Patent Publication Number: US-11381274-B2

Title: Transmit-and-receive module and communication device

Description:
This is a continuation of U.S. patent application Ser. No. 16/055,649 filed on Aug. 6, 2018. The contents of this application are incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to a transmit-and-receive module including a low-noise amplifier and a power amplifier and to a communication device including the transmit-and-receive module. 
     In accordance with a decreased size of a front-end module mounted on a mobile terminal, a transmit front-end unit, and a receive front-end unit are being integrated with each other (being formed into a module) by way of the integration of radio-frequency components. 
     Japanese Unexamined Patent Application Publication No. 2001-53544 discloses an antenna-integrated amplifier module including a duplexer for separating radio-frequency signals from each other according to the frequency, a low-noise amplifier, and a power amplifier (see FIG. 14 of the &#39;544 publication). 
     BRIEF SUMMARY 
     In the above-described antenna-integrated amplifier module, impedance matching is performed so that input/output impedance of the duplexer, input impedance of the low-noise amplifier, and output impedance of the power amplifier can match the characteristic impedance (about 50Ω, for example). 
     In the above-described antenna-integrated amplifier module, however, the mere adjustment between the input impedance of the low-noise amplifier and the output impedance of the receive side of the duplexer by using the characteristic impedance fails to optimize the noise figure (NF) of the low-noise amplifier. 
     To address the above-described problem, the present disclosure provides a transmit-and-receive module which is small in size and which can decrease the noise figure while a duplexer, a power amplifier, and a low-noise amplifier are integrated with each other, and provides a communication device including the transmit-and-receive module. 
     According to an embodiment of the present disclosure, there is provided a transmit-and-receive module including a duplexer, a power amplifier, and a low-noise amplifier. The duplexer includes a common terminal, a transmit terminal, a receive terminal, a transmit filter unit, and a receive filter unit. A radio-frequency transmit signal and a radio-frequency received signal are input into and output from the common terminal. A radio-frequency transmit signal is input into the transmit terminal. A radio-frequency received signal is output from the receive terminal. The transmit filter unit uses a transmit band as a pass band and is connected to the common terminal and the transmit terminal. The receive filter unit uses a receive band as a pass band and is connected to the common terminal and the receive terminal. The power amplifier amplifies a radio-frequency transmit signal and outputs the amplified radio-frequency transmit signal to the transmit terminal. The low-noise amplifier amplifies a radio-frequency received signal input and received from the common terminal via the duplexer and the receive terminal. The power amplifier and the low-noise amplifier are integrated with each other. In a Smith chart, impedance in the receive band of the receive filter unit seen from the receive terminal intersects a line connecting a center point of noise figure (NF) circles and a center point of gain circles. The center point of the NF circles represents the impedance at which a noise figure of the low-noise amplifier is minimized. The center point of the gain circles represents the impedance at which gain of the low-noise amplifier is maximized. 
     In the related art, a low-noise amplifier handling low-power signals and a power amplifier handling high-power signals are formed in different modules. In this configuration, impedance matching between the low-noise amplifier and a duplexer is performed by using the characteristic impedance of a front-end circuit. In this case, the output impedance of a receive filter unit is set so as to maximize the gain of the low-noise amplifier. In contrast, in the above-described configuration according to an embodiment of the disclosure, the low-noise amplifier and the power amplifier are integrated with each other in the same module. To perform impedance matching between the low-noise amplifier and the duplexer, instead of using the characteristic impedance, the output impedance of the receive filter unit is set so as to optimize both of the gain and the noise figure of the low-noise amplifier. It is thus possible to provide a transmit-and-receive module which is small in size and which achieves the optimized balance of the receiving noise figure and the receiving gain while a power amplifier, a duplexer, and a low-noise amplifier are integrated with each other. 
     The receive filter unit may have output impedance which intersects the line connecting the center point of the NF circles and the center point of the gain circles in the Smith chart. The transmit filter unit may have input impedance at which gain of the power amplifier is maximized. 
     With this configuration, the input impedance of the transmit filter unit is set so as to achieve the optimized balance of the gain and the efficiency of the power amplifier, while the output impedance of the receive filter unit is set to be a value different from the characteristic impedance so that both of the gain and the noise figure of the low-noise amplifier can be optimized. This eliminates the need to provide an impedance matching circuit between the receive filter unit and the low-noise amplifier to perform impedance matching therebetween by using the characteristic impedance. It is thus possible to provide a transmit-and-receive module which is small in size and which achieves the optimized balance of the receiving noise figure and the receiving gain while a power amplifier, a duplexer, and a low-noise amplifier are integrated with each other. 
     A value of receive impedance used for impedance adjustment between the receive filter unit and the low-noise amplifier may be different from a value of transmit impedance used for impedance matching between the transmit filter unit and the power amplifier. 
     Impedance matching between the power amplifier and the duplexer is performed by using the characteristic impedance of the front-end circuit so that the balance of the gain and the efficiency of the power amplifier can be optimized. In contrast, customized impedance deviating from the characteristic impedance is used for impedance adjustment between the low-noise amplifier and the duplexer so that both of the gain and the noise figure of the low-noise amplifier can be optimized. Accordingly, the value of the receive impedance and that of the transmit impedance are different from each other. It is thus possible to enhance isolation characteristics in a path from the input terminal (module transmit terminal) of the power amplifier to the output terminal (module receive terminal) of the low-noise amplifier via the duplexer. 
     The value of the receive impedance used for impedance adjustment between the receive filter unit and the low-noise amplifier may be higher than the value of the transmit impedance used for impedance matching between the transmit filter unit and the power amplifier. 
     With this configuration, the output impedance of the receive filter unit is higher than the input impedance of the transmit filter unit. It is thus possible to enhance isolation characteristics in a path from the input terminal of the power amplifier to the output terminal of the low-noise amplifier via the duplexer. 
     The value of the receive impedance may be higher than the value of the transmit impedance by a factor of about 1.4 or greater. 
     The present inventors have found that, when the receive impedance is higher than the transmit impedance by a factor of about 1.4 or greater, it is possible to enhance the isolation while decreasing the receiving noise figure, compared with when the receive impedance is equal to the transmit impedance. That is, when the center point of NF circles of the low-noise amplifier is located on the higher impedance side than the center point of gain circles is, by setting the impedance in the receive band of the receive filter unit to be higher than the input impedance of the transmit filter unit by a factor of about 1.4 or greater, the receiving noise figure can be decreased substantially without necessarily decreasing the receiving gain. Additionally, as the difference between the output impedance of the receive filter unit and the input impedance of the transmit filter unit (characteristic impedance, for example) is greater, the isolation can be improved to a higher level. 
     The value of the receive impedance may be higher than the value of the transmit impedance by a factor smaller than about 2.3. 
     The present inventors have found that it is possible to enhance the isolation while decreasing the noise figure when the receive impedance is higher than the transmit impedance by a factor smaller than about 2.3, compared with when the receive impedance is equal to the transmit impedance. The present inventors have also found that, when the center point of NF circles of the low-noise amplifier is located on the higher impedance side than that of gain circles is, if the output impedance in the receive band of the receive filter unit is set to be higher than the input impedance in the transmit band of the transmit filter unit by a factor of about 2.3 or greater, the gain of the low-noise amplifier is significantly decreased and the noise figure thereof is also increased. When the receive impedance is higher than the transmit impedance by a factor smaller than about 2.3, the receiving noise figure can be decreased substantially without necessarily decreasing the receiving gain. 
     The transmit-and-receive module may further include a second duplexer, a second power amplifier, and a second low-noise amplifier. The second duplexer includes a second common terminal, a second transmit terminal, a second receive terminal, a second transmit filter unit, and a second receive filter unit. A radio-frequency transmit signal and a radio-frequency received signal are input into and output from the second common terminal. A radio-frequency transmit signal is input into the second transmit terminal. A radio-frequency received signal is output from the second receive terminal. The second transmit filter unit uses a second transmit band different from the transmit band as a pass band and is connected to the second common terminal and the second transmit terminal. The second receive filter unit uses a second receive band different from the receive band as a pass band and is connected to the second common terminal and the second receive terminal. The second power amplifier amplifies a radio-frequency transmit signal and outputs the amplified radio-frequency transmit signal to the second transmit terminal. The second low-noise amplifier amplifies a radio-frequency received signal input and received from the second common terminal via the second duplexer and the second receive terminal. 
     With the above-described configuration, in a multiband-support front-end circuit, impedance matching between the duplexers and the low-noise amplifiers disposed in plural signal paths connected to the antenna is performed as follows. In each signal path, instead of using the characteristic impedance, customized impedance reflecting the amplifying characteristics and the noise figure characteristics of the low-noise amplifier is used for impedance matching between the duplexer and the low-noise amplifier. It is thus possible to provide a transmit-and-receive module which is small in size and which achieves the optimized balance between the receiving noise figure and the receiving gain according to the frequency band while plural power amplifiers, plural duplexers, and plural low-noise amplifiers supporting multiple bands are integrated with each other. 
     According to another embodiment of the present disclosure, there is provided a communication device including the above-described transmit-and-receive module and a radio-frequency signal processing circuit. The radio-frequency signal processing circuit processes a radio-frequency received signal input from the transmit-and-receive module and processes a radio-frequency transmit signal and outputs the processed radio-frequency transmit signal to the transmit-and-receive module. 
     It is thus possible to provide a transmit-and-receive module which is small in size and which achieves the optimized balance of the receiving noise figure and the receiving gain while a power amplifier, a duplexer, and a low-noise amplifier are integrated with each other. 
     According to an embodiment of the present disclosure, it is possible to decrease the size and the noise figure of a transmit-and-receive module and a communication device in which a duplexer, a power amplifier, and a low-noise amplifier are integrated with each other. 
     Other features, elements, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of embodiments of the present disclosure with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a transmit-and-receive module according to a first embodiment; 
         FIG. 2  is a circuit diagram of a transmit-and-receive module according to a comparative example; 
         FIG. 3A  illustrates customized impedance matching for a receive filter in the first embodiment; 
         FIG. 3B  illustrates characteristic impedance (about 50Ω) matching for a receive filter in the comparative example; 
         FIG. 4  illustrates the impedance of receive filters in transmit-and-receive modules according to first through third examples and a comparative example; 
         FIG. 5A  is a Smith chart illustrating the relationship of the in-band impedance of the receive filter to NF circles and gain circles in the first example; 
         FIG. 5B  is a Smith chart illustrating the relationship of the in-band impedance of the receive filter to NF circles and gain circles in the comparative example; 
         FIG. 6  is a graph illustrating comparison results of the noise figure of the transmit-and-receive module of the first example and that of the comparative example; 
         FIG. 7A  is a graph illustrating comparison results of isolation characteristics of a single duplexer of the first example and those of the comparative example; 
         FIG. 7B  is a graph illustrating comparison results of receive-band transmission characteristics of a single duplexer of the first example and those of the comparative example; 
         FIG. 8A  is a Smith chart illustrating the relationship of the impedance of the receive filter to NF circles and gain circles in the first example; 
         FIG. 8B  is a Smith chart illustrating the relationship of the impedance of the receive filter to NF circles and gain circles in the second example; 
         FIG. 8C  is a Smith chart illustrating the relationship of the impedance of the receive filter to NF circles and gain circles in the comparative example; 
         FIG. 9A  is a graph illustrating comparison results of the isolation between power amplifiers and low-noise amplifiers according to the first example, the second example, and the comparative example; 
         FIG. 9B  is a graph illustrating comparison results of receive-band transmission characteristics between duplexers and the low-noise amplifiers according to the first example, the second example, and the comparative example; 
         FIG. 10A  is a Smith chart illustrating the relationship of the impedance of the receive filter to NF circles and gain circles in the third example; 
         FIG. 10B  is a graph illustrating comparison results of receive-band transmission characteristics between the duplexers and the low-noise amplifiers according to the third example and the comparative example; and 
         FIG. 11  is a circuit diagram of a communication device and a transmit-and-receive module according to a second embodiment. 
         FIG. 12  is a circuit diagram of a communication device and transmit-and-receive module according to a third embodiment. 
         FIG. 13  is a circuit diagram of a communication device and transmit-and receive module according to a fourth embodiment. 
         FIG. 14  is a circuit diagram of a communication device and transmit-and receive module according to a fifth embodiment. 
         FIG. 15A  is a structure of a communication device and transmit-and receive module according to a first embodiment. 
         FIG. 15B  is a structure of a communication device and transmit-and receive module according to a first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Transmit-and-receive modules and a communication device according to embodiments of the present disclosure will be described below in detail through illustration of examples with reference to the drawings. All of the embodiments described below illustrate general or specific examples. Numeric values, configurations, materials, components, and positions and connection states of the components illustrated in the following embodiments are only examples, and are not described for limiting the present disclosure. Among the components illustrated in the following embodiments, the components that are not recited in the independent claims will be described as optional components. The sizes and dimensional ratios of the components in the drawings are not necessarily illustrated as actual sizes and ratios. 
     FIRST EMBODIMENT 
     1.1 Circuit Configuration of Transmit-and-Receive Module 
       FIG. 1  is a circuit diagram of a transmit-and-receive module  1  according to a first embodiment. The transmit-and-receive module  1  includes a duplexer  10 , a power amplifier (PA)  20 , a low-noise amplifier (LNA)  30 , a matching circuit  21 , an adjusting circuit  31 , a module common terminal  110 , a module transmit terminal  120 , and a module receive terminal  130 . 
     The duplexer  10  includes a common terminal  101 , a transmit terminal  102 , a receive terminal  103 , a transmit filter  10 T, and a receive filter  10 R. With this configuration, the duplexer  10  is able to simultaneously pass a radio-frequency (RF) transmit signal in a transmit band from the transmit terminal  102  to the common terminal  101  and a RF received signal in a receive band from the common terminal  101  to the receive terminal  103  by using the frequency-division duplexing (FDD) method. 
     The common terminal  101  is connected to the module common terminal  110 . The duplexer  10  transmits a RF transmit signal and receives a RF received signal through the common terminal  101 . The transmit terminal  102  is a terminal into which a RF transmit signal is input via the module transmit terminal  120 , the power amplifier  20 , and the matching circuit  21 . The receive terminal  103  is a terminal from which a RF received signal is output via the module common terminal  110 , the common terminal  101 , and the receive filter  10 R. 
     The transmit filter  10 T is a transmit filter unit using a transmit band as a pass band and being connected to the common terminal  101  and the transmit terminal  102 . The receive filter  10 R is a receive filter unit using a receive band as a pass band and being connected to the common terminal  101  and the receive terminal  103 . 
     The power amplifier  20  is a power amplifier circuit that amplifies a RF transmit signal input from the module transmit terminal  120  and outputs the amplified RF transmit signal to the transmit terminal  102  via the matching circuit  21 . 
     The low-noise amplifier  30  is a low-noise amplifier circuit that amplifies a RF received signal input from the module common terminal  110  via the receive filter  10 R and the adjusting circuit  31 . 
     The matching circuit  21  is a circuit for providing impedance matching between the power amplifier  20  and the transmit filter  10 T. The matching circuit  21  provides impedance matching so that the impedance of the power amplifier  20  seen from the transmit terminal  102  can match the characteristic impedance (about 50Ω, for example). 
     The adjusting circuit  31  is a circuit for adjusting the input impedance of the low-noise amplifier  30 . The adjusting circuit  31  adjusts the impedance of the low-noise amplifier  30  seen from the receive terminal  103  to the characteristic impedance (about 50Ω, for example). 
     In the transmit-and-receive module  1  according to the first embodiment, the provision of the matching circuit  21  and the adjusting circuit  31  may be omitted. 
     The module common terminal  110  is an external connecting terminal that can be connected to a communication medium, such as an antenna. The module transmit terminal  120  is an external connecting terminal for connecting the power amplifier  20  and a RF signal processing circuit (RF integrated circuit (RFIC)) (not shown) disposed subsequent to the power amplifier  20 . The module receive terminal  130  is an external connecting terminal for connecting the low-noise amplifier  30  and the RFIC (not shown). 
     To reduce the size of the transmit-and-receive module  1 , the power amplifier  20  handling high-power signals and the low-noise amplifier  30  handling low-power signals are integrated with each other. In the first embodiment, integrating of the power amplifier  20  and the low-noise amplifier  30  with each other is not restricted to forming of the power amplifier  20  and the low-noise amplifier  30  into one chip. An amplifier element forming the power amplifier  20  and an amplifier element forming the low-noise amplifier  30  may be made separately and be formed in the same package or be mounted on the same mounting substrate. Such a configuration is also included in integrating of the power amplifier  20  and the low-noise amplifier  30  with each other. 
     1.2 Circuit Configuration of Transmit-and-Receive Module According to Comparative Example 
     In some transmit-and-receive modules of the related art, a transmit module handling high-power signals and a receive module handling low-power signals are not integrated with each other. 
       FIG. 2  is a circuit diagram of a transmit-and-receive module according to a comparative example. As shown in  FIG. 2 , this transmit-and-receive module is constituted by a transmit module  500  and a receive module  600 . That is, unlike the transmit-and-receive module  1  according to the first embodiment, in the transmit-and-receive module of the comparative example, the transmit module  500  and the receive module  600  are formed as different chips and are not integrated with each other. The transmit-and-receive module of the comparative example is also different from the transmit-and-receive module  1  in the mode of impedance matching between a receive filter  510 R and the receive module  600 . 
     1.3 Comparison of Impedance Matching Between First Embodiment and Comparative Example 
       FIG. 3A  illustrates customized impedance matching for the receive filter  10 R in the first embodiment.  FIG. 3B  illustrates characteristic impedance (about 50Ω) matching for the receive filter  510 R in the comparative example. 
     In the transmit-and-receive module shown in  FIG. 2  in which the transmit module  500  handling high-power signals and the receive module  600  handling low-power signals are separately formed, impedance matching between a low-noise amplifier  530  and the receive filter  510 R of a duplexer  510  is performed by using the characteristic impedance (about 50Ω, for example) of a front-end circuit. In the transmit-and-receive module of the comparative example, as shown in  FIG. 3B , to maximize the gain of the low-noise amplifier  530 , a matching circuit  531  adjusts the impedance of the low-noise amplifier  530  so that the center point of gain circles (equal gain circles) of the low-noise amplifier  530  can be the characteristic impedance (about 50Ω). To maximize the gain of the low-noise amplifier  530 , the output impedance of the receive filter  510 R is set so that it can coincide with the center point (characteristic impedance) of the gain circles, as shown in  FIG. 3B . 
     In contrast, in the transmit-and-receive module  1  of the first embodiment, the low-noise amplifier  30  and the power amplifier  20  are integrated with each other. Instead of performing impedance matching between the low-noise amplifier  30  and receive filter  10 R by using the characteristic impedance (about 50Ω) of the front-end circuit, the impedance of the receive filter  10 R is set so as to increase the gain of the low-noise amplifier  30  and to decrease the noise figure (NF) thereof. That is, in the transmit-and-receive module  1  of the first embodiment, instead of maximizing the gain of the low-noise amplifier  30 , the impedance of the receive filter  10 R is set so as to optimize both of the gain and the noise figure of the low-noise amplifier  30 . More specifically, in the Smith chart shown in  FIG. 3A , the output impedance of the receive filter  10 R is set so that the impedance in the receive band of the receive filter  10 R seen from the receive terminal  103  can intersect a line connecting the center point of the NF circles and that of the gain circles. 
     The impedance, the NF circles, and the gain circles in the receive path of the first embodiment and those of the comparative example are based on the direction in which the receive filter is seen from the low-noise amplifier, as indicated in the lower sections of  FIGS. 3A and 3B . The NF circle (equal NF circle) represents the output impedance of the receive filter at which the noise figure of the low-noise amplifier  30  including the adjusting circuit  31  is equal. The gain circle (equal gain circle) represents the output impedance of the receive filter at which the gain of the low-noise amplifier  30  including the adjusting circuit  31  is equal. The center point of the NF circles represents the output impedance in the receive band of the receive filter at which the noise figure of the low-noise amplifier  30  including the adjusting circuit  31  is minimized. The center point of the gain circles represents the output impedance in the receive band of the receive filter at which the gain of the low-noise amplifier  30  including the adjusting circuit  31  is maximized. 
     With the configuration of the transmit-and-receive module  1  of the first embodiment, it is possible to provide a transmit-and-receive module which is small in size and which achieves the optimized balance of the receiving noise figure and the receiving gain while a power amplifier, a duplexer, and a low-noise amplifier are integrated with each other. 
     1.4 Characteristics Comparison of Transmit-and-Receive Modules According to Examples and Comparative Example 
       FIG. 4  illustrates the impedance of receive filters in transmit-and-receive modules according to first through third examples and a comparative example.  FIG. 4  shows that the impedance of the low-noise amplifier seen from the receive terminal  103  is Z 1  (Ω), the impedance of the receive filter seen from the receive terminal  103  is Z 2  (Ω), and the impedance of the other elements (the impedance of the power amplifier seen from the transmit terminal  102  and the impedance of the antenna seen from the common terminal  101 ) is all about 50Ω. 
     The table on the right side in  FIG. 4  shows that Z 2  of a transmit-and-receive module of a first example is about 80Ω, that of a second example is about 70Ω, and that of a third example is about 110Ω. Z 1  of the transmit-and-receive modules of the first through third examples and that of the comparative example are all about 50Ω. 
       FIG. 5A  is a Smith chart illustrating the relationship of the in-band output impedance of the receive filter  10 R to NF circles and gain circles in the first example.  FIG. 5B  is a Smith chart illustrating the relationship of the in-band output impedance of the receive filter to NF circles and gain circles in the comparative example. 
     Concerning the transmit-and-receive module of the comparative example in which the transmit module  500  and the receive module  600  are separately disposed, the Smith chart in  FIG. 5B  shows that the impedance in the receive band of the receive filter  510 R seen from the receive terminal  103  (duplexer Rx in-band impedance in  FIG. 5B ) does not intersect a line (L 1  in  FIG. 5B ) connecting the center point (NF in  FIG. 5B ) of the NF circles and that (Ga in  FIG. 5B ) of the gain circles. The impedance in the receive band of the receive filter  510 R is located at the center (characteristic impedance) of the Smith chart. 
     In contrast, concerning the transmit-and-receive module  1  of the first example, the Smith chart in  FIG. 5A  shows that the impedance in the receive band of the receive filter  10 R seen from the receive terminal  103  (duplexer Rx in-band impedance in  FIG. 5A ) intersects a line (L 1  in  FIG. 5A ) connecting the center point (NF in  FIG. 5A ) of the NF circles and that (Ga in  FIG. 5A ) of the gain circles. As a result, the impedance of the receive filter  10 R of the first example is separated farther from the center (characteristic impedance) of the Smith chart and is shifted toward the higher impedance side, and is thus located closer to the center point of the NF circles than that of the receive filter  510 R of the comparative example is. 
       FIG. 6  is a graph illustrating comparison results of the noise figure of the transmit-and-receive module  1  of the first example and that of the comparative example. More specifically, this graph illustrates comparison results of the noise figure in the receive band between the module common terminal  110  and the module receive terminal  130  of the first example and that of the comparative example. As a result of shifting the output impedance of the receive filter  10 R of the transmit-and-receive module  1  of the first example toward the center point of the NF circles, as shown in  FIG. 5A , the noise figure is decreased in the entire receive band by about 0 to 0.4 dB compared with that of the transmit-and-receive module of the comparative example. 
     With the above-described configuration of the transmit-and-receive module  1  of the first example, it is possible to decrease the receiving noise figure while the power amplifier  20 , the duplexer  10 , and the low-noise amplifier  30  are integrated with each other. 
     To form a small transmit-and-receive module by integrating a power amplifier and a low-noise amplifier with each other, the performance of circuit elements such as an inductor and a capacitor used for an adjusting circuit  31  is sacrificed. This may reduce the Q factor of the adjusting circuit  31 , which may lead to an increase in the receiving noise figure of the transmit-and-receive module. However, in the transmit-and-receive module  1  of the first embodiment, instead of maximizing the gain of the low-noise amplifier  30 , impedance adjustment is performed so as to achieve the optimized balance of the noise figure and the gain. This makes it possible to reduce the size of the transmit-and-receive module  1  without necessarily sacrificing the receiving performance even with a decrease in the Q factor of the adjusting circuit  31 . 
     The transmit-and-receive isolation characteristics and the transmission characteristics in the receive path of the transmit-and-receive modules will now be described below. 
       FIG. 7A  is a graph illustrating comparison results of the isolation characteristics of a single duplexer of the first example and those of the comparative example. More specifically, this graph illustrates isolation characteristics of a single duplexer between the transmit terminal  102  and the receive terminal  103  of the first example and those of the comparative example.  FIG. 7A  shows that the isolation characteristics of the transmit-and-receive module  1  of the first example are improved particularly in the transmit band, compared with the comparative example. 
     In the transmit-and-receive module  1  of the first example, the receive impedance used for impedance adjustment between the receive filter  10 R and the low-noise amplifier  30  is about 80Ω. The receive impedance used for impedance adjustment between the receive filter  10 R and the low-noise amplifier  30  is the output impedance in the receive band of the receive filter  10 R to optimize the balance between the noise figure and the gain of the low-noise amplifier  30 . The transmit impedance used for impedance matching between the transmit filter  10 T and the power amplifier  20  is set so that the input impedance of the transmit filter  10 T can achieve the optimized gain and efficiency of the power amplifier  20 , and more specifically, the input impedance of the transmit filter  10 T is set to be about 50Ω. The transmit impedance used for impedance matching between the transmit filter  10 T and the power amplifier  20  is, for example, the input impedance in the transmit band of the transmit filter  10 T for causing the transmit impedance to match the impedance of the matching circuit  21 . 
     That is, in the transmit-and-receive module  1  of the first example, the value of the receive impedance and that of the transmit impedance are different. More particularly, in the first example, the value of the receive impedance is higher than that of the transmit impedance. The isolation characteristics are thus improved, as shown in  FIG. 7A , because of the difference in the impedance between the transmit path and the receive path in the signal path from the transmit terminal  102  to the receive terminal  103 . 
       FIG. 7B  is a graph illustrating comparison results of receive-band transmission characteristics of a single duplexer of the first example and those of the comparative example. More specifically, this graph illustrates transmission characteristics of a single duplexer (receive filter) between the common terminal  101  and the receive terminal  103  of the first example and those of the comparative example.  FIG. 7B  shows that the insertion loss in the receive band of the transmit-and-receive module  1  of the first example is increased, compared with the comparative example. The reason for this is that the receive impedance in the first example deviates from the characteristic impedance which is about 50Ω, and this makes the in-band transmission characteristics appear to be decreased. However, the transmission characteristics in the path between the module common terminal  110  and the module receive terminal  130  including the low-noise amplifier  30 , the adjusting circuit  31 , and the duplexer  10  are maintained, which will be discussed later in detail with reference to  FIG. 9B . 
     The relationship between the above-described receive impedance and the isolation characteristics will be described below. 
       FIG. 8A  is a Smith chart illustrating the relationship of the output impedance of the receive filter  10 R to NF circles and gain circles in the first example.  FIG. 8B  is a Smith chart illustrating the relationship of the output impedance of the receive filter  10 R to NF circles and gain circles in the second example.  FIG. 8C  is a Smith chart illustrating the relationship of the output impedance of the receive filter  510 R to NF circles and gain circles in the comparative example. 
       FIG. 8A  illustrates out-of-band impedance characteristics of the receive filter  10 R in addition to the impedance characteristics of the transmit-and-receive module  1  (Z 2 =about 80Ω) according to the first example shown in FIG.  5 A.  FIG. 8B  illustrates the impedance characteristics of the receive filter  10 R of the transmit-and-receive module  1  (Z 2 =about 70Ω) according to the second example.  FIG. 8C  illustrates out-of-band impedance characteristics of the receive filter  510 R in addition to the impedance characteristics of the transmit-and-receive module (Z 2 =about 50Ω) according to the comparative example shown in  FIG. 5B . 
     As shown in  FIGS. 8A through 8C , as the receive impedance Z 2  increases such as about 50Ω (comparative example), about 70Ω (second example), and about 80Ω (first example), the receive-band output impedance of the receive filter approaches closer to the center point (NF) of the NF circles from the center point (Ga) of the gain circles. Additionally, the transmit-band impedance (Tx 1 -Tx 2  in  FIGS. 8A through 8C ) of the receive filter varies in accordance with a change in the receive impedance Z 2 . More specifically, impedance Z 0  of the receive filter  10 R of the first example corresponding to Tx 2  (862 MHz) is Z 0 (0.114+j1.400) (=70.2Ω). Impedance Z 0  of the receive filter  10 R of the second example corresponding to Tx 2  (862 MHz) is Z 0 (0.098+j1.196) (=60.0Ω). Impedance Z 0  of the receive filter  510 R of the comparative example corresponding to Tx 2  (862 MHz) is Z 0 (0.096+j1.146) (=57.5Ω). This shows that, as the receive impedance Z 2  increases, the output impedance in the transmit band of the receive filter separates farther from the characteristic impedance (about 50Ω) and farther from the center point (Ga) of the gain circles. That is, as the impedance of the receive filter  10 R is shifted toward the higher impedance side, the transmit-band impedance can separate farther from the center point of the gain circles. This reduces the gain in the transmit band of the receive filter  10 R, thereby making it possible to enhance the transmit-and-receive isolation characteristics. 
       FIG. 9A  is a graph illustrating comparison results of the isolation between the power amplifiers and the low-noise amplifiers according to the first example, the second example, and the comparative example. This graph illustrates the isolation characteristics (PA-LNA isolation) between the module transmit terminal  120  and the module receive terminal  130 .  FIG. 9A  shows that the isolation, particularly in the transmit-band isolation, of the transmit-and-receive module  1  of each of the first and second examples is improved, compared with the transmit-and-receive module of the comparative example. The isolation of the transmit-and-receive module  1  in the first example is improved to a higher level than that of the second example. This validates that the transmit-and-receive isolation characteristics are improved to a higher level as the difference between the receive impedance (output impedance of the receive filter  10 R) used for impedance adjustment between the receive filter  10 R and the low-noise amplifier  30  and the transmit impedance (input impedance of the transmit filter  10 T) used for impedance matching between the transmit filter  10 T and the power amplifier  20  is greater. 
       FIG. 9B  is a graph illustrating comparison results of the receive-band transmission characteristics between the duplexers and the low-noise amplifiers according to the first example, the second example, and the comparative example. This graph illustrates the transmission characteristics (DupRx-LNA transmission characteristics) between the module common terminal  110  and the module receive terminal  130 .  FIG. 9B  shows that, even with a difference between the receive impedance and the transmit impedance, the transmission characteristics in the receive path do not deteriorate, but are maintained, compared with the comparative example in which the receive impedance and the transmit impedance match the characteristic impedance (about 50Ω). 
     That is, as shown in  FIGS. 8A through 8C , by reducing the gain of the transmit band in the receive path as a result of separating the transmit-band impedance of the receive filter  10 R farther from the gain circles, the transmit-and-receive isolation characteristics can be improved while maintaining the transmission characteristics of the receive band. 
     The above-described comparison results of the first and second examples and the comparative example show that, when the transmit impedance is about 50Ω, the receive impedance can be 70Ω or higher. That is, the receive impedance can be higher than the transmit impedance by a factor of about 1.4 or greater. 
     The present inventors have found that, when the receive impedance is higher than the transmit impedance by a factor of about 1.4 or greater, as in the second example, it is possible to enhance the isolation while decreasing the receiving noise figure, compared with when the receive impedance is equal to the transmit impedance. That is, when the center point of NF circles of the low-noise amplifier is located on the higher impedance side than the center point of gain circles is, by setting the impedance in the receive band of the receive filter  10 R to be higher than the input impedance of the transmit filter  10 T by a factor of about 1.4 or greater, the receiving noise figure can be decreased substantially without necessarily decreasing the receiving gain. Additionally, as the difference between the output impedance of the receive filter  10 R and the input impedance of the transmit filter  10 T (characteristic impedance, for example) is greater, the isolation can be improved to a higher level. 
       FIG. 10A  is a Smith chart illustrating the relationship of the output impedance of the receive filter  10 R to NF circles and gain circles in the third example. 
       FIG. 10A  illustrates the impedance characteristics of the receive filter  10 R of the transmit-and-receive module  1  (Z 2 =about 110Ω) according to the third example. As shown in  FIG. 10A , when the receive impedance Z 2  is about 110Ω, the receive-band impedance (Rx 1  to Rx 2  in  FIG. 10A ) of the receive filter  10 R approaches even closer to the center point (NF) of the NF circles from the center point (Ga) of the gain circles than that of the first and second examples. In accordance with the increased receive impedance Z 2 , the transmit-band impedance (Tx 1  to Tx 2  in  FIG. 10A ) of the receive filter is farther shifted to the higher impedance side than that of the first and second examples. This makes it possible to further enhance the transmit-and-receive isolation characteristics to a higher level than those of the first and second examples. The noise figure can also be decreased to be smaller than that of the first and second example, and can be reduced to a minimal level. 
       FIG. 10B  is a graph illustrating comparison results of the receive-band transmission characteristics between the duplexers and the low-noise amplifiers according to the third example and the comparative example. This graph illustrates the transmission characteristics (DupRx-LNA transmission characteristics) between the module common terminal  110  and the module receive terminal  130 .  FIG. 10B  shows that, when the receive impedance is about 110Ω, the transmission characteristics are less likely to deteriorate than in the comparative example in which the receive impedance and the transmit impedance match the characteristic impedance (about 50Ω). That is, as shown in  FIGS. 10A and 10B , by reducing the gain of the transmit band in the receive path as a result of separating the transmit-band impedance of the receive filter  10 R even farther from the gain circles, the transmit-and-receive isolation characteristics can be improved while substantially maintaining the transmission characteristics of the receive band. 
     In the transmit-and-receive module  1  including the low-noise amplifier  30  having the above-described impedance characteristics, if the receive impedance is higher than about 110Ω (about 115Ω, for example), the insertion loss and the noise figure in the receive band are increased. 
     When the transmit impedance is about 50Ω, the receive impedance can be lower than about 115Ω. That is, the receive impedance can be higher than the transmit impedance by a factor smaller than about 2.3. 
     The present inventors have found that it is possible to enhance the isolation while decreasing the noise figure when the receive impedance is higher than the transmit impedance by a factor smaller than about 2.3, compared with when the receive impedance is equal to the transmit impedance. The present inventors have also found that, when the center point of NF circles of the low-noise amplifier is located on the higher impedance side than that of gain circles is, if the output impedance in the receive band of the receive filter  10 R is set to be higher than the input impedance in the transmit band of the transmit filter  10 T by a factor of about 2.3 or greater, the gain of the low-noise amplifier is significantly decreased and the noise figure thereof is also increased. When the receive impedance is higher than the transmit impedance by a factor smaller than about 2.3, the receiving noise figure can be decreased substantially without necessarily decreasing the receiving gain. 
     SECOND EMBODIMENT 
     In the first embodiment, a transmit-and-receive module for transmitting and receiving RF signals in a single frequency band has been discussed. In a second embodiment, a transmit-and-receive module for transmitting and receiving RF signals in multiple frequency bands will be discussed. 
       FIG. 11  is a circuit diagram of a communication device and a transmit-and-receive module  2  according to the second embodiment. The communication device according to the second embodiment includes the transmit-and-receive module  2  and a RF signal processing circuit  4 . The communication device may alternatively include the transmit-and-receive module  1  according to the first embodiment and the RF signal processing circuit  4 . 
     The RF signal processing circuit  4  performs signal processing, such as down-conversion, on a RF received signal received from an antenna  3  via a duplexer and a low-noise amplifier, and outputs the resulting received signal to a baseband signal processing circuit (not shown) subsequent to the RF signal processing circuit  4 . The RF signal processing circuit  4  also performs signal processing, such as up-conversion, on a transmit signal received from the baseband signal processing circuit, and outputs the resulting RF transmit signal to a power amplifier. An example of the RF signal processing circuit  4  is a RFIC. 
     The transmit-and-receive module  2  includes a switch  11 , a high-band transmitter-and-receiver  2 H, and a low-band transmitter-and-receiver  2 L. 
     In the switch  11 , a common terminal is connected to the antenna  3 , a first selection terminal is connected to the high-band transmitter-and-receiver  2 H, and a second selection terminal is connected to the low-band transmitter-and-receiver  2 L. With this configuration, the switch  11  connects the antenna  3  to the high-band transmitter-and-receiver  2 H or the low-band transmitter-and-receiver  2 L. Alternatively, the switch  11  may have a function of simultaneously connecting the antenna  3  to the high-band transmitter-and-receiver  2 H and the low-band transmitter-and-receiver  2 L. 
     The high-band transmitter-and-receiver  2 H includes a duplexer  10 H, a power amplifier (PA)  20 H, a low-noise amplifier (LNA)  30 H, a matching circuit  21 H, an adjusting circuit  31 H, a module transmit terminal  120 H, and a module receive terminal  130 H. The duplexer  10 H includes a common terminal  101 H, a transmit terminal  102 H, a receive terminal  103 H, a transmit filter  10 HT, and a receive filter  10 HR. The transmit filter  10 HT is a transmit filter unit using a first transmit band as a pass band and being connected to the common terminal  101 H and the transmit terminal  102 H. The receive filter  10 HR is a receive filter unit using a first receive band as a pass band and being connected to the common terminal  101 H and the receive terminal  103 H. 
     The high-band transmitter-and-receiver  2 H is the transmit-and-receive module  1  according to the first embodiment, for example. 
     To reduce the size of the high-band transmitter-and-receiver  2 H, the power amplifier  20 H handling high-power signals and the low-noise amplifier  30 H handling low-power signals are integrated with each other. In a Smith chart, the output impedance of the receive filter  10 HR is set so that the impedance in the receive band of the receive filter  10 HR seen from the receive terminal  103 H intersects a line connecting the center point of NF circles and that of gain circles. 
     The low-band transmitter-and-receiver  2 L includes a duplexer  10 L, a power amplifier (PA)  20 L, a low-noise amplifier (LNA)  30 L, a matching circuit  21 L, an adjusting circuit  31 L, a module transmit terminal  120 L, and a module receive terminal  130 L. The duplexer  10 L is a second duplexer including a common terminal  101 L (second common terminal), a transmit terminal  102 L (second transmit terminal), a receive terminal  103 L (second receive terminal), a transmit filter  10 LT, and a receive filter  10 LR. 
     The transmit filter  10 LT is a second transmit filter unit using a second transmit band which is lower than the first transmit band as a pass band and being connected to the common terminal  101 L and the transmit terminal  102 L. The receive filter  10 LR is a second receive filter unit using a second receive band which is lower than the first receive band as a pass band and being connected to the common terminal  101 L and the receive terminal  103 L. 
     The power amplifier  20 L is a second power amplifier that amplifies a RF transmit signal and outputs the amplified RF transmit signal to the duplexer  10 L via the transmit terminal  102 L. 
     The low-noise amplifier  30 L is a second low-noise amplifier that amplifies a RF received signal received from the antenna  3  via the receive terminal  103 L. 
     The low-band transmitter-and-receiver  2 L is the transmit-and-receive module  1  according to the first embodiment, for example. 
     To reduce the size of the low-band transmitter-and-receiver  2 L, the power amplifier  20 L handling high-power signals and the low-noise amplifier  30 L handling low-power signals are integrated with each other. In a Smith chart, the output impedance of the receive filter  10 LR is set so that the impedance in the receive band of the receive filter  10 LR seen from the receive terminal  103 L intersects a line connecting the center point of NF circles and that of gain circles. 
     With the above-described configuration, in a multiband-support front-end circuit, impedance matching between the duplexer  10 H and the low-noise amplifier  30 H and between the duplexer  10 L and the low-noise amplifier  30 L disposed in plural signal paths connected to the antenna  3  is performed as follows. Instead of using the characteristic impedance, customized impedance reflecting the impedance characteristics of the low-noise amplifier  30 H is used for impedance matching between the duplexer  10 H and the low-noise amplifier  30 H, and customized impedance reflecting the impedance characteristics of the low-noise amplifier  30 L is used for impedance matching between the duplexer  10 L and the low-noise amplifier  30 L. It is thus possible to provide a transmit-and-receive module which is small in size and which achieves the optimized balance between the receiving noise figure and the receiving gain according to the frequency band while plural duplexers, plural power amplifiers, and plural low-noise amplifiers supporting multiple bands are integrated with each other, and to provide a communication device including the transmit-and-receive module. 
     In the transmit-and-receive module  2  according to the second embodiment, at least one of the high-band transmitter-and-receiver  2 H and the low-band transmitter-and-receiver  2 L may have the configuration and the function of the transmit-and-receive module  1  according to the first embodiment. In the transmit-and-receive module  2 , three or more frequency bands may be used. That is, three or more signal paths, each of which is constituted by a transmit signal path and a received signal path, may be provided. In this case, at least one of the three or more transmitters-and-receivers has the configuration and the function of the transmit-and-receive module  1  according to the first embodiment. 
     THIRD EMBODIMENT 
     In the first and second embodiments, FDD-support transmit-and-receive module for transmitting and receiving RF signals having different frequencies has been discussed. 
     In a third embodiment, a time-division-duplexing (TDD) transmit-and-receive module for transmitting and receiving RF signals having the same frequencies will be discussed.  FIG. 12  is a circuit diagram of a communication device and a transmit-and-receive module  3 TDD according to the third embodiment. The communication device and a transmit-and-receive module  3 TDD of  FIG. 12  is capable of time division duplexing (TDD). 
     The communication device according to the third embodiment includes a TDD-support switch  11 TDD and a TDD-support band-pass filter  10 TDD, but does not include the duplexer  10  used in the first and second embodiments. 
     In the switch  11 TDD, a common terminal is connected to the band pass filter  10 TDD, a first selection terminal (or transmit terminal)  101 TDDT is connected to a matching circuit  21 TDD, and a second selection terminal (or receive terminal)  101 TDDR is connected to an adjusting circuit  31 TDD. The switch  11 TDD switches between a transmit path and a receive path in accordance with a switching timing at which a signal is transmitted and a signal is received. 
     The matching circuit  21 TDD is connected to an output terminal  102 TDDT of a power amplifier  20 TDD. The output impedance of the power amplifier  20 TDD seen from the output terminal  102 TDDT matches the input impedance of the band-pass filter  10 TDD. The output impedance of the power amplifier  20 TDD is converted by the matching circuit  21 TDD so as to match the input impedance of the band pass filter  10 TDD. Likewise, the adjusting circuit  31 TDD is connected to an input terminal  102 TDDR of a low-noise amplifier  30 TDD. The input impedance of the low-noise amplifier  30 TDD seen from the input terminal  102 TDDR matches the output impedance of the band pass filter  10 TDD. 
     Accordingly, as in the first embodiment, in a Smith chart, the output impedance of the band pass filter  10 TDD is set so that the impedance in the receive band of the band pass filter  10 TDD seen from the input terminal  102 TDDR of the low-noise amplifier  30 TDD can intersect a line connecting the center point of the NF circles and that of the gain circles. 
     With the above-described configuration for a TDD front-end circuit, instead of using a characteristic impedance (about 50Ω, for example), a customized impedance reflecting the impedance characteristics of the low-noise amplifier  30 TDD is used for impedance matching between the band-pass filter  10 TDD and the low-noise amplifier  30 TDD in a received signal path is used for impedance matching between the band pass filter  10 TDD and the low-noise amplifier  30 TDD. It is thus possible to provide impedance matching between the low-noise amplifier  30 TDD and the band-pass filter  10 TDD while achieving the optimized balance between the noise figure characteristics and the gain characteristics of the low-noise amplifier  30 TDD. 
     It is thus possible to provide a transmit-and-receive module which achieves the optimized balance between the receiving noise figure and the receiving gain according to the frequency band. 
     FOURTH EMBODIMENT 
     In the third embodiment, a TDD transmit-and-receive module for transmitting and receiving RF signals having the same frequencies has been discussed. 
     In a fourth embodiment, a description will be given of the configuration in which a TDD-support transmit-and-receive module and an FDD-support transmit-and-receive module are combined with each other. The points already discussed in the first through third embodiments will not be repeated. 
       FIG. 13  is a circuit diagram of a communication device and a transmit-and-receive module  3 TF according to the fourth embodiment, in which the transmit-and-receive module  3 TDD is combined with the transmit-and-receive module  1  of the first embodiment. 
     With the configuration, the characteristic impedance is not used for performing impedance matching of the low-noise amplifier  30 TDD. Similarly, in the FDD transmit-and-receive module, the characteristic impedance is not used for performing impedance matching of the low-noise amplifier  30 L. Instead of using the characteristic impedance, customized impedance reflecting the impedance characteristics of the low-noise amplifier disposed in each signal path, which intersects a line connecting the center point of the NF circles and that of the gain circles, is used for impedance matching of the low-noise amplifier. It is thus possible to set the output impedance of the band pass filter  10 TDD and the receive filter  10 LR so that the input impedance of the low-noise amplifiers  30 TDD and  30 L can be optimized to achieve the balance between the noise figure characteristics and the gain characteristics of the low-noise amplifiers  30 TDD and  30 L. To achieve the characteristics of each of the low-noise amplifiers  30 TDD and  30 L, the output impedance of the band pass filter  10 TDD and that of the receive filter  10 LR may be different from each other or may be the same. 
     It is thus possible to provide a transmit-and-receive module which achieves the optimized balance between the receiving noise figure and the receiving gain according to the frequency band. 
     FIFTH EMBODIMENT 
     In the fourth embodiment, the configuration in which a TDD-support transmit-and-receive module and an FDD-support transmit-and-receive module are combined with each other has been discussed. In the configuration of a fifth embodiment, switches  11 MUT and  11 MUR are added to the transmit-and-receive module  1  of the first embodiment, and a multiplexer  10 MU is provided instead of the duplexer  10 . 
     In the example in  FIG. 14 , the multiplexer  10 MU is a quadplexer. However, the multiplexer  10 MU may be of a size other than a quadplexer, such as a triplexer and an octaplexer. In other words, the multiplexer  10 MU is a device including two or more filters. 
     The multiplexer  10 MU includes four filters  10 MU 1 ,  10 MU 2 ,  10 MU 3 , and  10 MU 4 . The filters  10 MU 1  and  10 MU 2  are connected to an output matching circuit  21 MU via the switch  11 MUT. The filters  10 MU 3  and  10 MU 4  are connected to an adjusting circuit  31 MU via the switch  11 MUR. The output impedance of each of the filters  10 MU 3  and  10 MU 4  is set to intersect a line connecting the center point of the NF circles and that of the gain circles of a low-noise amplifier  30 MU. The output impedance of the filter  10 MU 3  and that of the filter  10 MU 4  may be different from each other or may be the same. As in the first embodiment in which the input impedance of the transmit filter is different from the output impedance of the receive filter, the input impedance of the filters  10 MU 1  and  10 MU 2  is different from the output impedance of the filters  10 MU 3  and  10 MU 4 . In this case, to achieve the characteristics of the low-noise amplifier  30 MU, the output impedance of the filter  10 MU 3  and that of the filter  10 MU 4  may be different from each other or may be the same. 
     In contrast to a regular multiplexer in which the impedance of each filter is set to be the characteristic impedance (about 50Ω, for example), in the configuration of the fifth embodiment, the impedance of the filters  10 MU 1  and  10 MU 2  is different from that of the filters  10 MU 3  and  10 MU 4 , thereby achieving high isolation characteristics between the transmit filters and the receive filters. If the impedance of the filter  10 MU 3  and that of the filter  10 MU 4  are different from each other, high isolation characteristics between the receive filters are also implemented. Although the switches  11 MUT and  11 MUR are separately provided in the example in  FIG. 14 , they may be integrated into one chip or may be formed in one circuit. 
     In some embodiments, the power amplifier and the low-noise amplifier may be integrated with each other as described above. 
     Mounting of Transmit-and-Receive Module 
       FIG. 15A  and  FIG. 15B  are schematic views of configurations in which the transmit-and-receive module  1  is mounted on a substrate. 
     In  FIG. 15A , the low-noise amplifier  30 , the power amplifier  20 , the duplexer  10 , the matching circuit  21 , and the adjusting circuit  31  are mounted on a front surface of a substrate  1 P. The transmit-and-receive module  1  is covered by resin  1 R. 
     In  FIG. 15B , the power amplifier  20 , the duplexer  10 , the matching circuit  21 , and the adjusting circuit  31  are mounted on the front surface of the substrate  1 P, and the low-noise amplifier  30  is mounted on the back surface of the substrate  1 P. Conversely, the low-noise amplifier  30  may be mounted on the front surface of the substrate  1 P, and the power amplifier  20  may be mounted on the back surface of the substrate  1 P. The transmit-and-receive module  1  is covered by resin  1 R. 
     OTHER EMBODIMENTS 
     The transmit-and-receive modules and the communication device according to embodiments of the disclosure have been discussed through illustration of the first and second embodiments. However, certain elements in the above-described first and second embodiments may be combined to realize other embodiments, and various modifications apparent to those skilled in the art may be made to the first and second embodiments without necessarily departing from the scope and spirit of the disclosure. Such embodiments and modified examples are also encompassed within the present disclosure. Additionally, various apparatuses integrating the transmit-and-receive module and the communication device described above therein are also encompassed within the present disclosure. 
     In the first and second embodiments, a transmit-and-receive module and a communication device including a duplexer have been discussed by way of example. The present disclosure is also applicable to a quadplexer and a hexaplexer in which plural duplexers are connected to each other. 
     The configuration in which the output impedance (receive impedance) of the receive filter  10 R is set to be higher than the characteristic impedance and the input impedance (transmit impedance) of the transmit filter  10 T is not restricted to a particular configuration. For example, if the receive filter  10 R is a surface acoustic wave filter including resonators constituted by plural interdigital transducer (IDT) electrodes, electrode parameters such as the pitch of electrode fingers forming an IDT electrode, the interdigital width of the electrode fingers, the number of pairs of electrode fingers, and the distance between the reflector and the IDT electrode may vary among the IDT electrodes. If the receive filter  10 R is constituted by ladder elastic wave resonators, the impedance of the elastic wave resonator located closest to the receive terminal  103  may be set to be higher than that of the other elastic wave resonators. With the above-described configurations, high impedance can effectively be implemented while maintaining filter characteristics of the receive filter  10 R. 
     Instead of setting the output impedance of the receive filter  10 R to be higher, the impedance of the adjusting circuit  31  seen from the low-noise amplifier  30  may be adjusted. 
     In the first embodiment, the receive band is a lower frequency side, and the transmit band is a higher frequency side. However, a transmit-and-receive module according to an embodiment of the disclosure may be applicable to a configuration in which the receive band is a higher frequency side and the transmit band is a lower frequency side. 
     In a transmit-and-receive module according to an embodiment of the disclosure, circuit elements, such as an inductor, a capacitor, and a resistor element, may be added between the module common terminal  110  and the module transmit terminal  120  or the module receive terminal  130 . 
     The present disclosure is widely applicable to communication terminals, such as cellular phones, as a high-gain, low-noise, small transmit-and-receive module and a communication device including such a transmit-and-receive module. 
     While preferred embodiments of the invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.