Abstract:
A high-frequency module prevents distortion in first and second diodes of a high-frequency switch in a communication system which is not selected without providing a negative power source, and a communication apparatus includes such a high-frequency module. The high-frequency module includes a diplexer having inductors and capacitors, and high-frequency switches including first and second diodes, transmission lines, inductors, and capacitors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a composite high-frequency switch, a high-frequency module, and a communication apparatus. More particularly, the present invention relates to a composite high-frequency switch, a high-frequency module, and a communication apparatus, which is used in a plurality of different communication systems. 
     2. Description of the Related Art 
     Presently, in Europe, as a communication apparatus, a dual-band portable phone which operates in a plurality of communication systems, for example, a DCS (Digital Cellular System) in which a 1.8-GHz band is used and a GSM (Global System for Mobile Communications) in which a 900-MHz band is used has been proposed. In contrast to a conventional portable phone which operates in only one communication system, this dual-band portable phone operates in two communication systems. This enables a user to select and to use a suitable communication system. 
     In the dual-band portable phone, a high-frequency module is operative to switch between a plurality of communication systems and to switch between a transmission circuit and a receiving circuit. A known high-frequency module is disclosed in Japanese Unexamined Patent Application Publication No. 11-168303. This conventional high-frequency module is defined by an antenna switch module which connects the two communication systems DCS and GSM to an antenna, and includes a diplexer and two high-frequency switches. The two high-frequency switches switch between a transmission circuit and a receiving circuit, and include first and second diodes, a transmission line, and an inductor. The anode of a first diode is connected to the antenna terminal side and its cathode is connected to the transmission terminal side, and an inductor is connected between the cathode and a ground. The transmission line is connected between an antenna terminal and a receiving terminal, and the cathode of a second diode is connected to the receiving terminal side and a capacitor is connected between the anode of the second diode and a ground. A control terminal is connected between the second diode and the capacitor. 
     When this high-frequency module operates, a positive voltage is applied to the control terminal of the high-frequency switch on the selected communication system side, and a zero voltage is applied to the control terminal of the high-frequency switch on the communication system side which is not selected. However, a problem arises in that a received signal or a transmission signal in the selected communication system side leaks to the communication system side which is not selected, and the first and second diodes of the high-frequency switch of the communication system which is not selected are distorted. To solve this problem, a method in which a negative voltage is applied to the control terminal of the high-frequency switch of the communication system side which is not selected, such that a reverse bias is applied to the first and second diodes of the high-frequency switch of the communication system which is not selected. 
     However, according to the above-described high-frequency module, since a negative voltage must be applied to the control terminal of the high-frequency switch on the communication system side which is not selected, a negative power source must be provided within the dual-band portable phone, which increases the complexity of the configuration of the circuit. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems described above, preferred embodiments of the present invention provide a composite high-frequency switch and a high-frequency module which prevents distortion in first and second diodes of a communication system which is not selected without providing a negative power source, and a communication apparatus including the high-frequency module. 
     A first preferred embodiment of the present invention provides a composite high-frequency switch including a plurality of high-frequency switches, each having a first terminal, a second terminal, a first control terminal, a second control terminal, a first diode, a second diode, and a transmission line, wherein the high-frequency switch is configured such that the first terminal, the transmission line, the first diode, and the second terminal are connected in series, the first terminal side of the transmission line is connected to a ground via the second diode, the first control terminal is connected to the ground side of the second diode, and the second control terminal is connected to the second terminal side of the first diode, and at least two of the plurality of high-frequency switches are configured such that the second control terminals are connected to each other and the connection point thereof is connected to a common control terminal via a resistor. 
     Another preferred embodiment of the present invention provides a composite high-frequency switch including a plurality of high-frequency switches, each having a first terminal, a second terminal, a first control terminal, a second control terminal, a first diode, a second diode, and a transmission line, wherein the high-frequency switch is configured such that the first terminal, the transmission line, the first diode, and the second terminal are connected in series, the first terminal side of the transmission line is connected to a ground via the second diode, the first control terminal is connected to the ground side of the second diode, the second control terminal is connected to the second terminal side of the first diode, and at least two of the plurality of high-frequency switches are configured such that the first control terminals are connected to each other and the connection point thereof is connected to a common control terminal via a resistor. 
     Still another preferred embodiment of the present invention provides a composite high-frequency switch including a plurality of high-frequency switches each having a first terminal, a second terminal, a first control terminal, a second control terminal, a first diode, a second diode, and a transmission line, wherein the high-frequency switch is configured such that the first terminal, the transmission line, the first diode, and the second terminal are connected in series, the first terminal side of the transmission line is connected to a ground via the second diode, the first control terminal is connected to the ground side of the second diode, the second control terminal is connected to the second terminal side of the first diode, and at least two of the plurality of high-frequency switches are configured such that the first control terminal of a high-frequency switch and the second control terminal of another high-frequency switch are connected to each other and the connection point thereof is connected to a common control terminal via a resistor. 
     Preferably, the composite high-frequency switch according to preferred embodiments of the present invention further includes a multilayer substrate having a plurality of sheet layers layered thereon, wherein a diode of the high-frequency switch is mounted on the multilayer substrate, and a transmission line of the high-frequency switch is provided in the multilayer substrate. 
     Another preferred embodiment of the present invention provides a high-frequency module including a diplexer having an inductor and a capacitor, the diplexer being connected to a connection point of the first diode and the transmission line of the high-frequency switch. 
     Preferably, the high-frequency module according to preferred embodiments of the present invention further includes a multilayer substrate having a plurality of sheet layers layered thereon, wherein a diode of the high-frequency switch is mounted on the multilayer substrate, and the inductor and the capacitor of the diplexer, and a transmission line of the high-frequency switch is provided in the multilayer substrate. 
     In the high-frequency module, preferably, a capacitor of the diplexer is connected to the ground, and a grounding electrode of the high-frequency switch is provided between the capacitor and the transmission line of the high-frequency switch. 
     Another preferred embodiment of the present invention provides a communication apparatus using the composite high-frequency switch according to preferred embodiments described above. 
     Still another preferred embodiment of the present invention provides a communication apparatus using the high-frequency module according to preferred embodiments of the present invention described above. 
     According to the composite high-frequency switch of various preferred embodiments of the present invention, by applying a positive voltage to the control terminal of a high-frequency switch in a selected communication system side, a forward bias is applied to the first and second diodes of the high-frequency switch on the selected communication system side, and at the same time, a reverse bias is applied to the first and second diodes of a high-frequency switch on the communication system side which is not selected. 
     According to the high-frequency module of preferred embodiments of the present invention, since a composite high-frequency switch having greatly improved distortion characteristics is provided, the distortion characteristics of the high-frequency module are also greatly improved. 
     According to the communication apparatus of preferred embodiments of the present invention, since a composite high-frequency switch or a high-frequency module, having greatly improved distortion characteristics, is provided, the transmission and receiving characteristics of the communication apparatus are greatly improved. 
     Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a first preferred embodiment of a high-frequency module according to the present invention. 
     FIG. 2 is a partially exploded perspective view showing a specific construction of the high-frequency module of FIG.  1 . 
     FIGS. 3A,  3 B,  3 C,  3 D,  3 E,  3 F,  3 G,  3 H,  3 I, and  3 J are top views of first to tenth sheet layers which define the high-frequency module of FIG.  2 . 
     FIGS. 4A,  4 B,  4 C,  4 D,  4 E, and  4 F are top views of eleventh to sixteenth sheet layers which define the high-frequency module of FIG. 2, and FIG. 4G is a bottom view of the sixteenth sheet layer. 
     FIG. 5 is a circuit diagram showing a modification of the high-frequency module of FIG.  1 . 
     FIG. 6 is a circuit diagram of a second preferred embodiment of a high-frequency module according to the present invention. 
     FIG. 7 is a circuit diagram showing a modification of the high-frequency module of FIG.  3 . 
     FIG. 8 is a circuit diagram of a third preferred embodiment of a high-frequency module according to the present invention. 
     FIG. 9 is a circuit diagram showing a modification of the high-frequency module of FIG.  5 . 
     FIG. 10 is a block diagram showing a portion of the configuration of a communication apparatus using the high-frequency module of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described below with reference to the drawings. 
     FIG. 1 is a circuit diagram of a first preferred embodiment of a high-frequency module according to the present invention. A high-frequency module  10  includes a diplexer  11 , and high-frequency switches  12  and  13 . 
     The diplexer  11  includes inductors L 11  and L 12  and capacitors C 11  to C 15 . A parallel circuit including the inductor L 11  and the capacitor C 11  is connected between a first terminal P 11  and a second terminal P 12 , and a second terminal P 12  side of the parallel circuit is connected to a ground via the capacitor C 12 . 
     Furthermore, the capacitors C 13  and C 14  are connected in series between the first terminal P 11  and a third terminal P 13 , and a connection point thereof is connected to a ground via the inductor L 12  and the capacitor C 15 . 
     The high-frequency switch  12  includes first and second diodes D 21  and D 22 , a transmission line TL 21 , inductors L 21  and L 22 , and a capacitor C 21 . The first diode D 21 , the transmission line TL 21 , the first diode D 21 , and the second terminal P 22  are connected in series. Furthermore, the first terminal P 21  side of the transmission line TL 21  is connected to a ground via the second diode D 22  and the capacitor C 21 , and a first control terminal V 21  is connected to the ground side of the second diode D 22 , that is, the anode thereof, via the inductor L 21 . 
     Furthermore, a second control terminal V 22  is connected to the second terminal P 22  side of the first diode D 21 , that is, the cathode thereof, via the inductor L 22 . A third terminal P 23  is provided between the transmission line TL 21  and the first diode D 21 . 
     The high-frequency switch  13  includes first and second diodes D 31  and D 32 , a transmission line TL 31 , inductors L 31  and L 32 , and a capacitor C 31 . The structure of the high-frequency switch  13  is the same as that of the high-frequency switch  12 . 
     The second control terminal V 22  of the high-frequency switch  12  and a second control terminal V 32  of the high-frequency switch  13  are connected to each other, and a connection point thereof is connected to a common control terminal Vc via a resistor R. 
     In the above-described construction, an antenna ANT is connected to the first terminal P 11  of the diplexer  11 , the third terminal P 23  of the high-frequency switch  12  is connected to the second terminal P 12 , and a third terminal P 33  of the high-frequency switch  13  is connected to the third terminal P 13 . Furthermore, a receiving circuit Rx is connected to the first terminal P 21  of the high-frequency switch  12  and a first terminal P 31  of the high-frequency switch  13 , and a transmission circuit Tx is connected to the second terminal P 22  of the high-frequency switch  12  and a second terminal P 32  of the high-frequency switch  13 . 
     The operation of the high-frequency module  10  having the circuit configuration of FIG. 1 will now be described. When a communication system on the high-frequency switch  12  side is to be selected, that is, when the high-frequency switch  12  is to be turned on, approximately 3 V is applied to the first control terminal V 21  of the high-frequency switch  12 , 0 V is applied to the first control terminal V 31  of the high-frequency switch  13 , and approximately 0.5 V is applied to the common control terminal Vc. 
     Just then, a forward bias is applied to the first and second diodes D 21  and D 22  of the high-frequency switch  12  on the selected communication system side, and forward current flows from the first control terminal V 21  of the high-frequency switch  12  toward the common control terminal Vc. Then, since a voltage drop occurs in the resistor R due to this forward current, the amount of the voltage drop is applied as a reverse bias to the first and second diodes D 31  and D 32  of the high-frequency switch  13 . This reverse bias causes the off capacity of the first and second diodes D 31  and D 32  of the high-frequency switch  13  on the communication system side which is not selected to be stabilized and decreased. As a consequence, the distortion characteristics are greatly improved. 
     Furthermore, since an inductor is inserted at a stage that precedes the resistor, the inductor shields a high-frequency signal. 
     FIG. 2 is a partially exploded perspective view showing the specific structure of the high-frequency module shown in FIG.  1 . The high-frequency module  10  includes a multilayer substrate  14 . Although not shown, inductors L 11  and L 12  and capacitors C 11  to C 15  which define the diplexer  11  (FIG.  1 ), and, transmission lines TL 21  and TL 31  and capacitor C 21  and C 31  which define the high-frequency switches  12  and  13  (FIG.  1 ), respectively, are provided in the multilayer substrate  14 . 
     The first and second diodes D 21 , D 22 , D 31 , and D 32 , and the inductors L 21 , L 22 , L 31 , and L 32 , which define the high-frequency switches  12  and  13  (FIG. 1) defined by chip components, and the resistor R, are mounted on the surface of the multilayer substrate  14 . 
     Furthermore,  12  external terminals T 1  to T 12  are preferably formed by screen printing, or other suitable methods, so as to extend from the side surfaces of the multilayer substrate  14  toward the bottom. The external terminal T 1  defines a second terminal P 32  of the high-frequency switch  13 . The external terminal T 3  defines a first terminal P 11  of the diplexer  11 . The external terminal T 4  defines a first control terminal V 31  of the high-frequency switch  13 . The external terminal T 5  defines a first terminal P 31  of the high-frequency switch  13 . The external terminal T 7  defines a first terminal P 21  of the high-frequency switch  12 . The external terminal T 8  defines a first control terminal V 21  of the high-frequency switch  12 . The external terminal T 11  defines a second terminal P 22  of the high-frequency switch  12 . The external terminals T 2 , T 6 , T 9 , T 10 , and T 12  define grounding terminals. 
     The second terminal P 12  of the diplexer  11  and the third terminal P 23  of the high-frequency switch  12 , and the third terminal P 13  of the diplexer  11  and the third terminal P 33  of the high-frequency switch  13  are connected to each other through via holes provided in the multilayer substrate  14 . 
     FIGS. 3A to  3 J, and FIGS. 4A to  4 F are top views of each sheet layer defining a multilayer substrate of the high-frequency module of FIG.  2 . FIG. 4G is a bottom view of the sheet layer of FIG. 4F. A multilayer substrate  14  is formed by sequentially layering, from the top, first to sixteenth sheet layers  14   a  and  14   p  made of ceramics, in which barium oxide, aluminum oxide, and silica are main ingredients, and by firing these sheet layers at a firing temperature of about 1000° C. or lower, after which these are turned upside down. That is, the sixteenth sheet layer  14   p  becomes the topmost layer of the multilayer substrate  14 , and the first sheet layers  14   a  becomes the bottommost layer of the multilayer substrate  14 . 
     The external terminals T 1  to T 12  are provided on the surface of the first sheet layer  14   a.  Grounding electrodes Gp 1  to Gp 4  are formed by screen printing, or other suitable method, on the surfaces of the second, fourth, eighth, and tenth sheet layers  14   b,    14   d,    14   h,  and  14   j.    
     Capacitor electrodes Cp 1  to Cp 9  are formed by screen printing, or other suitable method, on the surfaces of the third sheet layers  14   c,  and ninth to twelfth sheet layers  14   i  to  14   l.  Furthermore, stripline electrodes Lp 1  to Lp 10  are formed by screen printing, or other suitable method, on the surfaces of the fifth to seventh sheet layers  14   e  to  14   g.    
     Mounting electrodes La for mounting the first and second diodes D 21 , D 22 , D 31 , and D 32 , and the inductors L 21 , L 22 , L 31 , and L 32 , which define the high-frequency switches  12  and  13  (FIG.  1 ), and the resistor R are provided on the bottom surface (FIG. 4G,  14   pu ) of the sixteenth sheet layer. Furthermore, a plurality of via hole electrodes Vh are provided on the second to sixteenth sheet layers  14   b  to  14   p  so as to extend through each of the sheet layers  14   b  to  14   p.    
     At this time, the inductor L 11  of the diplexer  11  is defined by the stripline electrodes Lp 2 , Lp 4 , and Lp 10 . The inductor L 12  of the diplexer  11  is defined by the stripline electrodes Lp 1 , Lp 3 , and Lp 7 . The transmission line TL 21  of the high-frequency switch  12  is defined by the stripline electrodes Sp 5  and Sp 8 . The transmission line TL 31  of the high-frequency switch  13  is defined by the stripline electrodes Sp 6  and Sp 9 . 
     In addition, the capacitor C 11  of the diplexer  11  is defined by the capacitor electrodes Cp 3  and Cp 6 . The capacitor C 12  of the diplexer  11  is defined by the capacitor electrode Cp 2  and the grounding electrodes Gp 1  and Gp 2 . The capacitor C 13  of the diplexer  11  is defined by the capacitor electrodes Cp 6  and Cp 8 . The capacitor C 14  of the diplexer  11  is defined by the capacitor electrodes Cp 8  and Cp 9 . The capacitor C 15  of the diplexer  11  is defined by the capacitor electrode Cp 1  and the grounding electrodes Gp 1  and Gp 2 . 
     The capacitor C 21  of the high-frequency switch  12  is defined by the capacitor electrode Cp 7  and the grounding electrode Gp 4 . The capacitor C 31  of the high-frequency switch  13  is defined by the capacitor electrode Cp 5  and the grounding electrodes Gp 3  and Gp 4 . 
     FIG. 5 is a circuit diagram showing a modification of the high-frequency module of FIG. 1. A high-frequency module  10   a  is designed for a triple band and includes the diplexer  11  and the high-frequency switches  12   a  and  13   a.  The structure of the diplexer  11  is preferably the same as that of the high-frequency module  10  (FIG. 1) of the first preferred embodiment, and accordingly, a description thereof is omitted. 
     The high-frequency switch  12   a  includes first and second diodes Da 21   a  and D 22   a,  a transmission line TL 21   a,  inductors L 21   a  and L 22   a,  and a capacitor C 21   a.  A first terminal P 21   a,  a transmission line TL 21   a,  the first diode D 21   a,  and a second terminal P 22   a  are connected in series. 
     Furthermore, the first terminal P 21   a  side of the transmission line TL 21   a  is connected to a ground via the second diode D 22   a  and the capacitor C 21   a,  and a first control terminal V 21   a  is connected to the ground side of the second diode D 22   a,  that is, the anode thereof, via the inductor L 21   a.    
     In addition, a second control terminal V 22   a  is connected to the second terminal P 22   a  side of the first diode D 21   a,  that is, the cathode thereof, via the inductor L 22   a.  Furthermore, a third terminal P 23   a  is provided between the transmission line TL 21   a  and the first diode D 21   a.    
     The high-frequency switch  13   a  includes first to fourth diodes D 31   a  to D 34   a,  transmission lines TL 31  and TL 32   a,  inductors L 31   a  to L 33   a,  and capacitors C 31   a  to C 33   a.  A first terminal P 31   a,  the transmission line TL 31   a,  the first diode D 31   a,  and a second terminal P 32   a  are connected in series. 
     The first terminal P 31   a  side of the transmission line TL 31   a  is connected to a ground via the second diode D 32   a  and the capacitor C 31   a,  and a first control terminal V 31   a  is connected to the ground side of the second diode D 32   a,  that is, the anode thereof, via the inductor L 31   a.    
     In addition, a second control terminal V 32   a  is connected to the second terminal P 32   a  side of the first diode D 31   a,  that is, the cathode thereof, via the inductor L 32   a.  Furthermore, the capacitor C 32   a,  the third diode D 33   a,  the transmission line TL 32   a,  and the third terminal P 33   a  are connected in series to the connection point of the transmission line TL 31   a  and the first diode D 31   a.    
     Furthermore, the third terminal P 33   a  side of the transmission line TL 32   a  is connected to a ground via a fourth diode D 34   a  and a capacitor C 33   a,  and a third control terminal V 33   a  is connected to the ground side of the fourth diode D 34   a,  that is, the anode thereof, via an inductor L 33   a.    
     In addition, a fourth terminal P 34   a  is provided between the third diode D 33   a  and the transmission line TL 32   a,  and a fourth control terminal V 34   a  is connected between the capacitor C 32   a  and the third diode D 33   a  via the inductor L 34   a.    
     The second control terminals V 22   a  and V 32   a  of the high-frequency switches  12   a  and  13   a,  and the fourth control terminal V 34   a  of the high-frequency switch  13   a  are connected to each other, and a connection point thereof is connected to a common control terminal Vca via a resistor Ra. 
     In the above-described configuration, the third terminal P 23   a  of the high-frequency switch  12   a  is connected to the second terminal P 12  of the diplexer  11 , and the fourth terminal P 34   a  of the high-frequency switch  13   a  is connected to the third terminal P 13 . Although not shown, an antenna ANT is connected to the first terminal P 11  of the diplexer  11 . A receiving circuit Rx is connected to the first terminals P 21   a  and P 31   a  of the high-frequency switches  12   a  and  13   a.  A transmission circuit Tx is connected to the second terminals P 22   a  and P 32   a  of the high-frequency switches  12   a  and  13   a.  Both the receiving circuit Rx and the transmission circuit Tx are connected to the third terminal P 33   a  of the high-frequency switch  13   a.    
     Here, the operation of the high-frequency module  10   a  having the circuit configuration of FIG. 5 is described. When the communication system on the high-frequency switch  12   a  side is to be selected, that is, when the high-frequency switch  12   a  is to be turned on, approximately 3 V is applied to the first control terminal V 21   a  of the high-frequency switch  12   a,  0 V is applied to the first control terminal V 31   a  of the high-frequency switch  13   a,  and approximately 0.5 V is applied to the common control terminal Vca. 
     Just then, a forward bias is applied to the first and second diodes D 21   a  and D 22   a  of the high-frequency switch  12   a  on the selected communication system side, and forward current flows from the first control terminal V 21   a  of the high-frequency switch  12   a  toward the common control terminal Vca. Then, since a voltage drop occurs in the resistor Ra due to this forward current, the amount of the voltage drop is applied as a reverse bias to the first to fourth diodes D 31   a  to D 34   a  of the high-frequency switch  13   a.  This reverse bias causes the off capacity of the first to fourth diodes D 31   a  to D 34   a  of the high-frequency switch  13   a  on the communication system side which is not selected to be stabilized and decreased. As a consequence, the distortion characteristics are greatly improved. 
     FIG. 6 is a circuit diagram of a second preferred embodiment of a high-frequency module according to the present invention. A high-frequency module  20  includes a diplexer  11  and high-frequency switches  22  and  23 . The structure of the diplexer  11  is preferably the same as that of the high-frequency module  10  (FIG. 1) of the first preferred embodiment, and accordingly, a description thereof is omitted. 
     The high-frequency switch  22  includes first and second diodes D 21  and D 22 , a transmission line TL 21 , inductors L 21  and L 22 , and capacitors C 21  and c 22 . A first terminal P 21 , the transmission line TL 21 , the first diode D 21 , and a second terminal P 22  are connected in series. Furthermore, the first terminal P 21  side of the transmission line TL 21  is connected to a ground via the second diode D 22  and the capacitor C 21 , and a first control terminal V 21  is connected to the ground side of the second diode D 22 , that is, the anode thereof, via the second diode D 22  and the capacitor C 21 . 
     The second terminal P 22   a  side of the first diode D 21   a,  that is, the anode thereof, is connected to the ground via the inductor L 21  and the capacitor C 22 , and a second control terminal V 22  is connected to the ground side of the inductor L 21 . Furthermore, a third terminal P 23  is provided between the transmission line TL 21  and the first diode D 21 . 
     The high-frequency switch  23  includes first and second diodes D 31  and D 32 , a transmission line TL 31 , an inductor L 31 , and capacitors C 31  and C 32 . The structure of the high-frequency switch  23  is the same as that of the high-frequency switch  22 . 
     The first control terminals V 21  and V 31  of the high-frequency switches  22  and  23  are connected to each other, and a connection point thereof is connected to a common control terminal Vc via a resistor R. The inductor L 22  is connected between the first control terminal V 21  of the high-frequency switch  22  and the resistor R. However, the inductor may not be connected. 
     Here, the operation of the high-frequency module  20  having the circuit configuration of FIG. 6 is described. When the communication system on the high-frequency switch  22  side is to be selected, that is, when the high-frequency switch  22  is to be turned on, approximately 3 V is applied to the first control terminal V 22  of the high-frequency switch  22 , 0 V is applied to the first control terminal V 32  of the high-frequency switch  23 , and approximately 0.5 V is applied to the common control terminal Vc. 
     Just then, a forward bias is applied to the first and second diodes D 21  and D 22  of the high-frequency switch  22  on the selected communication system side, and forward current flows from the second control terminal V 22  of the high-frequency switch  22  toward the common control terminal Vc. Then, since a voltage drop occurs in the resistor R due to this forward current, the amount of the voltage drop is applied as a reverse bias to the first and second diodes D 31  and D 32  of the high-frequency switch  23 . This reverse bias causes the off capacity of the first and second diodes D 31  and D 32  of the high-frequency switch  23  on the communication system side which is not selected to be stabilized and decreased. As a consequence, the distortion characteristics are greatly improved. 
     FIG. 7 is a circuit diagram showing a modification of the high-frequency module of FIG. 6. A high-frequency module  20   a  is designed for a triple band and is defined by the diplexer  11  and the high-frequency switches  22   a  and  23   a.  The structure of the diplexer  11  is the same as that of the high-frequency module  10  (FIG. 1) of the first preferred embodiment, and accordingly, a description thereof is omitted. 
     The high-frequency switch  22   a  includes first and second diodes D 21   a  and D 22   a,  a transmission line TL 21   a,  an inductor L 21   a,  and capacitors C 21   a  and C 22   a.  A first terminal P 21   a,  the transmission line TL 21   a,  the first diode D 21   a,  and a second terminal P 22   a  are connected in series. Furthermore, the first terminal P 21   a  side of the transmission line TL 21   a  is connected to a ground via the second diode D 22   a  and the capacitor C 21   a,  and a first control terminal V 21   a  is connected to the ground side of the second diode D 22   a,  that is, the cathode thereof. 
     Furthermore, the second terminal P 22   a  side of the first diode D 21   a,  that is, the anode thereof, is connected to a ground via the inductor L 21   a  and the capacitor C 22   a,  and a second control terminal V 22   a  is connected to the ground side of the inductor L 21   a.  Furthermore, a third terminal P 23   a  is provided between the transmission line TL 21   a  and the first diode D 21   a.    
     The high-frequency switch  23   a  includes first to third diodes D 31   a  to D 33   a,  a transmission line TL 31   a,  inductors L 31   a  and L 32   a,  and capacitors C 31   a  to C 33   a.  A first terminal P 31   a,  the transmission line TL 31   a,  the first diode D 31   a,  and a second terminal P 32   a  are connected in series. 
     Furthermore, the first terminal P 31   a  side of the transmission line TL 31   a  is connected to a ground via the second diode D 32   a  and the capacitor C 31   a,  and a first control terminal V 31   a  is connected to the ground side of the second diode D 32   a,  that is, the cathode thereof. 
     In addition, the second terminal P 32   a  side of the first diode D 31   a,  that is, the anode thereof, is connected to a ground via an inductor L 31   a  and the capacitor C 32   a,  and a second control terminal V 32   a  is connected to the ground side of the inductor L 31   a.  Furthermore, a third diode D 33   a  and a third terminal P 33   a  are connected in series to a connection point of the transmission line TL 31   a  and the second diode D 31   a.    
     In addition, the third terminal P 33   a  side of the third diode D 33   a,  that is, the anode thereof, is connected to a ground via an inductor L 32   a  and the capacitor C 33   a,  and a third control terminal V 33   a  is connected to the ground side of the inductor L 32   a.  Furthermore, a fourth terminal P 34   a  is provided between the transmission line TL 31   a  and the first diode D 31   a.    
     The first control terminals V 21   a  and V 31   a  of the high-frequency switches  22   a  and  23   a  are connected to each other, and a connection point thereof is connected to a common control terminal Vca via a resistor Ra. The inductor L 22   a  is connected between the first control terminal V 21   a  of the high-frequency switch  22   a  and the resistor Ra. However, the inductor L 22   a  may not be connected. 
     In the above-described configuration, the third terminal P 23   a  of the high-frequency switch  22   a  is connected to the second terminal P 12  of the diplexer  11 , and the fourth terminal P 34   a  of the high-frequency switch  23   a  is connected to the third terminal P 23 . Although not shown, an antenna ANT is connected to the first terminal P 11  of the diplexer  11 . A receiving circuit Rx is connected to the first terminals P 21   a  and P 31   a  of the high-frequency switches  22   a  and  23   a.  A transmission circuit Tx is connected to the second terminals P 22   a  and P 32   a  of the high-frequency switches  22   a  and  23   a.  Both the receiving circuit Rx and the transmission circuit Tx are connected to the third terminal P 33   a  of the high-frequency switch  23   a.    
     The operation of the high-frequency module  20   a  having the circuit configuration of FIG. 7 will now be described. When the communication system on the high-frequency switch  22   a  side is to be selected, that is, when the high-frequency switch  22   a  is to be turned on, approximately 3 V is applied to the second control terminal V 22   a  of the high-frequency switch  22   a,  0 V is applied to the second control terminal V 32   a  of the high-frequency switch  23   a,  and approximately 0.5 V is applied to the common control terminal Vca. 
     Just then, a forward bias is applied to the first and second diodes D 21   a  and D 22   a  of the high-frequency switch  22   a  on the selected communication system side, and forward current flows from the second control terminal V 22   a  of the high-frequency switch  22   a  toward the common control terminal Vca. Then, since a voltage drop occurs in the resistor Ra due to this forward current, the amount of the voltage drop is applied as a reverse bias to the first to third diodes D 31   a  to D 33   a  of the high-frequency switch  23   a.  This reverse bias causes the off capacity of the first to third diodes D 31   a  to D 33   a  of the high-frequency switch  23   a  on the communication system side which is not selected to be stabilized and decreased. As a consequence, the distortion characteristics are greatly improved. 
     FIG. 8 is a circuit diagram of a third preferred embodiment of a high-frequency module according to the present invention. A high-frequency module  30  includes a diplexer  11 , and high-frequency switches  32  and  33 . The structure of the diplexer  11  is preferably the same as that of the high-frequency module  10  (FIG. 1) of the first preferred embodiment, and accordingly, a description thereof is omitted. 
     The high-frequency switch  32  includes first and second diodes D 21  and D 22 , a transmission line TL 21 , inductors L 21  and L 22 , and a capacitor C 21 . A first terminal P 21 , the transmission line TL 21 , the first diode D 21 , and a second terminal P 22  are connected in series. Furthermore, the first terminal P 21  side of the transmission line TL 21  is connected to a ground via the second diode D 22  and the capacitor C 21 , and a first control terminal V 21  is connected to the ground side of the second diode D 22 , that is, the anode thereof, via the inductor L 21 . 
     A second control terminal V 22  is connected to the second terminal P 22  side of the first diode D 21 , that is, the cathode thereof, via the inductor L 22 . Furthermore, a third terminal P 23  is provided between the transmission line TL 21  and the first diode D 21 . 
     The high-frequency switch  33  includes first and second diodes D 31  and D 32 , a transmission line TL 31 , an inductor L 31 , and capacitors C 31  and C 32 . A first terminal P 31 , the transmission line TL 31 , the first diode D 31 , and a second terminal P 32  are connected in series. The first terminal P 31  side of the transmission line TL 31  is connected to a ground via the second diode D 32  and the capacitor C 31 , and a first control terminal V 31  is connected to the ground side of the second diode D 32 , that is, the cathode thereof. 
     The second terminal P 32  side of the first diode D 31 , that is, the anode thereof, is connected to a ground via an inductor L 31  and the capacitor C 32 , and a second control terminal V 32  is connected to the ground side of the inductor L 31 . Furthermore, a third terminal P 33  is provided between the transmission line TL 31  and the first diode D 31 . 
     The second control terminal V 22  of the high-frequency switch  32  and the first control terminal V 31  of the high-frequency switch  33  are connected to each other, and a connection point thereof is connected to the common control terminal Vc via a resistor R. 
     The operation of the high-frequency module  30  having the circuit configuration of FIG. 8 will now be described. When the communication system on the high-frequency switch  32   a  side is to be selected, that is, when the high-frequency switch  32  is to be turned on, approximately 3 V is applied to the first control terminal V 21  of the high-frequency switch  32 , 0 V is applied to the second control terminal V 32  of the high-frequency switch  33 , and approximately 0.5 V is applied to the common control terminal Vc. 
     Just then, a forward bias is applied to the first and second diodes D 21  and D 22  of the high-frequency switch  32  on the selected communication system side, and forward current flows from the first control terminal V 21  of the high-frequency switch  32  toward the common control terminal Vc. Then, since a voltage drop occurs in the resistor R due to this forward current, the amount of the voltage drop is applied as a reverse bias to the first and second diodes D 31  and D 32  of the high-frequency switch  33 . This reverse bias causes the off capacity of the first and second diodes D 31  and D 32  of the high-frequency switch  33  on the communication system side which is not selected to be stabilized and decreased. As a consequence, the distortion characteristics are greatly improved. 
     FIG. 9 is a circuit diagram showing a modification of the high-frequency module of FIG. 8. A high-frequency module  30   a  is designed for a triple band and includes the diplexer  11  and the high-frequency switches  32   a  and  33   a.  The structure of the diplexer  11  is preferably the same as that of the high-frequency module  10  (FIG. 1) of the first preferred embodiment, and accordingly, a description thereof is omitted. 
     The high-frequency switch  32   a  includes first and second diodes D 21   a  and D 22   a,  a transmission line TL 21   a,  inductors L 21   a  and L 22   a,  and a capacitor C 21   a.  A first terminal P 21   a,  the transmission line TL 21   a,  the first diode D 21   a,  and a second terminal P 22   a  are connected in series. 
     Furthermore, the first terminal P 21   a  side of the transmission line TL 21   a  is connected to a ground via the second diode D 22  and the capacitor C 21   a,  and a first control terminal V 21   a  is connected to the ground side of the second diode D 22   a,  that is, the anode thereof, via the inductor L 21   a.    
     Furthermore, a second control terminal V 22   a  is connected to the second terminal P 22   a  side of the first diode D 21   a,  that is, the cathode thereof. Furthermore, a third terminal P 23   a  is provided between the transmission line TL 21   a  and the first diode D 21   a.    
     The high-frequency switch  33   a  includes first to third diodes D 31   a  to D 33   a,  a transmission line TL 31   a,  inductors L 31   a  and L 32   a,  and capacitors C 31   a  to C 33   a.  A first terminal P 31   a,  the transmission line TL 31   a,  the first diode D 31   a,  and a second terminal P 32   a  are connected in series. 
     Furthermore, the first terminal P 31   a  side of the transmission line TL 31   a  is connected to a ground via the second diode D 32   a  and the capacitor C 31   a,  and a first control terminal V 31   a  is connected to the ground side of the second diode D 32   a,  that is, the cathode thereof. 
     Furthermore, the second terminal P 32   a  side of the first diode D 31   a,  that is, the anode thereof, is connected to a ground via an inductor L 31   a  and the capacitor C 32   a,  and a second control terminal V 32   a  is connected to the ground side of the inductor L 31   a.  Furthermore, a third diode D 33   a  and a third terminal P 33   a  are connected in series to a connection point of the transmission line TL 31   a  and the first diode D 31   a.    
     In addition, the third terminal P 33   a  side of the third diode D 33   a,  that is, the anode thereof, is connected to a ground via an inductor L 32   a  and the capacitor C 33   a,  and a third control terminal V 33   a  is connected to the ground side of the inductor L 32   a.  Furthermore, a fourth terminal P 34   a  is provided between the transmission line TL 31   a  and the first diode D 31   a.    
     The second control terminals V 22   a  of the high-frequency switches  32   a  and the first control terminal V 31   a  of the high-frequency switch  33   a  are connected to each other, and a connection point thereof is connected to a common control terminal Vca via a resistor Ra. 
     In the above-described configuration, the third terminal P 23   a  of the high-frequency switch  32   a  is connected to the second terminal P 12  of the diplexer  11 , and the fourth terminal P 34   a  of the high-frequency switch  33   a  is connected to the third terminal P 13 . Although not shown, an antenna ANT is connected to the first terminal P 11  of the diplexer  11 . A receiving circuit Rx is connected to the first terminals P 21   a  and P 31   a  of the high-frequency switches  32   a  and  33   a.  A transmission circuit Tx is connected to the second terminals P 22   a  and P 32   a  of the high-frequency switches  32   a  and  33   a.  Both the receiving circuit Rx and the transmission circuit Tx are connected to the third terminal P 33   a  of the high-frequency switch  33   a.    
     The operation of the high-frequency module  30   a  having the circuit configuration of FIG. 9 will now be described. When the communication system on the high-frequency switch  32   a  side is to be selected, that is, when the high-frequency switch  32   a  is to be turned on, approximately 3 V is applied to the first control terminal V 21   a  of the high-frequency switch  32   a,  0 V is applied to the first control terminal V 31   a  of the high-frequency switch  33   a,  and approximately 0.5 V is applied to the common control terminal Vca. 
     Just then, a forward bias is applied to the first and second diodes D 21   a  and D 22   a  of the high-frequency switch  32   a  on the selected communication system side, and forward current flows from the first control terminal V 21   a  of the high-frequency switch  32   a  toward the common control terminal Vca. Then, since a voltage drop occurs in the resistor Ra due to this forward current, the amount of the voltage drop is applied as a reverse bias to the first to fourth diodes D 31   a  to D 34   a  of the high-frequency switch  33   a.  This reverse bias causes the off capacity of the first to fourth diodes D 31   a  to D 34   a  of the high-frequency switch  33   a  on the communication system side which is not selected to be stabilized and decreased. As a consequence, the distortion characteristics are greatly improved. 
     According to the high-frequency module of the above-described preferred embodiments, by applying a positive voltage to the control terminal of a high-frequency switch on the selected communication system side, a forward bias is applied to the first and second diodes of the high-frequency switch on the selected communication system side, and at the same time, a reverse bias is applied to the first and second diodes of a high-frequency switch on the communication system side which is not selected. As a result, a composite high-frequency switch having greatly improved distortion characteristics is obtained. Therefore, since a composite high-frequency switch having greatly improved distortion characteristics is used, the distortion characteristics of a high-frequency module are greatly improved. 
     Furthermore, since a diplexer and high-frequency switches which define a high-frequency module are integrally provided with a multilayer substrate having a plurality of sheet layers made of ceramics layered thereon, matching adjustment between the diplexer and the high-frequency switches is facilitated, and a matching circuit for performing matching adjustment between the diplexer and the high-frequency switches is not required. Therefore, the size of the high-frequency module is reduced. 
     In addition, the diplexer includes an inductor and a capacitor, the high-frequency switch includes a diode, an inductor, and a capacitor. The diplexer and the high-frequency switch are provided in or mounted on a multilayer substrate, and these are connected by connections provided in the multilayer substrate. Therefore, the high-frequency module is defined by one multilayer substrate, and a greatly reduced size is obtained. In addition, loss due to wiring between components is reduced, and as a result, loss of the entire high-frequency module is greatly reduced. 
     Furthermore, since the lengths of the stripline electrodes which define inductors are shortened due to a wavelength shortening effect, the insertion loss of these stripline electrodes is greatly reduced. As a result, a reduced size and a reduced loss of the high-frequency module are achieved. Therefore, a reduced size and greatly improved performance of a communication apparatus in which this high-frequency module is incorporated are achieved at the same time. 
     FIG. 10 is a block diagram showing a portion of the structure of a dual-band portable phone in which DCS of a 1.8-GHz band and GSM of a 900-MHz band are combined. A dual-band portable phone  40  includes an antenna  1  and a high-frequency module  10  (FIG.  1 ). 
     An antenna ANT is connected to a first terminal P 11  of the diplexer  11  which is a component of the high-frequency module  10 . A receiving circuit Rxg of the GSM system and a transmission circuit Txg of the GSM system are connected to the first and second terminals P 21  and P 22  of the high-frequency switch  12 . A receiving circuit Rxg of the DCS system and a transmission circuit Txg of the DCS system are connected to the first and second terminals P 31  and P 32  of the high-frequency switch  13 . 
     According to the above-described dual-band portable phone, since a composite high-frequency switch or a high-frequency module, having greatly improved distortion characteristics, is provided, the transmission and receiving characteristics of the dual-band portable phone are greatly improved. 
     In the above-described preferred embodiments, although a case in which approximately 0.5 V is applied to the common control terminal of a high-frequency module is described, 0 V may be applied. Since, in the case of 0 V, the common control terminal need not to extend as an external terminal on a multilayer substrate, the ease of use is greatly improved. 
     Furthermore, although a dual-band portable phone compatible with DCS and GSM is described, the communication apparatus is not limited to a combination of DCS and GSM. For example, the communication apparatus may be used in a combination of PCS (Personal Communications Service) and AMPS (Advanced Mobile Phone Service), a combination of DECT (Digital European Cordless Telecommunication) and GSM, or a combination of PHS (Personal Handyphone System) and PDC (Personal Digital Cellular), or may be used in a triple-band portable phone. 
     In addition, although the high-frequency module of FIG. 1 used in a dual-band portable phone is described, the same advantages are obtained when the high-frequency module of FIGS. 5 to  8  is used. 
     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 the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.