand, active mixers have conversion gain that acts to reduce the noise contribution from an intermediate frequency (IF) stage. It is the active mixers on which we will concentrate hereinafter. 
     The majority of the active mixers are based on the “Gilbert cell” well known in the art and some down-conversion mixers choose a single-balanced type configuration. The key specifications of the mixer are the conversion gain, noise figure, and the linearity. 
     Referring to  FIG. 1 , there is shown a conventional basic single balanced mixer  10  based on CMOS transistors. The single balanced mixer  10  downconverts a single-ended radio frequency (RF) signal having a predetermined center frequency value (ω RF ) to a lower center frequency value (ω IF ) by mixing the RF signal with a balanced local oscillator (LO) signal having a positive phase portion LO +  and a balanced local oscillator signal with a negative phase portion LO − . 
     The single balanced mixer  10  includes load resistors R 11  and R 12 , a differential pair of MOS transistors M 12  and M 13  as a switching pair and a MOS transistor M 11  as a driver stage. The load resistors R 11  and R 12  are connected to a supply voltage V DD  and drains of the pair of transistors M 12  and M 13 , respectively. Each of sources of the pair of transistors M 12  and M 13  is connected in parallel to a drain of the transistor M 11 . A source of the transistor M 11  is grounded. Intermediate frequency (IF) output terminals IF +  and IF −  are formed between the load resistor R 11  and the transistor M 12  and between the load resistor R 12  and the transistor M 13 , respectively. 
     The transistor M 11  is operated as a transconductance amplifier and the transistors M 12  and M 13  perform switching functions. A radio frequency (RF) signal and the balanced LO signals LO +  and LO −  are inputted to a gate of the transistor M 11  and respective gates of transistors M 12  and M 13 , respectively. The RF signal inputted to the transistor M 11  is amplified and then mixed with the differential LO signals LO+ and LO− applied to the transistors M 12  and M 13 , respectively, to thereby output respective downconverted IF signals at the IF output terminals IF+ and IF−. 
     Assuming an ideal LO switching at each LO terminal and using a long-channel device expressions for the drain currents, it can be shown that the differential output current of the mixer  10  shown in  FIG. 1  is given by Eq. 1 as follows: 
                     i     out   ,   conv       =       ⁢       i     out   ,   conv     +     -     i     out   ,   conv     -                   =       ⁢           4   ⁢     I   D1       π     ⁢   cos   ⁢           ⁢     ω   LO     ⁢   t     +         2   ⁢     g   mn1     ⁢     v   RF       π     ⁢     cos   ⁡     (       ω   LO     ±     ω   RF       )       ⁢   t     +                     ⁢     higher   ⁢           ⁢   order   ⁢           ⁢   terms                   Eq.1             
 
where i out,conv  is the differential output current of the conventional single-balanced mixer  10 , I D1  and g mn1  are a drain current and a transconductance of the transistor M 11 , respectively, ν RF  is a voltage amplitude of the RF signal applied to the transistor M 11 , ω RF  and ω LO  are frequencies of the RF signal and the LO signal, respectively. As shown in Eq. 1, since the output signal IF (i out,conv  ) is generated from the differential signals IF +  and IF − , a frequency component (ω RF ) of the RF signal is cancelled in the IF signal. However, a problem rises in which a separate notch or band stop filter is needed to remove the LO component from the IF signal because a frequency component of the LO signal remains, e.g., 
           4   ⁢     I   D1       π     ⁢   cos   ⁢           ⁢     ω   LO     ⁢     t   .         
 
     Also, the active mixer linearity tends to be dominantly determined at the driver stage, where the DC bias current plays a major role. For the single-balanced mixer  10  in  FIG. 1 , the increase in the driver stage current forces a reduction of the load resistance. The driver stage current may be increased to improve the linearity of the mixer  10 , and then, however, it is difficult to increase the conversion gain because the conversion gain depends on the load resistors R 11 , R 12  and the transconductance of the MOS transistor M 11 . 
     Another prior art mixer design, the so-called double-balanced mixer, typically shown by  20  in  FIG. 2 , eliminates the need for the band stop filter by using two differential pairs cross-coupled so as to cancel the LO component from the IF output signal. The operation and structure of the double-balanced or “Gilbert” mixer, as it is known in the art, are well known. Although the Gilbert mixer does cancel the LO component in the IF output signal, it does little to improve the basically-nonlinear performance of the single-balanced mixer  10  of FIG.  1 . Moreover, the Gilbert mixer requires a matching network at its RF input to achieve low noise. 
     FIG.  3  and  FIG. 4  represent conventional CMOS based single-balanced and double-balanced mixers  30  and  40  with current bleedings I bld  as current sources, respectively. The mixers  30  and  40  described above further include bleedings I bld  in addition to structures of the single-balanced and the double-balanced mixers  10  and  20  shown in  FIGS. 1 and 2 , respectively. 
     The bleedings I bld  can allow a higher conversion gain through a higher load resistor as some of the driver stage currents are being steered away from the switching transistors. Furthermore, with bleedings I bld , the switching transistors could be operated at a lower gate-source voltage or smaller size transistors could be used. In either case, for a given amount of the LO signal, the bleeding I bld  helps to improve the conversion efficiency as the smaller amount of currents are necessary to turn them on and off. 
     However, the bleeding I bld  can degrade the high frequency performance at the driver stage, especially with MOS transistors and/or with too much bleeding. The smaller DC current through the switching transistors reduces the transconductance of them such that the higher impedances are presented at the output of the driver stage. The major down side of the current bleeding is the addition of noise signals, especially with the current source I bld  implemented with active devices. The reduction of the transconductance of the switching transistors and the additional noise signals may be factors that increase the noise figure. 
     Referring to  FIG. 5 , there is provided a conventional double-balanced mixer  50  with a series connection of a parallel LC tank circuit in addition to the mixer  20  shown in FIG.  2 . The LC tank circuit is used to eliminate the addition of the noise signals generated by the bleeding circuit. However, not only can the LC tank circuit increases the die area significantly, but it does not stop many other frequency components of the noise signals down converted to the desired signal. 
     Therefore, there is a demand for circuit configuration featuring an increased conversion gain and linearity as well as a reduced noise figure in the mixer. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a mixer circuit capable of providing a better performance in conversion gain, linearity, noise figure, and LO isolation. 
     In accordance with a preferred embodiment of the present invention, there is provided a mixer, comprising: 
     a first transistor for amplifying a radio frequency (RF) signal; 
     a second and a third transistor, each connected to the first transistor, for receiving a balanced local oscillator (LO) signal to mix it with the RF signal; 
     a first and a second load element connected between a supply voltage source and the second transistor and the supply voltage source and the third transistor, respectively; and 
     a fourth transistor, connected between the supply voltage source and the first transistor, for amplifying the RF signal and bleeding a current from the supply voltage source. 
     In accordance with another preferred embodiment of the present invention, there is provided a mixer, comprising: 
     a first differential circuit including a first and a second transistor connected differentially to each other, for amplifying a balanced radio frequency (RF) signal; 
     a second differential circuit including a third and a fourth transistor connected differentially to each other, the third and the fourth transistor being connected to the first and the second transistor, respectively, for receiving a balanced local oscillator (LO) signal to mix it with the balanced RF signal; 
     a third differential circuit including a fifth and a sixth transistor connected differentially to each other, the fifth and the sixth transistor being connected to the first and the second transistor and cross-coupled to the third and the fourth transistor, respectively, for receiving a balanced local oscillator (LO) signal to mix it with the balanced RF signal; 
     a first and a second load element connected between a supply voltage source and the third transistor and the supply voltage source and the sixth transistor, respectively; and 
     a fourth circuit including a seventh and an eighth transistor, connected to the supply voltage source and the first differential circuit, for amplifying the RF signal and bleeding a current from the supply voltage source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a circuit diagram of a conventional single-balanced mixer; 
         FIG. 2  depicts a circuit diagram of a conventional double-balanced mixer; 
         FIG. 3  illustrates a circuit diagram of a conventional single-balanced mixer with a current bleeding; 
         FIG. 4  provides a circuit diagram of a conventional double-balanced mixer with current bleedings; 
         FIG. 5  represents a circuit diagram of a conventional double-balanced mixer with LC tank circuits in addition to the current bleedings; 
         FIG. 6  presents a circuit diagram of a single-balanced mixer with a current-reusing bleeding in accordance with a preferred embodiment of the present invention; 
         FIG. 7  depicts a circuit diagram of the single-balanced mixer of the  FIG. 6  further including a coupling capacitor in accordance with a preferred embodiment of the present invention; 
         FIG. 8  sets forth a circuit diagram of a double-balanced mixer with current-reusing bleeding in accordance with another embodiment of the present invention; 
         FIG. 9  pictures a circuit diagram of a double-balanced mixer with coupling and bypassing capacitors in accordance with another embodiment of the present invention; 
         FIG. 10  is a circuit diagram of the conventional single-balanced mixer for simulation; and 
         FIG. 11  shows a circuit diagram of the single-balanced mixer for simulation in accordance with the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 6 , there is provided a current-reusing bleeding single-balanced mixer  60  in accordance with a first preferred embodiment of the present invention. The current-reusing bleeding single-balanced mixer  60  downconverts a single-ended radio frequency (RF) signal having a predetermined center frequency value to a lower center frequency value by mixing the RF signal with a balanced local oscillator (LO) signal having a positive phase portion LO +  and with a balanced local oscillator signal having a negative phase portion LO − . 
     The current-reusing bleeding single-balanced mixer  60  includes load resistors R 61  and R 62 , a differential pair of MOS transistors M 63  and M 64  as a switching pair and a MOS transistor M 61  as a driver stage and a current-reusing bleeding MOS transistor M 62 . The load resistors R 61  and R 62  are connected to a supply voltage V DD  and respective drains of the pair of MOS transistors M 63  and M 64 . Each of sources of the pair of transistors M 63  and M 64  is connected in parallel to a drain of the transistor M 61 . A source of the transistor M 61  is grounded. A source of the transistor M 62  is connected to the V DD  and its drain is connected to the sources of the transistors M 63  and M 64  and the drain of the transistor M 61 . A gate of the transistor M 62  is connected to the gate of the transistor M 61  to which RF signal is inputted. Each of the transistors M 61 , M 63  and M 64  is an n-channel MOS transistor and the transistor M 62  is a p-channel MOS transistor. Intermediate frequency (IF) output terminals IF +  and IF −  are formed between the load resistor R 61  and the transistor M 63  and between the load resistor R 52  and the transistor M 64 , respectively. 
     The transistor M 61  is operated as a transconductance amplifier and the transistors M 63  and M 64  perform switching functions. The transistor M 62  is operated as a bleeding current source for DC as well as a transconductace amplifier for AC. A radio frequency (RF) signal and the balanced LO signals LO +  and LO −  are inputted to a gate of the transistor M 61  and M 62  and respective gates of transistors M 63  and M 64 , respectively. The RF signal inputted to the transistors M 61  and M 62  is amplified and then mixed with the differential LO signals LO+ and LO− applied to the transistors M 63  and M 64 , respectively, to thereby output downconverted intermediate frequency (IF) signals at the IF output terminals IF+ and IF−. 
     Assuming an ideal LO switching and using the long-channel device expressions for the drain currents, it can be shown that the differential output current of the mixer  60  shown in  FIG. 6  is given by Eq. 2 as follows: 
                           i     out   ,   bld       =       ⁢       i     out   ,   bld     +     -     i     out   ,   bld     -                   =       ⁢             4   ⁢     (       I   D61     -     I   D62       )       +       (       β   n61     -     β   p62       )     ⁢     v   RF   2         π     ⁢   cos   ⁢           ⁢     ω   LO     ⁢   t     +                     ⁢           2   ⁢     (       g   mn61     +     g     m   ⁢           ⁢   p62         )     ⁢     v   RF       π     ⁢     cos   ⁡     (       ω   LO     ±     ω   RF       )       ⁢   t     +                     ⁢     higher   ⁢           ⁢   order   ⁢           ⁢   terms                   Eq.2               
 
where i out,bld  is the differential output current of the current-reusing single-balanced mixer  60 , g mn61  and g mp62  are transconductances of the transistor M 61  and M 62 , respectively, β n61  and β p62  are KP·W/L of the transistor M 61  and M 62 , wherein the KP represents the transconductance parameter, and W and L the channel width and length of the MOS transistors, respectively. ν RF  represents the voltage amplitude of the applied RF signal, ω LO  and ω RF  represent the LO and RF signals frequencies, respectively. I D61  and I D62  are drain currents of the transistors M 61  and M 62 , respectively.
 
     As shown in Eq. 2, since the IF output signal i out,bld  is generated from the differential signals IF+ and IF−, a frequency component of the RF signal is cancelled in the IF signal. Also, the current-reusing bleeding I bld  can suppress the LO signal at the IF terminal, in a same way as in the conventional double-balanced mixer  20  of the FIG.  2 . 
     Furthermore, from Eq. 2, contrary to the conventional mixer  10  shown in  FIG. 1 , the current-reusing mixer  60  provides complete LO isolation if 4(I D1 −I D2 )+(β n1 −β p2 )ν 2   RF =0 . For a small RF signal V RF , it is possible to nearly cancel the LO signal at the output by making I D1 =I D2 . For I D1 =I D2 , the switching pair M 63  and M 64  operate like a passive mixer. Be on the active mixer, generally I D1 &gt;I D2 , therefore a partial cancellation. Even if it may not be a complete cancellation, reducing the LO signal at the output of the single-balanced mixer  60  has a definite advantage because the large LO signal applied to the transistors M 63  and M 64  tends to push the transistors M 63  and M 64  into linear operational region. 
     Comparing the mixers  10  and  60  shown in  FIGS. 1 and 6 , the effective DC currents of the driver stages are I D11  and I D61 +I D62  (each of M 61  and M 62  is effectively a single transistor), respectively. Because the increase in the transconductance of the driver stage leads to lower noise figure, for the same overall supply current I D11 =I D61 &lt;I D61 +I D62 , the noise figure of the proposed mixer  60  is lower than that of the conventional mixer  10 . In addition, the input third order intercept point (IP 3 ) of the current-reusing bleeding mixer  60  is expected to be higher than that of the conventional mixer  10  as the IP 3  increases with bias current. 
     A class AB operation of the single-balanced mixer for high P1 dB (1 dB compression point) is introduced in “A Class AB Monolithic Mixer for 900-MHz Applications” IEEE J. Solid-State Circuits, Vol. 32, No. 8, pp. 1166-1172, Aug. 1997. by K. Fong, C. D. Hull and R. G. Meyer. Under class AB operation, the DC current of the driver stage rises with the increase in the RF input power. One additional feature of the current-reusing bleeding mixer  60  is that under the class AB operation of the driver stage, when the DC current through transistor M 61  increases, so does the current through transistor M 62 . Therefore, the amount of bleeding tends to increase by following the increase in the main driver amplifier current I D61  to thereby maintain an initial percentage ratio. 
     Referring now  FIG. 7 , there is provided a current-reusing bleeding single-balanced mixer  70  in accordance with a second preferred embodiment of the present invention. The current-reusing bleeding single-balanced mixer  70  further includes a capacitor C coupling  in addition to the structures in the current-reusing bleeding single-balanced mixer  60  shown in FIG.  6 . The capacitor C coupling  is connected between a gate of the transistor M 71  and a gate of the transistor M 72  and decouples DC contained in the RF signal. Therefore, a predetermined DC voltage can be applied to the gate of the transistor M 72  irrespective of the transistor M 71 . The operation of the mixer  70  is similar to that of the mixer  60  shown in FIG.  6 . 
       FIG. 8  represents a current-reusing bleeding double-balanced mixer  80  in accordance with a third preferred embodiment of the present invention. The current-reusing bleeding double-balanced mixer  80  further includes current-reusing bleeding MOS transistors M 84  and M 88  in addition to a Gilbert mixer as it is well known. Sources of the current-bleeding transistors M 84  and M 88  are connected to V DD  and their drains and gates are connected to drains and gates of driver amplifiers M 81  and M 85 , respectively. Operations of the current-reusing bleeding transistors M 84  and M 88  are similar to those of current-reusing single-balanced mixer  60  in FIG.  6 . 
       FIG. 9  shows a mixer  90  with current-reusing bleeding transistors M 92  and M 94  in accordance with a fourth preferred embodiment of the present invention. The mixer  90  further includes current-reusing bleeding MOS transistors M 92  and M 94  in addition to a mixer described in “A 2.7-v 900-MHz CMOS LNA and Mixer” IEEE J. of Solid-State Circuits, Vol. 31, No. 12, Dec. 1996 by Karanicolas. As shown in  FIG. 9 , by adding the bleeding transistors M 92  and M 94 , all the merits described above can be achieved. Further, since transistors M 91  and M 93  are implemented coupled with the transistors M 92  and M 94 , respectively, the circuit in  FIG. 9  has a much better symmetry than the circuit proposed by Karanicolas. Still further, since the features of the transistor pairs M 91 -M 92  and M 93 -M 94  get more symmetrical, the improvement of the linearity in an entire drive stage can be expected. 
     EXAMPLE 
       FIGS. 10 and 11  are circuit diagrams for simulation to compare the conventional single-balanced mixer  10  to the current-reusing bleeding single-balanced mixer  60 . A performance comparison has been carried out between the conventional mixer  10  shown in FIG.  1  and the mixer  60  shown in FIG.  6 . The 900 MHz down-conversion mixers are designed by using a 0.35 μm CMOS process. The 300 and 400 μm sizes are used for the n- and p-channel devices, respectively. The details of the bias conditions and the component values are shown in  FIGS. 10 and 11  in the parenthesis. For example, M 12  (300/0.35) shown in  FIG. 10  represents that the MOS transistor M 12  is an n-channel device of 300 μm width and 0.35 μm length. Also, M 72  (400/0.35) shown in  FIG. 11  depicts that the MOS transistor is a p-channel device of 400 μm width and 0.35 μm length. Both circuits are biased to operate at the same DC supply currents and voltages. The bleeding current is adjusted to be 50% of the total current. 
     Table 1 summarizes the simulation results. 
     
       
         
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Current-reused 
               
               
                   
                 Specifications 
                 Conventional mixer 
                 bleeding mixer 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Simulation conditions 
                 f RF  = 900 MHz, f LO  = 1 GHz, 
                   
               
               
                   
                   
                 LO input power = 0 dBm 
               
             
          
           
               
                   
                 Conversion power gain 
                 0 
                 4 
               
               
                   
                 [dB] 
               
               
                   
                 Noise figure [dB] 
                 12.1 
                 11.2 
               
               
                   
                 LO power at IF-port 
                 −3.6 
                 −6.9 
               
               
                   
                 [dBm] 
               
               
                   
                 Output IP3 [dBm] 
                 −4 
                 −1.6 
               
               
                   
                   
               
             
          
         
       
     
     As can be seen from Table 1, the current-reusing bleeding mixer  60  in accordance with the present invention demonstrates 4 dB higher conversion gain, 0.9 dB lower noise figure, 2.4 dB higher IP 3 , and 3.3 dB lower LO power at the output over that of the conventional mixer. 
     Without losing the advantages discussed above, the proposed idea can be applied to other types of the mixer topologies including the Gilbert cell type, as well as the up-conversion applications. In fact, the up-conversion mixers are expected to show the advantages more clearly, since the p-channel device performances are more comparable to the n-channel at the significantly lower input frequencies. The proposed idea can be applied to the complimentary bipolar processes as well. 
     While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.