Abstract:
Briefly, an apparatus having a first capacitor-inductor-capacitor impedance converter operably coupled to a second capacitor-inductor-capacitor impedance converter. The first and second capacitor-inductor-capacitor impedance converter may combine a first and second signals of first and second outphasing power amplifiers and may provide a matched output impedance to a desired load.

Description:
BACKGROUND OF THE INVENTION 
   Outphasing transmitters may be used in stations of wireless communication systems such as, for example, base stations, mobile stations of cellular communication system and/or mobile unit and access point of wireless local area network (WLAN) and/or other types of wireless communication systems, if desired. 
   Outphasing techniques may combine two nonlinear radio frequency (RF) power amplifiers (PA&#39;s) into a linear power amplifier system. The two PA&#39;s may be driven with signals of different phases, and the phases may be controlled to provide an output signal with the desired amplitude. 
   The linear power amplifier system may include a combiner to combine the signal provided by the two nonlinear PA&#39;s. The combiner may include two transmission line couplers with shunt reactance. The power and efficiency of the outphasing transmitter may depend on the characteristics of the components and the architecture of the two transmission line couplers with shunt reactance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanied drawings in which: 
       FIG. 1  is a schematic illustration of a wireless communication system according to an exemplary embodiment of the present invention; 
       FIG. 2  is a block diagram of an outphasing amplifier according to an exemplary embodiment of the present invention; and 
       FIG. 3  is a schematic illustration of graphs helpful in demonstrating the efficiency of an outphasing amplifier according to an exemplary embodiment of the present invention. 
   

   It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
   Some portions of the detailed description, which follow, are presented in terms of algorithms and symbolic representations of operations on data bits or binary digital signals within a computer memory. These algorithmic descriptions and representations may be the techniques used by those skilled in the data processing arts to convey the substance of their work to others skilled in the art. 
   It should be understood that the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the circuits and techniques disclosed herein may be used in many apparatuses such as transmitters of a radio system. Transmitters intended to be included within the scope of the present invention include, by a way of example only, cellular radiotelephone transmitters, two-way radio transmitters, digital system transmitters, wireless local area network transmitters, wideband transmitters, ultra wideband transmitters, and the like. 
   Type of cellular radiotelephone transmitters intended to be within the scope of the present invention include, although not limited to, Code Division Multiple Access (CDMA), CDMA-2000 and wide band CDMA (WCDMA) cellular radiotelephone transmitters for receiving spread spectrum signals, transmitters for global system for mobile communication (GSM), transmitters for third generation cellular systems (3 G), orthogonal frequency division multiplexing (OFDM) transmitters and the like. 
   Turning first to  FIG. 1 , a schematic illustration of a wireless communication system  100  according to an exemplary embodiment of the present invention is shown. Although the scope of the present invention is not limited to this example, wireless communication system  100  may include at least one base station  110  and at least one mobile station  140 . In some embodiments of the invention base station  110  may include a transmitter  120  and mobile station  140  may include a transmitter  150 . At least one of transmitters  120  and  150  may be an outphasing transmitter with reactive termination. Reactive termination may be implemented, for example, in the form of a line coupler with shunt resistance, although the scope of the present invention is in no way limited to this respect. 
   Although the scope of the present invention is not limited in this respect, in some embodiments of the present invention, wireless communication system  100  may be a cellular communication system. Thus, base station  110  and mobile station  140  may be a base station and a mobile station of a cellular communication system. In other embodiments of the present invention, wireless communication system  100  may be a WLAN communication system Thus, base station  110  may be an access point (AP) and mobile station  140  may be a mobile unit such as, for example, a laptop computer, a tablet computer, a handheld device and the like. 
   Turning to  FIG. 2 , a block diagram of an outphasing transmitter  200  according to an exemplary embodiment of the present invention is shown. Although the scope of the present invention is not limited in this respect, outphasing transmitter  200  may include nonlinear PA&#39;s  210 ,  220 , a combiner  230 , impedance transformer  280 , a battery  285 , and an antenna  290 . In some embodiments of the invention, combiner  230  may include active devices, for example transistors (Q)  240 ,  245  and passive devices, for example, capacitors (C)  250 ,  255 , inductors (L)  260 ,  265 , and capacitor (C)  270 . 
   Although the scope of the present invention is not limited in this respect, types of antennas that may be used for antenna  290  may include an internal antenna, a dipole antenna, an omni-directional antenna, a monopole antenna, an end fed antenna, a circularly polarized antenna, a micro-strip antenna, a diversity antenna and the like. 
   Although the scope of the present invention is not limited in this respect, impedance transformation  280  may transform, from example, the antenna impedance and/or load impedance (Zload), for example, Zload=50 Ohm to intermediate impedance (Zintermidiate) for example, Zintermidiate=20 Ohm. In this exemplary embodiment, battery  285  may provide direct current (DC) feed to active devices  240 ,  245  through the impedance transformer  280 . 
   Although the scope of the present invention is not limited in this respect, combiner  230  may include two C-L-C PI (π) converters. The first π converter may include C  250  (C_A), L  260  (L_PI) and a portion of C  270  (C_PI). The second π converter may include C  255  (C_B), L  265  (L_PI) and a portion of C  270  (C_PI). The first and the second π converters may convert the impedance of Zintermidiate to the transistors  240 ,  245  impedance (Z PA ). In some embodiments of the invention C  270  may be expressed as C_PI=2*Cπ. The capacitance of C Π  and the inductance of inductor  260  or inductor  265  (L Π ) may be expressed calculated using the following equations: 
                     C   π     =       1       ω   CENTER     ·       2   ·     Z   INTER     ·     Z   PA             ⁢           ⁢     -     ⁢   π   ⁢     -     ⁢   section   ⁢           ⁢   capacitor       ;           (   1   )                   L   π     =           2   ·     Z   INTER     ·     Z   PA             ω   CENTER     ·       ⁢           ⁢     -     ⁢   π   ⁢     -     ⁢   section   ⁢           ⁢   inductor       ;           (   2   )               
wherein ω CENTER  may be the center frequency of the signal that received from PA&#39;s  210  and  220 .
 
   Although the scope of the present invention is not limited in this respect, in some alternate embodiments of the present invention, the first and the second π converters may include second harmonic traps (not shown), which may be used to remove the second harmonic of transistors  240 ,  245 , thus reducing the voltage peaking at the transistors. Although the scope of the present invention is not limited in this respect, other harmonic components may be filtered by π-section capacitor C_A (referenced  250 ) and/or capacitor C_B (referenced  255 ). 
   Although the scope of the present invention is not limited in this respect, shunt reactance may cause admittance shifts (±j*BS) wherein, BS is an amount of reactive admittance shift measured in mhos (e.g. 1/Ω). For example, positive admittance shift +j*BS may be accomplished by providing a shunt capacitor with the capacitance equal to BS/ω CENTER  Farads. In the same fashion, negative admittance shift, −j*BS, may be accomplished by providing a shunt inductor with an inductance equal to 1/(BS*ω CENTER ) Henry. In embodiments of the present invention, the admittance shifts may be added to capacitors C_A and C_B. These shifts may be defined in terms of K BS  which is the ratio of shift impedance to maximum power PA load impedance Z PA . K BS  may be expressed as follows: 
                   K   BS     =       1   /   BS       Z   PA               (   3   )               
wherein K BS  represents BS in terms of Z PA . For example, K BS  may be about 4 and Z PA  may be related to the optimum PA load at maximum output power.
 
   Although the scope of the present invention is not limited in this respect, capacitor C_A may be calculated according to the following equation: 
                 C_A   =       C   π     -     1     3   ⁢       ω   1   2     ·     L   RES           -         Z     PA   ⁢           ⁢   1       ·     K   BS         ω   1                 (   4   )               
wherein ω 1  is the fundamental harmonic of the input signal, L RES  may be the resonance of the second harmonic trap, and Z PA1  may be the output impedance of transistor  240 . In embodiments of the invention, capacitor C_A may be designed to have a positive value.
 
   Although the scope of the present invention is not limited in this respect, capacitor C_B may be calculated according to the following equation: 
                 C_B   =       C   π     -     1     3   ⁢       ω   1   2     ·     L   RES           -         Z     PA   ⁢           ⁢   2       ·     K   BS         ω   1                 (   5   )               
wherein Z PA2  is the output impedance of transistor  245 . In some embodiments of the invention, the term
 
           1     3   ⁢       ω   1   2     ·     L   RES               
in Equations (4) and (5) may represent compensation for the admittance shift of the second harmonic resonator, although the scope of the present invention is not limited in this respect. In some other embodiments of the present invention, the second harmonic may not be used. For those embodiments, the term
 
           1     3   ⁢       ω   1   2     ·     L   RES               
in Equations (4) and (5) may be omitted.
 
   Although the scope of the present invention is not limited in this respect, transistors  240  and  245  may include bipolar transistors, field effect transmitters (FET), metal oxide substrate field effect transistors (MOSFET), Heterojunction Bipolar Transistors (HBT), Complementary Metal Oxide Semiconductors (CMOS), High Electron Mobility Transistors (HEMT), Laterally Diffused Metal Oxide Semiconductors (LDMOS), tubes, or the like. In some embodiments of the invention, transistors  240  and  245  may be bipolar transistors and equivalent to a collector-emitter capacitance C CE , which may be expressed as 
             C   CE     =         Z   PA     ·     K   BS         ω   1             
and may be absorbed in capacitor C_B. An equivalent to a collector-emitter inductance L CE  may be expressed as
 
             L   CE     =       ω   1         Z   PA     ·     K   BS               
and may be absorbed in capacitor C_A in the form of equivalent negative capacitance
 
             -     C   CE       =     -           Z   PA     ·     K   BS         ω   1       .             
Although the scope of the present invention is not limited in this respect, the selection of K BS  and an intermediate transformation ratio may not result in the negative capacitor C_A value in Equation (4).
 
   Reference is now made to  FIG. 3 , which schematically illustrates graphs  310 ,  320  helpful in demonstrating the efficiency of an outphasing transmitter according to an exemplary embodiment of the present invention. Graph  310  and  320  depict the efficiency of transmitter  200  as a function of variations in the output power. Both graphs indicate an increase in efficiency when the output power is increased. The first graph  310  represents exemplary simulation results while the second graph represents results of actual measurements performed on a transmitter according to embodiments of the invention. It should be noted that graphs  310 ,  320  represent merely examples of efficiency curves and that actual efficiency curves of embodiments of the present invention may vary according to specific designs and implementations. It should be understood that the scope of the present invention is in no way limited to those examples. 
   While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications, substitutions, changes and equivalents as may fall within the true spirit of the invention.