Abstract:
The invention relates to an impedance-matching circuit for a multiband power amplifier, which uses an impedance-matching circuit with one-to-multiple path to efficiently transmit a radio frequency (RF) from an input port to the corresponding multi-ports without spurious effect. The impedance-matching circuit includes an input port for receiving a plurality of frequency band signals; a plurality of output ports for outputting the plurality of frequency band signals; and a plurality of frequency paths in which each path has an impedance matching network for matching an input port&#39;s impedance with its output port&#39;s impedance in a desired band; and a short circuit for filtering out RF signals in other remaining bands.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an impedance-matching circuit for a multiband power amplifier, using an impedance-matching circuit with one-to-multiple path to efficiently transmit a radio frequency (RF) from an input port to the corresponding multi-ports without spurious effect.  
           [0003]    2. Description of Related Art  
           [0004]    In wireless communication, multiband operation is widely employed. Typically, a power amplifier is used to operate in different bands, as shown in FIG. 1. In addition, the structure of one power amplifier per band can be used (not shown). As shown in FIG. 1, a dual-band GSM/DCS radiotelephone with a power amplifier  120  can use both the Global System for Mobile Communications (GSM), which operates at 900 MHz, and the Digital Communications System (DCS), which is similar to GSM except that it operates at 1800 or 1900 MHz. Such a structure can save costs but the circuitry is complicated. In this example, the amplifier  120  outputs the proper impedance, e.g. 50Ω, matched to the impedance of the multiband antenna through the matching circuit  140  so that the input signal  110  generates the proper operating power output  151 ,  152 . For this purpose, switches or resonators are added to the matching circuit. Thus, the circuit becomes more complicated.  
           [0005]    Accordingly, multiple output ports are further employed. For this example, the use of multiple output ports has the advantage of easily connecting different filters and transmitting/receiving signal from switch. To build up a multiband-matching network with multiple output ports, a separate, switched highpass- and lowpass-matching network is utilized with output from an amplifier. However, such a structure requires a switch with power handling capability and lower insertion loss, occupying more space and raising costs.  
           [0006]    Therefore, a solution is further subjected to the switch problem. Different operating band resonators are used in the multiband application of multiple transmitting paths (U.S. Pat. No. 5,969,582). However, a problem in this circuitry design is lack of flexibility.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, an object of the invention is to provide a multiband matching circuit with an input port and multiple output ports, which has the feature of circuitry flexibility to create the best impedance matching.  
           [0008]    The invention provides a multiband matching circuit with an input port and multiple output ports. The matching circuit includes a plurality of frequency paths; and a short circuit for filtering out the RF signals in other remaining bands. The reactance used in every path provides different bands with different amplitudes and signs (inductance or capacitance) so that the buffer resonator is generated with the preferred flexibility of multiband match. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention will become apparent by referring to the following detailed description of a preferred embodiment with reference to the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a schematic diagram illustrating a typically partial transmitter having a multiband matching circuit and a power amplifier;  
         [0011]    [0011]FIG. 2 is a block diagram illustrating a multiband impedance-matching circuit of the invention;  
         [0012]    [0012]FIG. 3( a ) is an embodiment of a dual band matching circuit of the invention;  
         [0013]    [0013]FIG. 3( b ) is an equivalent circuit of FIG. 3 a  operated in one band;  
         [0014]    [0014]FIG. 3( c ) is an equivalent circuit of FIG. 3 a  operated in the other band;  
         [0015]    [0015]FIG. 4 is another embodiment of a dual band matching circuit of the invention; and  
         [0016]    [0016]FIG. 5 is a further embodiment of a dual band matching circuit of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The following numbers denote the same elements throughout the description and drawings.  
         [0018]    [0018]FIG. 2 is a block diagram illustrating a multiband impedance-matching circuit of the invention. In FIG. 2, a dual band impedance-matching circuit is explained as an example for the sake of clarity. The circuit includes three parts: a buffer resonator ( 212 ,  222 ); a short circuit to ground ( 214 ,  224 ); and a matching network ( 218 ,  228 ). The buffer resonator provides different signs of reactance in different bands for multiple functions. The buffer resonator can be a resonant circuit, rather than the use of an open or short circuit in the undesired band as seen in the prior art, as a bridge or buffer between the short circuit to ground and the input port to prevent the desired signals from being reflected by the short circuit to ground. The short circuit to ground is a trap circuit used to filter out radio frequency (RF) signals in the undesired band and can be implemented by a series resonant circuit to ground, a quarter wavelength open stub or a half wavelength short stub or others. The matching network is used to transfer RF signals in the desired band and can be an L type, pi type, T type, ladder type network and others.  
         [0019]    As shown in FIG. 2, the signal  130  from the power amplifier of FIG. 1 is separated into a first band path  220  and a second band path  210 . The first band path  220  passes through a first buffer resonator  222 , a first short circuit to ground  224  at the frequency f 1 , and a first matching network  228  at f 2 , to output a first operating band  152 . Likewise, the second band path  210  passes through a second buffer resonator  212 , a second short circuit to ground  214  at f 2 , and a second matching network  218  at f 1 , to output a second operating band  151 . Examples of the first and second operating band  152 ,  151  are a dual band radiotelephone with the operating band 900/1800 MHz for Taiwan and the 900/1900 MHz for North America. The following circuits are embodied in detail according to the invention.  
         [0020]    [First Embodiment] 
         [0021]    [0021]FIG. 3( a ) shows one embodiment of the present invention for impedance matching in a dual-band power amplifier. The dual-band matching circuit  300  has a first path  310  to transfer RF signals in f 1  band to an output port  151  and a second path  320  to transfer RF signals in f 2  band to an output port  152 . Frequency f 1  is lower than f 2 .  
         [0022]    Path  310  includes a parallel resonant circuit  311  composed of inductor L 1  and capacitor C 1 , a series resonant circuit  322  composed of inductor L 2  and capacitor C 2 , and a matching network including a series inductor L 5  and a shunt capacitor C 5 . Resonant circuit  322  in path  310  resonates at f 2  band and provides a bandstop notch at a predetermined frequency of f 2  band. Appropriate notch bandwidth should be applied according to the system specification. This series resonant circuit  322  provides a high reflection to f 2  band and hence prevents f 2  band signal coupled to output port  151 . Furthermore, it also isolates the load connected at port  151  from path  320  such that the impedance presented in input port  130  at f 2  band is not affected by the load at port  151 . On the other hand, resonant circuit  322  exhibits capacitance in f 1  band, which is then viewed as a matching element. In resonant circuit  311 , inductor L 1  and capacitor C 1  are chosen such that circuit  311  resonates between f 1  and f 2  and consequently presents inductance in f 1  band and capacitance in f 2  band.  
         [0023]    Path  320  includes a series resonant circuit  333  composed of inductor L 3  and capacitor C 3 , a series resonant circuit  344  including inductor L 4  and capacitor C 4 , and a matching network  399  including a series inductor L 6  and two shunt capacitors C 6  and C 7 . Similarly to path  310 , resonant circuit  344  in path  320  resonates in f 1  band and provides a bandstop notch at a predetermined frequency of f 1  band, which provides a high reflection to fband and hence prevents f 1  band signal coupled to output port  152 . Furthermore, it also isolates the load connected at port  152  from path  310  such that the impedance presented to input port  130  in f 1  band is not affected by the load at port  152 . Meanwhile, resonant circuit  344  exhibits inductance in f 2  band, which is then viewed as a matching element. In resonant circuit  333 , inductor L 3  and capacitor C 3  are selected such that circuit  333  resonates between f 1  and f 2  and consequently presents capacitance in f 1  band and inductance in f 2  band.  
         [0024]    When a signal in f 1  band is input to port  130  of FIG. 3( a ), the equivalent circuit is shown in FIG. 3( b ). The buffer-resonator  311  in path  310  formed by inductor L 1  and capacitor C 1  functions as a series inductor L 11 . The series resonant circuit  322  formed by inductor L 2  and capacitor C 2  functions as a shunt capacitor C 22 . In path  320 , the series resonant circuit  344  formed by inductor L 4  and capacitor C 4  resonates as a short circuit to ground and prevents f 1  signal coupled to output port  152 . Therefore, the buffer-resonator circuit  333  formed by inductor L 3  and capacitor C 3  functions as a shunt capacitor C 33 . It is noted that f 1  signal will short-circuit at the input port  130  in the absence of buffer-resonator circuit  333 . As a result, the series inductor L 11  and shunt capacitors C 22 , C 33  combined with network L 5  and CS form a low pass matching network to provide a 50 ohm impedance match to the desired load at port  130  in f 1  band.  
         [0025]    When a signal in f 2  band is input to port  130  of FIG. 3( a ), the equivalent circuit is shown in FIG. 3( c ). In path  320 , the buffer-resonator  333  formed by inductor L 3  and capacitor C 3  functions as a series inductor L 33 , while the series resonant circuit  344  formed by inductor L 4  and capacitor C 4  functions as a shunt inductor L 44 . In path  310 , the series resonant circuit  322  formed by inductor L 4  and capacitor C 4  resonates as a short circuit to ground and prevents f 2  signal coupled to output port  151 . Therefore, the buffer-resonator circuit  311  formed by inductor L 1  and capacitor C 1  functions as a shunt capacitor C 11 . It is obvious that f 2  signal would be short-circuited at the input port  130  under the absence of buffer-resonator circuit  311 . As a result, the series inductor L 33  and shunt capacitors C 11  combined with network L 44 , C 6 , L 6  and C 7  form a low pass matching network to provide a 50 ohm impedance match to the desired load at port  130  in f 2  band.  
         [0026]    The buffer-resonator circuit  311  in path  310  formed by inductor L 1  and capacitor C 1  is applied to resonate between f 1  and f 2  operating bands. Therefore, circuit  311  functions like a series inductor L 11  in f 1  band as shown in FIG. 3( b ), and like a series capacitor C 11  in f 2  band as shown in FIG. 3( c ) . Similarly, the buffer-resonator circuit  333  in path  320  formed by inductor L 3  and capacitor C 3  is also designed to resonate between two operating bands f 1  and f 2 . Circuit  333  functions like a series capacitor C 33  in f 1  band as shown in FIG. 3( b ), and like a series inductor L 33  in f 2  band as shown in FIG. 3( c ). Appropriate choice of C 1 , L 1 , C 3 , and L 3  can provide desired values of L 11 , C 11 , L 33 , and C 33  for optimum impedance matching. The degree of design flexibility is thus increased and the optimization of circuit performance can be achieved more easily. A dual-band GSM (900 MHz)/DCS (1800 MHz) system is described in the following example.  
         [0027]    The impedance at port  130  seen by the power amplifier is 4.2 ohm, while the load impedance at port  151  and port  152  is 50 ohm.  
                                               L1 = 0.63 nH;   C1 = 21 pF;   L2 = 0.7 nH;   C2 = 12 pF;       L3 = 1.6 nH;   C3 = 9 pF;   L4 = 15 nH ;   C4 = 2 pF;       L5 = 3.84 nH;   C5 = 5.8 pF;   L6 = 1.8 nH ;   C6 = 2.63 pF;       C7 = 7.8 pF;                  
 
         [0028]    The simulation result is shown in Appendix A, which presents low insertion loss and good performance in both GSM and DCS bands.  
         [0029]    [Second Embodiment] 
         [0030]    [0030]FIG. 4 illustrates the second embodiment of the present invention for impedance matching in a dual-band power amplifier. The dual-band matching circuit  400  has a first path  410  to transfer RF signals in f 1  band to an output port  151  and a second path  420  to transfer RF signals in f 2  band to an output port  152 . Frequency f 1  is lower than f 2 . Paths  410  and  420  are substantially similar to paths  310  and  320  shown in FIG. 3( a ), respectively, except that resonant LC circuit  322  is replaced with a shunt open stub T 1  with a quarter wavelength at f 2  and resonant LC circuit  344  is replaced with a shunt open stub T 2  with a quarter wavelength at f 1 . An additional capacitor C 8  in matching network  488  is used to facilitate impedance matching. The shunt open stub T 1  can be viewed as a series resonator connected to ground, which resonates in f 2  band and exhibits capacitance in f 1  band. The shunt open stub T 2  can also be viewed as a series resonator connected to ground, which resonates in f 1  band and may exhibit inductance or capacitance in f 2  band. The behavior of both the parallel resonant circuit composed of inductor L 1  and capacitor C 1 , and the series resonant circuit composed of inductor L 3  and capacitor C 3 , is identical to the previous descriptions associated with FIG. 3( a ).  
         [0031]    An example based on FIG. 4 for a dual-band GSM 900 MHz/DCS 1800 MHz system follows. The impedance at port  130  seen by the power amplifier is 4.2 ohm, while the load impedance at port  151  and port  152  is 50 ohm. T 1  and T 2  are realized using microstrip line structures.  
                                                               L1 = 0.61 nH;   C1 = 21 pF;   L3 = 1.7 nH;   C3 = 8 pF           L5 = 4.19 nH;   C5 = 6 pF;   C8 = 10 pF;           L6 = 1.8 nH ;   C6 = 2.63 pF;   C7 = 7.8 pF;                      
 
         [0032]    Substrate thickness of T 1  and T 2 =14.7 mil;  
         [0033]    Relative dielectric constant of substrate=4.6;  
         [0034]    T 1  width=T 2  width=20 mil; T 1  length=920 mil;  
         [0035]    T 2  length=1780 mil;  
         [0036]    The simulation results are plotted in Appendix B, which presents low insertion loss and good performance to meet the requirement of GSM/DCS dual-band applications.  
         [0037]    [Third Embodiment] 
         [0038]    [0038]FIG. 5 illustrates the second embodiment of the present invention for impedance matching in a dual-band power amplifier. The dual-band matching circuit  500  has a first path  510  to transfer RF signals in f 1  band to an output port  151  and a second path  520  to transfer RF signals in f 2  band to an output port  152 . Frequency f 1  is lower than f 2 . Paths  510  and  520  are substantially similar to paths  410  and  420  as shown in FIG. 4, respectively, except that open stub  422  is replaced with a shunt short stub of a half wavelength at f 2  and open stub  444  is replaced with a shunt short stub of a half wavelength at f 1 . The shunt short stub T 1  can be viewed as a series resonator connected to ground, which resonates in f 2  band and may exhibit capacitance or inductance in f 1  band. The shunt short stub T 2  can also be viewed as a series resonator connected to ground, which resonates in f 1  band and may exhibit inductance or capacitance in f 2  band. It should be noted that, in applying the dual-band structure of FIG. 5, one must avoid a situation in which one operating frequency is a plurality of another operating frequency.  
         [0039]    Although the present invention has been described in its preferred embodiment, it is not intended to limit the invention to the precise embodiment disclosed herein. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.