Abstract:
The present invention relates to balanced power amplifier network in combination with outphasing techniques such as Chireix. The object of the present invention is to provide a solution to the problem to combine balanced amplifiers like the current mode class D (CMCD) or class E/F with a LINC network. The main problem is that some power amplifiers have balanced output and the LINC network is single-ended so that a high power low loss transformer that works at several impedance levels is needed, which is hard to realize at cellular frequencies.

Description:
TECHNICAL FIELD 
   The present invention relates to a balanced amplifier network in combination with outphasing technique. 
   BACKGROUND OF THE INVENTION 
   In radio transmitters for broadcast, cellular and satellite systems the power amplifier in the transmitter has to be very linear in addition to being able to simultaneously amplify many radio channels (i.e. frequencies) spread across a wide bandwidth. High linearity is required since nonlinear amplifiers would cause leakage of interfering signal energy between channels and distortion within each channel. 
   In radio transmitter stations for cellular systems, amplifiers in class A and B have been suggested for use in combination with LINC (LINC, Linear Amplification using Nonlinear Components) or Chireix outphasing methods providing high linearity and efficiency, and wide bandwidth. 
   The outphasing method, which will be described in more detail in  FIG. 1 , resolves an envelope-modulated bandpass waveform s in  in a signal component separator into two out-phased constant envelope signals s 1  and s 2 , which are applied to power amplifiers. The outputs of the power amplifiers are combined in a hybrid arrangement recovering the envelope-modulated waveform. The output amplitude of the amplified outsignal s out  is a result of the phase shift between the signals s 1  and s 2 . When the signals are in phase amplitude maximum is achieved and when in anti-phase a minimum amplitude is achieved. The hybrid is constructed in order for the amplifier to see an impedance load. Hence, the efficiency ratio will be reciprocally proportional to the ratio between peak power and mean power. By replacing the impedance load by a compensating reactance network, known as the Chireix method, the region of high efficiency is extended to include lower output power levels. 
   LINC and Chireix networks are sensitive systems that fit well with unbalanced amplifier like unbalanced class A, B, C, E and F amplifiers. 
   Documents WO2004/023647 and WO2004/057755 describe composite amplifier structures comprising several Chireix pairs of unbalanced power amplifiers for use in radio terminals such as mobile radio terminals and base stations. 
   A current-mode class-D power amplifier achieving high efficiency at radio frequencies is described in “Current-Mode Class-D Power Amplifiers for High-Efficiency RF Applications”, IEEE Transactions on Microwave Theory and Techniques, vol. 49, no 12, December 2001, pp. 2480-2485. However, due to new progress in semiconductor electronics, especially production methods of integrated circuits in Gallium Nitride (GaN) techniques, it has been interesting to use balanced class B, E/F and current mode class D (CMCD) amplifiers instead. Said amplifiers are non-linear, but provide high efficiency. 
   No specific solution exists as to combine balanced amplifiers like the CMCD or class E/F with a LINC network. The main problem is that the CMCD amplifier and class E/F have a balanced output and the LINC network is single-ended so that a high power low loss transformer that works at several impedance levels is needed, which is hard to realize at cellular frequencies. 
   BRIEF DESCRIPTION OF THE INVENTION 
   An object of the present invention is to provide a balanced power amplifier network in combination with a LINC or Chireix outphasing technique. 
   One advantage with the present invention is that the use of said amplifiers, which are balanced, in combination with a LINC or Chireix outphasing method results in high linearity and wide bandwidth. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will in the following be described in more detail with reference to enclosed drawings wherein: 
       FIG. 1  is a block diagram of a typical prior art Chireix outphasing amplifying system: 
       FIG. 2  shows a block diagram of a preferred embodiment of a Chireix outphasing amplifying system with balanced amplifiers and a balanced lossless output combination network; 
       FIG. 3  shows a block diagram of another preferred embodiment of a Chireix outphasing amplifying system with balanced amplifiers; 
       FIG. 4  shows a Lumped-element balun device; and 
       FIG. 5  shows a two cascaded Lumped-elements balun arrangement. 
       FIG. 6  is a schematic illustration of a WCDMA network architecture comprising base stations, node B, for mobile radio telecommunications. 
       FIG. 7  illustrates schematically a mobile radio terminal for mobile radio telecommunications. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a block diagram of a typical prior art outphasing amplifier system  100 , i.e. a Chireix amplifier. A signal s in  is connected via an input  105  to a signal component separator (SCS)  110 , which comprises two outputs  115  and  116 . The signal strength of s in  will be represented as a phase difference between the two output signals s 1  and s 2  on the outputs  115  and  116 , respectively. Each output is connected to a power amplifier  120  and  130 , respectively, as shown in the figure. The amplifiers  120 ,  130 , are typical of any type of unbalanced amplifiers of class B. Hence, two out-phased constant envelope signals s 1  and s 2  are resolved from an envelope-modulated bandpass waveform s in  in a signal component separator  110 . The outputs of the power amplifiers are combined to form an amplified linear signal in a Chireix type output network. The phase difference of these constant-amplitude out-phased signals are determined by the SCS  110  so that the result from their vector-summation yields the desired amplitude of the output signal s out . 
   As shown in  FIG. 1 , the amplifiers  120 ,  130  are connected to an output combination network  150 , including two transmission lines  140 ,  145  (one line for each amplifier) with electrical length λ/4 and impedance R, where λ denotes the wavelength of the center frequency of the frequency band at which the amplifier is operated. R is the chosen output impedance for the amplifier providing maximum power efficiency. In addition, the network comprises two compensating reactances, a capacitor C  125  and an inductor L  135 , which are used to extend the region of high efficiency to include lower output levels. Said transmission lines are connected in a connection point  160  from which an output  165  conducts the resulting outsignal s out  to a load R L    170 . The transformed antenna impedance R L  ( 170 ) equals the parallel connection of the optimal loads of all amplifiers, i.e. R L =R/2. 
   An appropriate combination network for the Chireix amplifying system not only reinserts the amplitude modulation to the signal, it also provides a dynamic adjustment of the impedance presented to each amplifier (out-phasing). This out-phasing adjustment of the impedances is such that the DC current through each active device decreases as the combined output amplitude decreases, thereby maintaining high efficiency. 
   An advantage of the Chireix amplifying system is the ability to change the efficiency curve to suit different peak-to-average power ratios, by changing the size of the reactances. 
   As mention new semiconductor techniques provide amplifiers of class B, E/F and CMCD with high efficiency at radio frequencies. The problem with class B, E/F and CMCD amplifiers is that they are balanced. 
     FIG. 2  shows a block diagram of a preferred embodiment of a Chireix outphasing amplifying system  200  with balanced amplifiers  220 ,  230  and a balanced lossless output combination network  250 . A signal s in  is connected via an input  205  to a signal component separator  210 , which comprises four outputs  215   a ,  215   b ,  216   a  and  216   b . The signal strength of s in  will be represented as a phase difference between the four output signals s 1a , s 1b , s 2a  and s 2b  on the outputs  215   a ,  215   b ,  216   a  and  216   b , respectively. Outputs  215   a  and  215   b  are connected to a first power amplifier  220  and outputs  216   a  and  216   b  are connected to a second power amplifier  230 , as shown in the figure. The amplifiers  220 ,  230 , could be of any type of balanced amplifiers of class B, E/F and CMCD. Hence, four out-phased constant envelope signals s 1a , s 1b , s 2a , s 2b  are resolved from an envelope-modulated bandpass waveform s in  in a signal component separator  110 . The first and second outputs  225   a ,  225   b  of the first power amplifier and the first and second outputs  235   a ,  235   b  of the second power amplifier are combined to form an amplified linear signal in a balanced lossless output combination network  250  without distorting the systems ability to change the efficiency curve to suit different peak-to-average power ratios. 
   As shown in  FIG. 2 , the amplifiers  220 ,  230  are connected to an output combination network  250 , including four transmission lines  240   a ,  240   b ,  245   a  and  245   b  (two lines for each amplifier) with electrical length λ/4 and impedance R/2, where λ denotes the wavelength of the center frequency of the frequency band at which the amplifier is operated. R is the chosen output impedance for the amplifier providing maximum power efficiency. In addition, the network comprises two compensating reactances, a capacitor C  227  and an inductor L  237 , which are used to extend the region of high efficiency to include lower output levels. The first transmission line  240   a  and the third transmission line  245   a  are connected to a first connection point  260   a , whereto the processed signal s 3a  is outputted. Similarly, the second transmission line  240   b  and fourth transmission line  245   b  are connected to a second connection point  260   b , whereto the processed signal s 3b  is outputted. The connection points  260   a  and  260   b  are connected via a first input  262   a  and a second input  262   b  to a balun arrangement  280  with two input terminals. The balun arrangement converts from balanced to unbalanced operation and outputs via conductor  265  the resulting outsignal s out , where s out  equals s 3a -s 3b , to a load R L    270 . The transformed antenna impedance R L  equals the parallel connection of the optimal loads of all amplifiers, i.e. R L =R/2. 
   Another way of implementing the output networks of the Chireix outphasing network are by using transmission lines only. In such a distributed implementation the reactance and quarter wave combination is replaced by shortened and lengthened transmission lines. Instead of the capacitor ( 227 ) a lengthened transmission line could be used and instead of the inductor ( 237 ) a shortened transmission line could be used. 
   The connection points  260   a  and  260   b  situated after the four transmission lines provide the possibility to connect a balun arrangement at the output load. The configuration enables the use of standard balun arrangements like a transformer coupling such as the hybrid ring when converting from balanced to unbalanced. i.e. single-ended, operation. A narrow-band balun device could be used. It doesn&#39;t have any impact on the amplifiers load. Connecting the balun arrangement at the connection points  260   a  and  260   b  implies that the systems ability to change the efficiency curve to suit different peak-to-average power ratios is preserved, providing high linearity and efficiency, and wide bandwidth. 
     FIG. 3  shows a block diagram of another preferred embodiment of a Chireix outphasing amplifying system  300  with balanced power amplifiers  320 ,  330  and a balanced lossless output combination network  350 . A signal s in  is connected via an input  305  to a signal component separator  310 , which comprises four outputs  315   a ,  315   b ,  316   a  and  316   b . The signal strength of s in  will be represented as a phase difference between the four output signals s 1a , s 1b , s 2a  and s 2b  on the outputs  315   a ,  315   b ,  316   a  and  316   b , respectively. Outputs  315   a  and  315   b  are connected to a first power amplifier  320  and outputs  316   a  and  316   b  are connected to a second power amplifier  330 , as shown in the figure. The amplifiers  320 ,  330 , could be of any type of balanced amplifiers of class B, E/F and CMCD. Hence, four out-phased constant envelope signals s 1a , s 1b , s 2a , s 2b  are resolved from an envelope-modulated bandpass waveform s in  in a signal component separator  310 . The first output  325   a  and the second output  325   b  of the first power amplifier  320  and the first output  335   a  and the second output  335   b  of the second power amplifier are combined to form an amplified linear signal in a balanced lossless output combination network  350  without distorting the systems ability to change the efficiency curve to suit different peak-to-average power ratios. 
   As shown in  FIG. 3 , the amplifiers  320 ,  330  are connected to an output combination network  350 , including two balun arrangements  380   a  and  380   b , which will be further described in connection to  FIGS. 4 and 5 , and two transmission lines  340 ,  345  with electrical length λ/4 and impedance R, where λ denotes the wavelength of the center frequency of the frequency band at which the amplifier is operated. Furthermore, R is the chosen output impedance for the amplifier providing maximum power efficiency. Said transmission lines are connected in a connection point  360  from which an output  365  conducts the resulting outsignal s out  to a load R L    370 . 
   Hereinafter, the principle of the function of the balun arrangement  380  will be described in more detail with reference to  FIGS. 4 and 5 . 
   A balun device is designed to have a precise 180-degree phase shift, with minimum loss and equal balanced impedances.  FIG. 4  shows a Lumped-element balun device  40 . The balun device  40  is essentially a bridge and consists of four elements, of which two are capacitors −jX B4  ( 42 ,  44 ) and two are inductors jX B4  ( 46 ,  48 ), constructed in a lattice network. If the impedance of each element is chosen to be of the same size, the impedance Z in  at the input ( 41 ) of the balun device will equal the inverted impedance Z at the output ( 49 ) of the balun device. The resulting balun device impedance at the balanced input ( 41 ) Z in  can be expressed by
 
 Z   in   =X   B4   2   /Z  
 
where X B4  is the impedance of each element in the balun device  40 . Moreover, a second Lumped-element balun device  50 , as shown in  FIG. 5 , is connected to the input of the first Lumped-element balun device  40  forming a two cascaded Lumped-elements balun arrangement  500 . The balun device  50  is also essentially a bridge and consists of four elements, of which two are capacitors −jX B5  ( 52 ,  54 ) and two are inductors jX B5  ( 56 ,  58 ), constructed in a lattice network. If the impedances of each element are chosen to be of the same size, the impedance Z incase  at the input ( 51 ) of the balun arrangement will equal the inverted impedance Z in  at the input ( 41 ) of the balun device  40  as the impedance Z in  of the first balun device  40  is inverted by the same principle as described above with reference to  FIG. 4  by said second Lumped-element balun device  50 . The resulting balun arrangement impedance at the balanced input ( 51 ) Z incase  can be expressed by
 
 Z   incase   =X   B5   2   /X   B4   2   ×Z  
 
where X B5  is the impedance of each element in the second balun device  50 . Thus, if X B5  equals X B4  the cascaded impedance equals the load Z at output of the first balun device  40 .
 
Z incase =Z
 
   Furthermore, the power amplifiers  320  and  330 , as shown in  FIG. 3 , are connected to two balun arrangement  380   a  and  380   b  with the same function and design as the described balun arrangement  500 . That is, the first output ( 325   a ) of the first power amplifier ( 320 ) is connected to a first input of a first balun arrangement ( 380   a ) and the second output ( 325   b ) of the first power amplifier ( 320 ) is connected to a second input of the first balun arrangement ( 380   a ). The first output ( 335   a ) of the second power amplifier ( 330 ) is connected to a first input of the second balun arrangement ( 380   b ), and the second output ( 335   b ) of the second power amplifier ( 330 ) is connected to a second input of the second balun arrangement ( 380   b ). Further, a single-ended output of the first balun arrangement ( 380   a ) is connected to the first transmission line ( 340 ) and a second single-ended output of the second balun arrangement ( 380   b ) is connected to the second transmission line ( 345 ). In addition, the network comprises two compensating reactances, a capacitor C  327  and an inductor L  337 , which are used to extend the region of high efficiency to include lower output levels. Further, the first and the second transmission lines are connected to a connection point ( 360 ) to which the output load R L  ( 370 ) is connected. 
   The transformed antenna impedance R L  equals the parallel connection of the optimal loads of all amplifiers, i.e. R L =R/2. In accordance with the described function of the balun arrangement  500  the amplifiers  320 ,  330  will see the impedance R as the balun arrangement could be arranged to have no impact on the impedance at its output. Thus, the systems ability to change the efficiency curve to suit different peak-to-average power ratios is preserved, providing high linearity and efficiency, and wide bandwidth. 
   This embodiment is advantageous when integrated on an ASIC. 
   Similarly as in previous described embodiment, a distributed implementation could be used, i.e. the reactance and quarter wave combination is replaced by shortened and lengthened transmission lines. Instead of the capacitor ( 327 ) a lengthened transmission line could be used and instead of the inductor ( 337 ) a shortened transmission line could be used. 
   The present invention also relates to a radio terminal comprising the composite power amplifier system for amplifying a signal to be transmitted via an aerial or antenna device. Said radio terminal may be a mobile radio terminal handset, a base station, or a satellite comprising transceiver or transmitter device(-s)/arrangement for transmitting the power amplified signal over the air interface to a receiving device. In the following  FIGS. 6 and 7 , different embodiments of radio terminals comprising the invented power amplifier system are schematically illustrated and described. 
     FIG. 6  is a schematic illustration of a WCDMA network architecture  600  comprising base stations  620 , node B. The network comprises a Core network  605  to which a number of Radio Network Controllers  610  (RNC) are connected and other networks  608 , e.g. Public Switched Telephony Network (PSTN), Public data Networks, Internet, Integrated Services Digital Network (ISDSN), other Public Land Mobile Networks (PLMN). Satellite telecommunication systems, etc. The RNC  610  controls at least one dedicated node B  620  (Base Station in GSM networks). The RNC  620  controls and handles the uplink and downlink communications over the air interface between a node B  620  (connected to the RNC) and subscriber units, such as radio handset terminals  630 . A node B comprises, among other units 8not shown), Base Band units  622 , Radio Units  624 . Filter Units  626  and an antenna system  628 . The Base Band unit  622  transforms digital messages into I and Q vectors, which are transferred to the Radio Unit  624 . In the Radio Unit, said vectors are modulated on carriers resulting in signals s in  (see  FIGS. 2-6 ). Before transmission, the signals s in  are power amplified. Therefore, the Radio Unit is equipped with at least one composite power amplifier system  200  according to the invention. The amplifier system  200  power amplifies s in  and provides an output signal s out  to the antenna system  628  for transmission over a radio channel to a subscriber unit  630 , e.g. a radio terminal handset. 
   It is also possible to use the composite power amplifier system  200  according to the invention in a satellite  650  for mobile radio telecommunications. The satellite will then operate as a relay station comprising transmitters and/or transceivers in the Radio Units  624  and antenna systems  628  for handling the communication with a control station in a Satellite telecommunication systems  608  and satellite radio terminals  630  located on the earth. 
     FIG. 7  illustrates schematically a mobile radio terminal, also denoted handset,  700  for mobile radio telecommunications. The terminal comprises a microphone  705  for transforming voice and audio to an electrical signal M(t). Said signal M(t) is processed before being modulated and power amplified by a first signal processing block  710  comprising A/D-converter unit, Speech and channel coder units, and digital formatting devices for arranging the signal into a suitable transmitting format for mobile radio telecommunication systems like GSM/(EDGE) GPRS, UMTS (WCDMA), och CDMA-systems. In the transmitter  715 , or Radio Unit  720 , the processed signal s in , which is generated by the block  710 , is modulated in a modulator (not shown) and thereafter power amplified by the composite power amplifier system  200  according to the present invention, generating an output signal s out , which is transmitted via the aerial/antenna  728  to a base station  620  in a telecommunication network  600  (see  FIG. 6 ). Preferably, a filter arrangement (not shown) is also inserted between the amplifier system and the antenna. 
   Said terminal  700  also comprises at least one controller  730 , such as a microprocessor or central processing unit, for controlling the units of the terminal by using stored, readable and executable software. 
   Further, the terminal  700  comprises a receiving unit  735  connected to the antenna  728  for receiving transmitted signals and transform them in a second signal processing block  737 . If the received signal contains voice, the voice signal is decoded by decoder units and converted by a D/A-converter to an audio signal before being transformed to sound by a loudspeaker  707 . The terminal comprises a keyboard  745 , a display  747  and a Man-Machine-Interface block (MMI)  740  that allows a user to interactively control the terminal, write and read text messages, initiate telephone calls, etc. 
   As obvious to person skilled in the art, the radio terminal may also comprise a number of other blocks and units that provides other services and functions, e.g. Short Message Service (SMS), Multi Media Service (MMS), etc. Said units, which have been chosen to not be illustrated in  FIG. 7  for reasons of simplifying the presentation of a terminal according to the present invention, generate digital information signal that are possible to power amplify using the invented composite power amplifier system. 
   The present embodiments have been described as Chireix outphasing circuits or networks. However, any other LINC technique may be applicable as realized by a person skilled in the art. 
   The present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein: rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention as defined by the enclosed set of claims.