Abstract:
A circuit for amplifying the power of signal, the circuit comprising a power amplifier, a transformer and a load; wherein the transformer comprises a primary inductor and a secondary inductor, the first inductor being coupled to, and capable of being driven by, the power amplifier, and the secondary inductor coupled to, and capable of driving, the load; wherein a first one of the primary and secondary inductors is a variable inductor whose inductance is variable responsive to a control input in order to change the output power of the amplifier.

Description:
FIELD OF THE INVENTION 
     The invention relates to the amplification of the power of electrical signals. Circuits for amplifying signal power are included in transmitters for boosting the power of signals that are to be transmitted wirelessly. 
     BACKGROUND 
       FIG. 1  is a schematic illustration of a power amplification circuit  10  in a transmitter. As shown, the circuit  10  comprises a power amplifier  12 , a balun  14  and an antenna  16 . 
     The power supply connections of the amplifier  12  are indicated  18  and  20 . As shown, the amplifier  18  has a power supply voltage of V dd . The signal that is to be transmitted from the antenna  16  is presented as a differential signal across the input terminals  22  and  24  of the amplifier  12 . The amplifier  12  amplifies that differential signal and outputs it across lines  26  and  28 , which form the input to the balun  14 . 
     A balun is a transformer that is designed to convert a differential signal into a single-ended signal (or vice versa in other scenarios). The balun  14  comprises a primary inductor  30  across which is applied the differential signal that is output by the amplifier  12 . The balun  14  also comprises a secondary inductor  32  that is linked to the primary inductor  30  by a shared magnetic flux, indicated by the dotted arrows, such that a voltage is induced across the secondary inductor  32 . The voltage that is developed across the secondary inductor  32  is the output signal of the balun  14  and is applied across the antenna  16  by means of lines  34  and  36 . The voltage that the balun  14  produces across its output terminals  34  and  36  is the voltage that is applied across its inputs  26  and  28  scaled up by a factor of n. That is to say, the balun  14  has a transformation ratio of 1:n. Where the primary inductor has an inductance L 1 , the secondary inductor has and inductance L 2  and the primary and secondary inductors  30  and  32  have a coupling factor of k, then n in the transformation ratio is given by: 
     
       
         
           
             
               
                 
                   n 
                   = 
                   
                     
                       1 
                       k 
                     
                     ⁢ 
                     
                       
                         
                           L 
                           2 
                         
                         
                           L 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
     If the impedance, from the point of view of the output of the amplifier  12 , the effective impedance of the balun  14  and the antenna  16  is X E , then it can be shown that the output power P OUT  of the amplifier  12  is: 
     
       
         
           
             
               
                 
                   
                     P 
                     OUT 
                   
                   = 
                   
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         V 
                         dd 
                         2 
                       
                     
                     
                       ⁡ 
                       
                         [ 
                         
                           X 
                           E 
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
     
     In equation 2,            [X E ] is the real part of X E . (Classically, P=V 2 /R, but here the signal is differential so V=2V dd  and R=2         [X E ].)
     Typically, it is required that the output power of a power amplifier in a transmit chain is adjustable. It will be apparent from equation 2 that this adjustability can be achieved in the case of amplifier  12  by altering V dd . In the case where a conventional regulator is used to derive V dd  from a voltage V BAT  supplied by a battery, the regulator could be controlled to adjust V dd  in a manner that provides the desired control over P OUT . However, associated with the use of a regulator, there would be a power loss P LOSS  of:
 
 P   LOSS   =I   PA ( V   BAT   −V   DD )  [3]
 
     In equation 3, I PA  is the current consumed by the power amplifier  12 . 
     Where power efficiency is a concern, a switched mode power supply (SMPS) could be used instead of a regulator. That is to say, rather than make the magnitude of V dd  continuously variable, V dd  can be periodically switched from a constant value to zero, for adjustable interludes. However, a SMPS will consume an undesirably large amount of space when implemented on a silicon chip and would still require a large off-chip inductor. 
     SUMMARY OF THE INVENTION 
     According to one aspect, an embodiment of the invention provides a circuit for amplifying the power of signal, the circuit comprising a power amplifier, a transformer and a load. The transformer comprises a primary inductor and a secondary inductor, the first inductor being coupled to, and capable of being driven by, the power amplifier, and the secondary inductor coupled to, and capable of driving, the load. A first one of the primary and secondary inductors is a variable inductor whose inductance is variable responsive to a control input in order to change the output power of the amplifier. In such an arrangement, it is possible to control the output power of the amplifier in a relatively simple way, by adjusting the inductance of the variable inductor. 
     In certain embodiments, the variable inductor comprises a plurality of excludable inductors and a switching arrangement operable to selectively exclude one or more of the excludable inductors from contributing to the variable inductor in order to alter the inductance of the variable inductor. 
     In certain embodiments, the variable inductor comprises first and second ends and the switching arrangement comprises first and second switching mechanisms connected, respectively, to the first and second ends and the plurality of excludable inductors are connected in parallel between the first and second switching mechanisms. 
     In certain embodiments, each of the plurality of excludable inductors is a conductive loop whose ends are connected to respective ones of the first and second switching mechanisms. 
     In certain embodiments, the excludable inductors do not cross each other. 
     In certain embodiments, the excludable inductors are concentric. 
     In certain embodiments, a second one of the first and second inductors is nested with the excludable inductors. 
     In certain embodiments, the first and second switching mechanisms are configured to provide a state in which all of the excludable inductors are connected between the first and second ends. 
     In certain embodiments, the first and second switching mechanisms are configured to provide a state in which only a subset of the excludable inductors are connected between the first and second ends. 
     In certain embodiments, the transformer is a balun. In certain embodiments, the load is an antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example only, certain embodiments of the invention will now be described by reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of an amplification circuit; 
         FIG. 2  is an equivalent circuit model for the circuit of  FIG. 1 ; 
         FIG. 3  is an alternative equivalent circuit model for the circuit of  FIG. 1 ; 
         FIG. 4  is an illustration of a balun that can be used in the circuit of  FIG. 1 ; and 
         FIG. 5  illustrates an arrangement of conductive tracks that can provide the inductors of the balun shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows an equivalent circuit model  38  for circuit  10 . Elements that are carried over from  FIG. 1  retain the same reference numerals in  FIG. 2 . 
     In model  38 , the balun  14  is modelled by an ideal (lossless) transformer  40  with a transformation ratio of 1:n, an inductor  42  in series with the input side of the ideal transformer  40 , an inductor  44  in parallel with the input side of the ideal transformer  40  and a capacitor  46  in parallel with the output side of the ideal transformer  40 . The inductor  42  has an inductance of (1−k 2 )L 1  and represents the leakage inductance of the primary inductor  30  of the balun  14 . The inductor  44  has an inductance of k 2 L 1  and represents the magnetization inductance of the primary inductor  30  of the balun  14 . The capacitor  46  has a capacitance C 2  and represents the stray capacitance of the secondary inductance  32  of the balun  14 . The model  38  also represents the antenna  16  as a resistor  48  of resistance R L . 
       FIG. 3  shows an alternative equivalent circuit model  50  for circuit  10 . Elements that are carried over from  FIGS. 1 and 2  retain the same reference numerals in  FIG. 3 . 
     Model  50  is derived from model  38  by replacing the ideal transformer  40  with a voltage controlled voltage source  52  whose output voltage is simply connected across the resistor  48  that represents the antenna  16 . To compensate for this change to the model, the network  54  representing the primary side of the balun  14  is adjusted by including in parallel with inductor  44  a resistor  56  of resistance R L /n 2  and a capacitor  58  of capacitance C 2 n 2 . It will also be noted that the model  50  has been augmented by including a capacitor  60  connected across the inputs  26  and  28  of the balun  14 . Capacitor  60  has a capacitance C 1  and represents the stray capacitance of the primary inductor  30  of the balun  14 . It is the impedance of the network  54  that the amplifier  12  sees when driving the antenna  16  via the balun  14 . That is to say, the impedance of network  54  is X E . 
     An expression for X E  can therefore be derived from network  54 . There is a resonant frequency, ω o , of the input signal applied across the inputs  22  and  24  of the amplifier  12 , at which that expression becomes simplified, such that its real part is given by: 
     
       
         
           
             
               
                 
                   
                     ⁡ 
                     
                       [ 
                       
                         X 
                         E 
                       
                       ] 
                     
                   
                   = 
                   
                     
                       
                         R 
                         L 
                       
                       ⁢ 
                       
                         
                           L 
                           1 
                         
                         
                           L 
                           2 
                         
                       
                       ⁢ 
                       
                         k 
                         2 
                       
                     
                     + 
                     
                       
                         
                           
                             ω 
                             o 
                             2 
                           
                           ⁢ 
                           
                             L 
                             2 
                           
                           ⁢ 
                           
                             L 
                             1 
                           
                         
                         
                           R 
                           L 
                         
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               1 
                               k 
                             
                             - 
                             k 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   4 
                   ] 
                 
               
             
           
         
       
     
     The parasitic capacitances C 1  and C 2  can be supplemented by additional capacitors (not shown) connected, respectively, across the primary and secondary inductors of the balun  14 , in order to alter ω o  to a desired operating frequency for the amplifier  12 . 
     From equation 4, it will be apparent that the output power of the amplifier  12  can be controlled by adjusting k and L 1 , since P OUT =f(X E ) and, in turn, X E =f(L 1 , k). 
       FIG. 4  schematically illustrates a form for balun  14  in which k and L 1  are adjustable for the purpose of controlling the output power of the power amplifier  12 . Elements that are carried over from  FIGS. 1 ,  2  and  3  retain the same reference numerals in  FIG. 4 . 
     In  FIG. 4 , the balun  14  is shown in a form that is intended for fabrication on a semiconductor chip. In  FIG. 4 , the primary and secondary inductors of the balun  14  are provided by conductive tracks laid out in loops. The track providing the secondary inductor is indicated  32  and the output connections  34  and  36  to the antenna are indicated once more. The primary inductor  30 , however, is provided by a pair of tracks  30   a  and  30   b . The tracks  30   a  and  30   b  are both formed into loops and nested between them is the looped track forming the secondary inductor  32 . The track providing the secondary inductor  32  takes on a dashed form where it crosses tracks  30   a  and  30   b . In practice, these crossing points would be constructed using vias. One end of inductor  30   a  is connected to output  26  of the amplifier  12  by a MOSFET switch  62  and the other end of the inductor  30   a  is attached to output  28  of the amplifier  12  by a MOSFET switch  68 . Similarly, the ends of inductor  30   b  are attached to outputs  26  and  28  by MOSFET switches  64  and  66 , respectively. In practical implementations, switches  62 ,  64 ,  66  &amp;  68  can be also conveniently implemented by the cascode devices already present in the PA  12  in  FIG. 3 . The inductance of the primary inductor  30  and the coupling factor k can be selected by choosing the states of the switches  62  to  68 , as will now be explained. 
     The switches  62  to  68  are restricted to occupying just three states, namely:
         a first state in which switches  62  and  68  are closed and switches  64  and  66  are open. In this state, loop  30   a  alone provides the primary inductor and k is determined by the spatial relationship between loop  30   a  and inductor  32 .   a second state in which switches  62  and  68  are open and switches  64  and  66  are closed. In this state, loop  30   b  alone provides the primary inductor and k is determined by the spatial relationship between loop  30   b  and inductor  32 .   a third state in which all the switches  62  to  68  are closed. In this state, loops  30   a  and  30   b  together provide the primary inductor since they are connected in parallel between the outputs  26  and  28  of the amplifier  12 . The coupling factor k is determined by the spatial relationship between loops  30   a ,  32  and  30   b.          

       FIG. 5  illustrates how the tracks making up the inductors of the balun  14  might be laid out in a structure fabricated on a silicon chip in practice. In  FIG. 5 , the MOSFET switches are omitted for clarity. It will be observed that the secondary inductor  32  includes two turns that run along the periphery of track  30   b . This is done in order to increase the coupling factor of loop  30   b  with the secondary inductor  32 . The secondary inductor  32  runs through a lower layer of the structure on the chip between vias  70  and  72  and between vias  74  and  76  so that the turns of the secondary inductor can cross the loop  30   b , and each other. This means that the looped tracks  30   a  and  30   b  providing the primary inductor can run uninterruptedly on one layer of the structure on the chip, and that layer can be the top metal layer of the chip. This is advantageous in all technologies with a single thick top metal layer capable of carrying high current density with no electromigration problems, since inductors  30   a  and  30   b  are the ones that carry a DC constant (in addition to a modulated AC current) while inductor  32  carries just an AC modulated current. Considering that electromigration limits are more stringent with DC current, it is clear that having inductors  30   a  and  30   b  run uninterruptedly on the single thick top metal layer of the chip is a significant advantage. Also, the fact that the inductors  30   a  and  30   b  can run uninterruptedly means that there is no need for them to include vias, since vias also have electromigration limits. 
     It will be appreciated that various modifications may be made to the embodiments described herein without departing from the scope of the invention, which is defined by the appended claims.