Patent Document

CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2007-167376 filed in the Japanese Patent Office on Jun. 26, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multiband radio technique using a plurality of radio frequency (RF) bands in a radio communication system, such as a cellular phone, a wireless LAN (WLAN), and the like. More particularly, the present invention relates to a radio communication apparatus which can be adapted to a plurality of RF bands. 
     2. Description of the Related Art 
     In recent years, an increase in the number of users of cellular phones has made it difficult to accommodate new users only in the frequency bands allocated for the third-generation cellular phones at first. 
     In order to solve this problem, it is necessary to develop a mobile terminal corresponding to a plurality of frequency bands, which are produced, for example, by allocation of a new frequency band, replacement of frequency bands that have been allocated to the second-generation, and the like. 
     As shown in  FIG. 1 , 3GPP, which has produced the specifications of the third-generation cellular phones, defines ten frequency bands between the band I and the band X. 
     In general, in the receiving section of a mobile telephone terminal, a direct conversion method is used, because the receiver using that method can be implemented with a fewer number of parts than the receiver using a superheterodyne method (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-246323). 
       FIG. 2  is a diagram illustrating an example of a configuration of a communication apparatus having a receiving system of a cellular phone using a direct conversion method. 
     As shown in  FIG. 2 , the communication apparatus  1  has an antenna (ANT)  2 , a switch (SW)  3 , a duplexer (DUP)  4 , a transmission power amplifier (PA)  5 , a low-noise amplifier (LNA)  6 , a filter  7 , a local oscillator (LO)  8 , a divider (phase shifter)  9 , mixers (MIX)  10 I and  10 Q, low-path filters (LPF)  11 I and  11 Q, and a baseband circuit  12 . 
     In the communication apparatus  1 , an RF signal received by the antenna  2  is demodulated through the switch  3 , the duplexer  4 , the LNA  6 , the filter  7 , the mixers  10 I and  10 Q, the LPFs  11 I and  11 Q, and the baseband circuit  12 . 
     Here, in  FIG. 2 , the LNA  6 , the filter  7 , and the mixers  10 I and  10 Q, which are surrounded by a broken line, are requested to have a different frequency characteristic depending on each receiving frequency. 
     SUMMARY OF THE INVENTION 
     In  FIG. 2 , although the LNA  6 , the filter  7 , and the mixers  10 I and  10 Q, which are surrounded by a broken line, are requested to have a different frequency characteristic depending on each receiving frequency, the filter  7  used here is requested to process a high frequency and to have a sharp attenuation characteristic, and thus it is difficult to achieve this function by an IC internal circuit. 
     As a result, in order to process a plurality of receiving frequencies, it is necessary for a mobile telephone terminal to include a plurality of filter parts in addition to an IC, and thus it is not appropriate for miniaturization of the mobile telephone terminal. 
     It is desirable to provide a radio communication apparatus which can dispense with filter parts, can prevent an increase in the number of parts in the case of having a multiband capability, can be miniaturized, and can achieve receiving processing with high precision. 
     According to an embodiment of the present invention, there is provided a radio communication apparatus using a direct conversion method capable of receiving a radio signal having a predetermined frequency band, the radio communication apparatus including: a low-noise amplifier section including one or a plurality of low-noise amplifiers receiving input of a receiving signal having a predetermined frequency band; and a mixer section including in-phase and quadrature mixers demodulating an output of the low-noise amplifier into in-phase-component and quadrature-component signals, respectively, wherein the mixer section includes a capacitor in an input section, separates the in-phase component and the quadrature component in direct current by the capacitor, and supplies the components to the corresponding in-phase and quadrature mixers, respectively. 
     In the embodiment of the present invention, the low-noise amplifier section preferably includes a bias circuit generating a bias signal biassing a signal-input terminal of the low-noise amplifier; and a filter reducing noise of an output signal of the bias circuit and supplying the signal to the signal-input terminal. 
     In the embodiment of the present invention, each of the plurality of low-noise amplifiers is preferably formed by a differential pair of transistors, and each of the differential pairs of transistors has a differential inductor for degeneration in common at a reference potential side. 
     Also, in the embodiment of the present invention, each of the plurality of low-noise amplifiers is preferably formed by a differential pair of transistors, and each of the differential pairs of transistors has cascode-connected transistors and a load inductor in common at an output side. 
     In the embodiment of the present invention, the low-noise amplifier section preferably has a switch selectively supplying the bias signal through the filter to input of the low-noise amplifier corresponding to input of the frequency signal in response to a receiving frequency. 
     In the embodiment of the present invention, the in-phase and quadrature mixers are preferably formed individually by a Gilbert cell mixer including a differential pair of transistors, and the input section of the mixer section supplies an output signal of the low-noise amplifier section to a differential connection section of the transistors of the corresponding cell through a capacitor. 
     By the present invention, a signal having been subjected to the amplification operation by a predetermined low-noise amplifier of the low-noise amplifier section is directly input into the mixer section. 
     The mixer section includes a capacitor in an input section, separates an in-phase component and a quadrature component in direct current by the capacitor, and supplies the components to the corresponding in-phase mixer and quadrature mixer, respectively. 
     By the present invention, it is possible to provide a radio communication apparatus which can make filter parts unnecessary, can prevent an increase in the number of parts in the case of having a multiband capability, can be miniaturized, and can achieve receiving processing with high precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a list of transmission and receiving frequencies for third-generation cellular phones; 
         FIG. 2  is a diagram illustrating an example of a configuration of a communication apparatus, primarily on a receiving system, of a cellular phone using a direct conversion method; 
         FIG. 3  is a diagram illustrating an example of a configuration of a radio communication apparatus according to an embodiment of the present invention; 
         FIG. 4  is a circuit diagram illustrating an example of a specific configuration of a receiving circuit in  FIG. 3 ; 
         FIG. 5  is a circuit diagram illustrating another example of a specific configuration of the receiving circuit in  FIG. 3 ; and 
         FIG. 6  is a circuit diagram illustrating still another example of a specific configuration of the receiving circuit in  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a description will be given of embodiments of the present invention with reference to drawings. 
       FIG. 3  is a diagram illustrating an example of a configuration of a radio communication apparatus according to an embodiment of the present invention. Also,  FIG. 4  is a circuit diagram illustrating an example of a specific configuration of a receiving circuit in  FIG. 3 . 
     The radio communication apparatus  100  of  FIG. 3  illustrates an example of a configuration of a communication apparatus including mainly a receiving system of a cellular phone using a direct conversion method. 
     As shown in  FIG. 3 , the radio communication apparatus  100  of the present embodiment has an antenna (ANT)  101 , switches (SW)  102  and  103 , duplexers (DUP)  104  and  105 , transmission power amplifiers (PA)  106  and  107 , LNAs (low-noise amplifiers)  108  and  109 , a local oscillator (LO)  110 , a divider (phase shifter)  111 , mixers (MIX)  112 I and  112 Q, low-path filters (LPF)  113 I and  113 Q, and a baseband circuit  114 . 
     The LNAs  108  and  109 , mixers (MIX)  112 I and  112 Q constitute a receiving circuit  120  in the radio communication apparatus  100 . 
     This receiving circuit  120  is integrated into one chip. 
     The receiving circuit  120  basically has a plurality of (two in the example in  FIG. 3 ) input terminals T 1  and T 2  corresponding to a plurality of receiving bands, input terminals T 3  and T 4  of local oscillation signals SloI and SloQ having a phase difference of 90 degrees, and output terminals T 5  and T 6  of baseband signals SbbI and SbbQ, to the LPFs  113 I and  113 Q, having a phase difference of 90 degrees. 
     The mixer and the LNA in the receiving circuit  120  of the present embodiment have a characteristic configuration as described below. 
     The mixers  112 I and  112 Q have a capacitor in the input section receiving the output of the LNA, and have a configuration which prevents secondary distortion from occurring by separating an in-phase component (I) and a quardrature component (Q) in direct current. 
     Also, in the bias circuit of the LNAs  108  and  109 , noise of the bias signal from the current source is reduced by the LPF, and thus the LNA is configured to have little NF (Noise Figure) deterioration at large input signal time. 
     The LNAs  108  and  109  have an input section with a differential configuration having two inputs or more, and a degeneration differential inductor at emitter (source) section, whose middle point is grounded, and have cascode-connected transistors and a load inductor in common. 
     In the present embodiment, these circuits are implemented in an IC, and it becomes unnecessary to have a SAW filter, which has been necessary between the LNA and the mixer (MXER). Also, it is possible to achieve a direct conversion receiver for communication or broadcasting, which has a characteristic of not increasing the number of parts in the case of having a multiband capability. 
     In the receiving circuit  120  of the present embodiment, which has such a characteristic, a filter circuit is not necessary between the LNA and the mixer. Thus, by providing the IC with individual LNA input terminals in accordance with a frequency band, it is possible to receive a plurality of frequency bands without increasing external filter parts. 
     For a specific configuration of the receiving circuit  120 , a detailed description will be given below in relation to  FIG. 4 . 
     Here, a description will be given of two points, one point is the reason that a filter part becomes necessary between an LNA and a mixer, and the other point is the performance of a circuit which does not need a filter. 
     One of the characteristics of the third-generation cellular phone using the WCDMA method is the point that a transmission signal can be output simultaneously with a receiving operation. 
     The transmission signal is amplified by a PA (Power Amplifier), and is supplied to an antenna through a filter circuit and switch circuit called a duplexer. 
     Also, a signal transmitted from a base station and received by the antenna is supplied to a LNA through the duplexer. The level of the transmission signal input into the duplexer is as high as +20 dBm, and thus the isolation (a signal leakage from the input terminal of the transmission signal to the output terminal of the receiving signal) is about 50 dB. Accordingly, a transmission signal of about −30 dBm is applied to the LNA input. 
     When this high-level transmission signal is applied to a mixer, a receiving signal, which is a weak signal, is suppressed, and it becomes difficult to correctly perform demodulation. 
     It is therefore necessary to dispose a filter circuit between the LNA and the mixer in order to attenuate the transmission signal so as not to cause suppression. For this purpose, a filter circuit is used. 
     The main reason why a strong signal causes suppression in the mixer is secondary distortion of the mixer. Accordingly, like the present embodiment, if the input section receiving the output of the LNA has a capacitor, and has a configuration which can keep the generation level of secondary distortion within a desired value by separating an in-phase component (I) and a quardrature component (Q) in direct current, it becomes possible to dispense with a filter between the LNA and the mixer. 
     Next, a description will be given of a specific configuration and functions of the receiving circuit  120  according to the present embodiment with reference to  FIG. 4 . 
     The receiving circuit  120  has an LNA section (low-noise amplifier section)  121  and a mixer section  122 . 
     Also, in  FIG. 4 , each signal is a differential signal, and thus a mark p (positive) or n (negative) is added to terminals T 1  to T 6 . 
     The LNA section  121  has transistors Q 1  to Q 7  constituted by npn bipolar transistors, transistors Q 8  and Q 9  constituted by p-channel MOS transistors, resistor elements R 1  to R 6 , capacitors C 1  to C 5 , a differential inductor for degeneration (in the following, called a degeneration inductor) L 1 , a load differential inductor (in the following, called a load inductor) L 2 , a buffer B 1 , a switch S 1 , and a current source I 1 . 
     The mixer section  122  has transistors Q 11  to Q 15  constituted by n-channel MOS transistors, transistors Q 21  and Q 28  constituted by npn bipolar transistors, capacitors C 11  to C 14 , resistor elements R 21  to R 24 , capacitors C 21  to C 24 , and a current source I 21 . 
     Also, a power source voltage Vdd is supplied from the power sources V 1  and V 2  to the LNA section  121  and the mixer section  122  of the receiving circuit  120 , respectively. 
     In the LNA section  121 , the emitter of the transistor Q 1  is connected to one terminal of the degeneration inductor L 1  and the emitter of the transistor Q 3 . The collector of the transistor Q 1  is connected to the emitter of the transistor Q 5  and the collector of the transistor Q 3 . The base of the transistor Q 1  is connected to one terminal of the resistor element R 2 , and to the input terminal T 1   p  through a DC cut capacitor C 2 . 
     The emitter of the transistor Q 1  is connected to the other terminal of the degeneration inductor L 1  and the emitter of the transistor Q 4 . The collector of the transistor Q 2  is connected to the emitter of the transistor Q 6  and the collector of the transistor Q 4 . The base of the transistor Q 2  is connected to one terminal of the resistor element R 3 , and to the input terminal T 1   n  through a DC cut capacitor C 3 . 
     The emitter of the transistor Q 3  is connected to one terminal of the degeneration inductor L 1  and the emitter of the transistor Q 1 . The collector of the transistor Q 3  is connected to the emitter of the transistor Q 5  and the collector of the transistor Q 1 . The base of the transistor Q 3  is connected to one terminal of the resistor element R 4 , and to the input terminal T 2   pn  through a DC cut capacitor C 4 . 
     The emitter of the transistor Q 4  is connected to the other terminal of the degeneration inductor L 1  and the emitter of the transistor Q 2 . The collector of the transistor Q 4  is connected to the emitter of the transistor Q 6  and the collector of the transistor Q 2 . The base of the transistor Q 4  is connected to one terminal of the resistor element R 5 , and to the input terminal T 2   n  through a DC cut capacitor C 5 . 
     The collector of the transistor Q 5  is connected to one terminal of the load inductor L 2 , and the connection point thereof forms one node, ND 1 , of a differential output of the LNA section  121 . The collector of the transistor Q 6  is connected to the other terminal of the load inductor L 2 , and the connection point thereof forms the other node, ND 2 , of the differential output of the LNA section  121 . 
     The middle point of the degeneration inductor L 1  is connected to a ground line LG 1  connected to a reference voltage (for example, a ground voltage). 
     Also, the base of the cascode-connected transistors Q 5 , Q 6  and the middle point of the load inductor L 2  are connected to a power-source line LV 1  connected to a power source V 1 . 
     The LNAs  108 ,  109  are constituted by the transistors Q 1  to Q 6 , the resistor elements R 2  to R 5 , the degeneration inductor L 1 , and the load inductor L 2 , which have such a connection relationship. 
     In this example, the LNAs  108 ,  109  use (have) the degeneration inductor L 1 , the load inductor L 2 , and the cascode-connected transistors Q 5 , Q 6  in common. 
     A switch S 1  has a fixed contact point a and operation contact points b and c. The fixed contact point a is connected to the output of the buffer B 1 , and the fixed contact point b is connected to the other terminals of the resistor elements R 2  and R 3 , and the fixed contact point c is connected to the other terminals of the resistor elements R 4  and R 5 . 
     The sources of the transistors Q 8 , Q 9  are connected to the power-source line LV 1 , the drain of the transistor Q 8  is connected to the collector of the transistor Q 7 , one terminal of the resistor element R 1 , and one terminal of the resistor element R 6 . 
     Individual gates of the transistors Q 8  and Q 9  are connected to each other. The drain of the transistor Q 9  is connected to the connection point of the individual bases and the current source I 1 , and the current source I 1  is connected to the ground line LG 1 . 
     The other terminal of the resistor element R 6  is connected to the base of the transistor Q 7 , and the emitter of the transistor Q 7  is connected to the ground line LG 1 . 
     The other terminal of the resistor element R 1  is connected to the input terminal of the buffer and a first electrode of the capacitor C 1 , and a second electrode of the capacitor C 1  is connected to the ground line LG 1 . 
     A bias circuit  1211  of the LNAs  108  and  109  of a current-mirror type is constituted by the transistors Q 8  and Q 9 , the current source I 1 , the transistor Q 7 , and the resistor element R 6 , which have such a connection relationship. 
     Also, a LPF (low-pass filter)  1212  is constituted by the resistor element R 1  and the capacitor C 1 . 
     In the mixer section  122 , first electrodes of the capacitors C 11  and C 12  are connected to the output node ND 1  of the LNA section  121 , and first electrodes of the capacitors C 13  and C 14  are connected to the output node ND 2  of the LNA section  121 . 
     These capacitors C 11  to C 14  constitute an input section  1221  of the mixer section  122 . 
     The sources of the transistors Q 11  to Q 15  are commonly connected to a ground line (reference voltage line) LG 2 . The gates of the transistors Q 11  to Q 15  are commonly connected, the connection point of the gates thereof are connected to the drain of the transistor Q 11  and a current source I 21 , and the current source I 21  is connected to the power-source line LV 2 . 
     The collector of the transistor Q 12  is connected to a second electrode of the capacitor C 11  of the input section  122 I, and is commonly connected to the emitters of the transistors Q 21  and Q 22 , thereby forming a node ND 11  by these connection points. 
     The drain of the transistor Q 13  is connected to a second electrode of the capacitor C 13  of the input section  122 I, and is commonly connected to the emitters of the transistors Q 23  and Q 24 , thereby forming a node ND 12  by these connection points. 
     The drain of the transistor Q 14  is connected to a second electrode of the capacitor C 12  of the input section  122 I, and is commonly connected to the emitters of the transistors Q 25  and Q 26 , thereby forming a node ND 13  by these connection points. 
     The drain of the transistor Q 15  is connected to a second electrode of the capacitor C 14  of the input section  122 I, and is commonly connected to the emitters of the transistors Q 27  and Q 28 , thereby forming a node ND 14  by these connection points. 
     A current source  1222  of a current-mirror type is constituted by the transistors Q 11  and Q 15 , and the current source I 1 , which have such a connection relationship. 
     Individual emitters of the transistors Q 21  and Q 22  are connected to each other, and are connected to the node ND 11 . The collector of the transistor Q 21  is connected to an output terminal T 5   n  of a baseband signal SbbI to the LPF  113 I, and the collector of the transistor Q 23 . Also, the collector of the transistor Q 21  is connected to the power-source line LV 2  through the resistor element R 21  and the capacitor C 21 , which are disposed in parallel. 
     Individual emitters of the transistors Q 23  and Q 24  are connected to each other, and are connected to the node ND 12 . The collector of the transistor Q 24  is connected to an output terminal T 5   n  of a baseband signal SbbI to the LPF  113 I, and the collector of the transistor Q 22 . Also, the collector of the transistor Q 24  is connected to the power-source line LV 2  through the resistor element R 22  and the capacitor C 22 , which are disposed in parallel. 
     The bases of the transistors Q 21  and Q 24  are connected to an input terminal T 3   n  of the local oscillation signal SloI, and the bases of the transistors Q 22  and Q 23  are connected to an input terminal T 3   p  of the local oscillation signal SloI. 
     An I-side mixer  112 I is constituted by the transistors Q 21  to Q 24 , the resistor elements R 21  and R 22 , the capacitors C 21  and C 22 , the transistors Q 11  to Q 13 , and the current source I 21 , which have such a connection relationship. 
     Individual emitters of the transistors Q 25  and Q 26  are connected to each other, and are connected to the node ND  13 . The collector of the transistor Q 25  is connected to an output terminal T 6   p  of a baseband signal SbbQ to the LPF  113 Q, and the collector of the transistor Q 27 . Also, the collector of the transistor Q 25  is connected to the power-source line LV 2  through the resistor element R 23  and the capacitor C 23 , which are disposed in parallel. 
     Individual emitters of the transistors Q 27  and Q 28  are connected to each other, and are connected to the node ND 14 . The collector of the transistor Q 28  is connected to an output terminal T 6   n  of a baseband signal SbbQ to the LPF  113 Q, and the collector of the transistor Q 26 . Also, the collector of the transistor Q 28  is connected to the power-source line LV 2  through the resistor element R 24  and the capacitor C 24 , which are disposed in parallel. 
     The bases of the transistors Q 25  and Q 28  are connected to an input terminal T 4   p  of the local oscillation signal SloQ, and the bases of the transistors Q 26  and Q 27  are connected to an input terminal T 4   n  of the local oscillation signal SloQ. 
     An Q-side mixer  112 Q is constituted by the transistors Q 25  to Q 28 , the resistor elements R 23  and R 24 , the capacitors C 23  and C 24 , the transistors Q 11 , Q 14 , and Q 15 , and the current source I 21 , which have such a connection relationship. 
     Next, a description will be given of the operation of the receiving system of the radio communication apparatus having the configuration of  FIGS. 3 and 4 . 
     In principle, as shown in  FIG. 3 , in the radio communication apparatus  100 , an RF signal received by the antenna  101  passes through the switches  102  and  103  and the duplexers  104  and  105 , and is input into the LNAs  108  and  109  of the receiving circuit  120  included in an IC. 
     The switch S 1  is switched in accordance with the receiving frequency by a control system not shown in the figure, an amplified signal Srf either by the LNA  108  or the LNA  109  is multiplied by the local oscillation signals SloI and SloQ by the mixers  112 I and  112 Q, respectively, and the signals are converted into the baseband signals SbbI and SbbQ, respectively. 
     Here, the local oscillation signals SloI and SloQ are obtained by dividing the oscillation signal of the local oscillator  110  into signals having ½ the original frequency, and the signals applied to the input terminals T 3  and T 4  have a phase difference of 90 degrees, thus constituting a quadrature mixer by the mixer  112 I and the mixer  112 Q. 
     Accordingly, the baseband signals SbbI and SbbQ having a phase difference of 90 degrees can be obtained at the output terminals T 5  and T 6 , respectively. 
     More specifically, in the receiving circuit  120 , the LNA  108  includes differential input transistors Q 1  and Q 2 , the degeneration inductor L 1 , the cascode-connected transistors Q 5  and Q 6 , and the load inductor L 2 . 
     By employing a cascode connection in this manner, it is possible to restrain the influence of so-called mirror effect. 
     The LNA  109  receives input at the bases of the differential transistors Q 3  and Q 4  uses the degeneration inductor L 1 , the cascode-connected transistors Q 5  and Q 6 , and the load inductor L 2  by sharing the same circuit with the LNA  108 . 
     As shown in  FIG. 4 , individual duplexers  104  and  105  corresponding to the receiving frequencies are connected to the bases of the differential transistors Q 1  and Q 2 , and the transistors Q 3  and Q 4 , which constitute both input sections, through the DC cutting capacitors C 2  and C 3 , and capacitors C 4  and C 5 . 
     In the example in  FIG. 4 , the duplexer  104  is for the band I, and duplexer  105  is for the band II. 
     The bias circuit  1211  of the LNAs  108  and  109  is constituted by the current source I 1 , the transistors Q 8 , Q 9 , and Q 7 , and the resistor element R 6 , which constitute a current-mirror. The LNA section  121  further includes the LPF  1212  including the resistor element R 1  and the capacitor C 1  for attenuating noise generated from the bias circuit (regulator circuit)  1211 , and the buffer B 1 . 
     Either the LNA  108  or the LNA  109  is biased by the position of the switch S 1  by the bias circuit  1211 . The switch is controlled, for example, such that the fixed contact point a and the operation contact point b are connected by a switching signal from a control system not shown in the figure in the case of the band I. Also, in the case of the band II, the switch is controlled such that the fixed contact point a and the operation contact point c are connected by the switching signal from the control system not shown in the figure. 
     A self-transmitting signal of about −30 dBm is input to the LNA  108  and the LNA  109  as a blocking signal. 
     The input of such a large input signal increases noise, in the receiving frequency band, occurred from the current regulator circuit of the bias circuit  1211 , deteriorating the NF in the receiving frequency band of the LNA  108  and the LNA  109 . 
     In the present embodiment, by inserting the LPF  1212  between the regulator and the buffer B 1 , noise from the current regulator is prevented, and the deterioration of the NF in the receiving frequency band is prevented. 
     Also, the bases of the differential input transistors Q 1  and Q 2 , or the transistors Q 3  and Q 4  are biased through the bias circuit  1211 , the LPF  1212 , the buffer B 1 , and the switch S 1 . In this case, for example, 0.8 V is applied to the bases, and the connection side of the resistor elements R 2  and R 3 , and the resistor elements R 4  and R 5  with the switch S 1  becomes about 0.9 V. 
     In response to this, 0.8 V is also applied to the base of the transistor Q 7  of the bias circuit  1211 , and the potential of the connection point between the resistor element R 6  and the collector of the transistor Q 7  becomes 0.9 V. 
     That is to say, it becomes possible to apply more stable and correct bias by providing the bias circuit  1211  with the configuration to go into a substantially equivalent state to the bias state of the LNA  108  or the LNA  109  to be actually amplified. 
     The signal that has been subjected to the amplification operation by the LNA  108  or the LNA  109  is output from the nodes ND 1  and ND 2  to the mixer section  122 . 
     The signal that has been amplified by the LNA  108  or the LNA  109  in the mixer section  122  passes through the capacitors C 11 , C 12 , C 13 , and C 14 , and is input to the mixers  112 I and  112 Q of grounded-emitter transistors Q 21  to Q 24 , and Q 25  to Q 28 . 
     The signal that has passed through the capacitor C 11  is supplied to the transistor Q 21  connected to the node ND 11  and the emitter of the transistor Q 22 . The signal that has passed through the capacitor C 12  is supplied to the transistor Q 25  connected to the node ND 13  and the emitter of the transistor Q 26 . The signal that has passed through the capacitor C 13  is supplied to the transistor Q 23  connected to the node ND 12  and the emitter of the transistor Q 24 . The signal that has passed through the capacitor C 14  is supplied to the transistor Q 27  connected to the node ND 14  and the emitter of the transistor Q 28 . 
     By inputting an RF signal from the emitter side of a mixer constituted by a so-called Gilbert cell mixer, the mixer having a small inter-modulation distortion is achieved. 
     In the mixer section  122  of the present embodiment, the coupling, together with DC cut, of the emitters of the I-side mixer  112 I and the Q-side mixer  112 Q with the LNA output is carried out by individual capacitors (capacitance). 
     The main cause of the secondary distortion that occurs in the mixers  112 I and  112 Q is the voltage offset between the base and emitter (BE) of the pair of transistors of the Gilbert cell mixer. 
     Like the present embodiment, by capacity coupling of the emitters, it is possible to prevent an increase in the secondary distortion by the direct-current voltage offset impacting from the I-side to the Q-side or from the Q-side to the I-side. 
     As described above, in the present embodiment, in the LNA section  121  of the receiving circuit  120 , individual duplexers  104  and  105  corresponding to the receiving frequencies are connected to the bases of the differential transistors Q 1  and Q 2 , and transistors Q 3  and Q 4 , which constitute both input sections of the LNAs  108  and  109  through the DC cutting capacitors C 2  and C 3 , and capacitors C 4  and C 5 . The LNA  108  and the LNA  109  share the degeneration inductor L 1 , the cascode-connected transistors Q 5  and Q 6 , and the load inductor L 2 . The bias circuit  1211  of the LNAs  108  and  109  is constituted by the current source I 1 , the transistors Q 8 , Q 9  and Q 7 , and the resistor element R 6 , which constitute a current-mirror. The LNA section  121  further includes the LPF  1212  including the resistor element R 1  and the capacitor C 1  for attenuating noise generated from the bias circuit  1211 . 
     The signal that has been amplified by the LNA  108  or the LNA  109  in the mixer section  122  passes through the capacitors C 11 , C 12 , C 13 , and C 14 , and is input to the mixers  112 I and  112 Q of grounded-emitter transistors Q 21  to Q 24 , and Q 25  to Q 28 . 
     Thus, according to the present embodiment, in the mixer section  122 , by capacity coupling of the emitters, it is possible to prevent an increase in the secondary distortion by the direct-current voltage offset impacting from the I-side to the Q-side or from the Q-side to the I-side. Also, in the LNA section  121 , it is possible to prevent noise generated from the current regulator, and to prevent the deterioration of the NF in the receiving frequency band by inserting the LPF  1212  between the bias circuit (regulator) and the buffer B 1 . 
     As a result, it is possible to dispense with filter parts disposed between the LNA and the mixer, and to prevent an increase in the number of parts in the case of having a multiband capability, to be miniaturized, and to achieve receiving processing with high precision. 
     Also, the following advantages are obtained in sharing the degeneration inductor L 1 , the cascode-connected transistors Q 5  and Q 6  in the output section, and the load inductor L 2  by the LNAs  108  and  109 . 
     An inductor occupies an extremely larger area compared to a transistor in an IC, and it is difficult to reduce the size thereof by semiconductor miniaturization. Accordingly, the benefit of sharing the degeneration inductor and the load inductor by a plurality of LNAs is great, and thus there is a great benefit in the miniaturization of the receiving circuit of a cellular phone, which is requested to have a multiband capability. 
     Also, it is not necessary to dispose a filter between the LNA and the mixer, and thus there is no need to increase the number of external parts. It is therefore possible to have a multiband capability, to reduce cost, and to achieve miniaturization. 
     Accordingly, a radio communication apparatus according to the present embodiment can be applied not only to a third-generation cellular phone, but also to a direct-conversion receiving circuit for broadcasting. Thus, the radio communication apparatus advantageously has a broad range of applications. 
     In this regard, the receiving circuit of  FIG. 4  has a configuration including a bipolar transistor and a field-effect transistor (MOS transistor). However, the receiving circuit is not limited to this configuration. 
     For example, as shown in  FIG. 5 , instead of constituting the transistors Q 8 , Q 9 , and Q 11  to Q 15  by field-effect transistors, it is possible to constitute them by bipolar transistors. 
     In this case, the transistors Q 8  and Q 9  can be formed by pnp bipolar transistors, and the transistors Q 11  to Q 15  can be formed by npn bipolar transistors. 
     Also, as shown in  FIG. 6 , instead of constituting the transistors Q 1  to Q 7  and Q 21  to Q 26  by bipolar transistors, it is possible to constitute them by field-effect transistors. 
     In this case, the transistors Q 1  to Q 7  and Q 21  to Q 26  can be formed by n-channel MOS transistors. 
     Also, the number of signal inputs of the receiving circuit is not limited to two, and it is possible to have three inputs or more. 
     In this case, LNAs corresponding to the number of signal inputs are provided, and the number of operation contact points of the switch S 1  is set in accordance with the number of inputs. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.

Technology Category: 5