Abstract:
The invention proposes a radio frequency circuit which comprises a transformer with a first primary circuit, a second primary circuit, and a secondary circuit, the secondary circuit comprising an integrated first inductor, this first inductor being positioned, on an axis orthogonal to a surface of the circuit, between an integrated second and third inductor respectively comprised in the first primary circuit and second primary circuit.

Description:
TECHNICAL FIELD 
     This invention relates to the transformers used in integrated electronic components. 
     More particularly, it concerns the transformers used in radio frequency (RF) integrated circuits. 
     TECHNOLOGICAL BACKGROUND 
     In order to linearize radio transmissions, radio frequency transmitters and receivers use differential architectures. Antennas, as well as the power amplifiers associated with them, do not have this differential structure, however. A transformation of the differential signals from communication circuits should therefore be performed before they are delivered to the power amplification stage and the antenna (antenna stage, for short). 
     The radio transmitters and receivers of today operate in different modes and on different frequency bands. In addition, transmitters and receivers currently have different outputs for various applications. For example, some devices can operate in compliance with different standards, and therefore have different antenna stages (for example operating in WCDMA and in GSM/EDGE). 
     Magnetic transformers are used to perform this transformation and also to match the output from these circuits to a particular antenna amplifier. 
     These transformers comprise inductors, and including these inductors in an integrated circuit requires a large silicon surface area. 
     A large amount of space is therefore consumed in the integrated circuit, as each antenna stage requires a circuit that has a transformer. 
     A prior art radio frequency circuit will now be described with reference to  FIG. 1 . 
     A matching circuit MATCH is positioned between two communication circuits CIRC 1  and CIRC 2  on the one hand and two antenna amplifiers AMPLI 1  and AMPLI 2  on the other hand. The antenna amplifiers APMLI 1  and APMLI 2  are respectively connected to the antennas ANT 1 , ANT 2 . 
     The circuit MATCH comprises two transformers TRSFO 1  and TRSFO 2 , respectively coupled to the circuits CIRC 1  and CIRC 2 . 
     The circuits CIRC 1  and CIRC 2  have a differential structure. The two outputs from each circuit are connected to the two terminals of the transformers&#39; primary circuits. The transformers&#39; secondary circuits have a terminal connected to the ground and a terminal connected to each of the amplifiers AMPLI 1  and AMPLI 2 . 
     Two separate transformers are therefore used in this structure. Each of these transformers occupies space on the silicon substrate. 
     SUMMARY OF THE INVENTION 
     A need therefore exists for a radio frequency circuit which allows matching between a differential stage and a single input stage, and occupies a reduced silicon surface area. 
     A radio frequency circuit is proposed for this purpose, comprising a transformer with a first primary circuit, a second primary circuit, and a secondary circuit. The secondary circuit comprises a first integrated inductor which is positioned, on an axis orthogonal to a surface of the circuit, between a second integrated and a third inductors respectively comprised in the first primary circuit and second primary circuit. 
     The present invention takes advantage of the large number of layers in integrated circuits as they are currently produced. 
     For example, the circuit is a CMOS circuit. 
     In the present invention, different primary inductors are associated with the same secondary inductor. 
     This allows obtaining different transformation ratios on the surface area occupied by a single inductor. 
     For example, the inductors used are substantially flat and each is substantially comprised in a layer of the circuit. 
     When creating the inductor, it may be necessary to create a bridge on an adjacent layer in order to intersect two tracks of the inductor. Thus, “substantially comprised in a layer of the circuit” is understood to mean that the major portion of the inductor is located in a layer and that, for example, the parts acting as bridges are comprised in adjacent layers. 
     For example, the inductors comprise a given number of concentric turns. 
     The turns may be formed in the same layer of the circuit or in several layers of the circuit. 
     In addition, the turns may have different forms. For example, they may have a square, octagonal, circular, figure-eight, or other form. 
     In some embodiments, the transformer is positioned in a matching stage between a radio frequency circuit and an antenna. 
     There can be different transformation ratios for the different primary circuits. For example, the transformation ratios are obtained by creating the first and second inductors with a different number of turns. 
     The circuit of the invention can comprise more than two primary circuits for the same secondary circuit. 
     In some embodiments of the invention, the circuit comprises a control unit for the primary circuits. This control unit is configured to cause only one primary circuit to operate at a time. 
     For example, the control circuit allows choosing a transformation ratio. For example, the control unit comprises current-controlled variable impedance circuits or switches to electrically and selectively isolate one of the first and second primary circuits. 
     Another object of the invention is a communication terminal comprising a circuit of the invention. 
     The terminal has at least the same advantages as those provided by the circuit of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Other features and advantages of the invention will be apparent from reading the following description. This description is purely illustrative and is to be read with reference to the attached drawings, in which, in addition to  FIG. 1 ; 
         FIG. 2  illustrates a radio frequency circuit of one embodiment; 
         FIG. 3  schematically illustrates the arrangement of the inductors of a transformer of a circuit according to one embodiment; 
         FIGS. 4 and 5  illustrate two control units according to two embodiments; and 
         FIG. 6  illustrates a terminal according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A radio frequency circuit according to one embodiment will now be described with reference to  FIG. 2 . 
     The structure in  FIG. 2  reuses the elements CIRC 1 , CIRC 2 , AMPLI 1 , AMPLI 2 , ANT 1 , ANT 2  of the circuit previously described with reference to  FIG. 1 . 
     In the present embodiment, the matching circuit MATCH, positioned between the circuits CIRC 1  and CIRC 2  and the amplifiers AMPLI 1  and AMPLI 2 , comprises a transformer TRSFO which has a secondary circuit SECOND comprising an inductor IND with one terminal connected to the first amplifier AMPLI 1  and the other terminal connected to the second amplifier AMPLI 2 . In addition, the transformer comprises two primary circuits PRIM 1  and PRIM 2  magnetically coupled to the secondary circuit SECOND. The primary circuits PRIM 1  and PRIM 2  respectively comprise the inductors IND 1  and IND 2 . The two output terminals of the circuit PRIM 1  are connected to the terminals of the circuit CIRC 1  and the two terminals of the circuit PRIM 2  are connected to the two output terminals of the circuit CIRC 2 . 
     It is therefore possible to make use of the primary circuit PRIM 1  or the primary circuit PRIM 2 , depending on whether the antenna ANT 1  or the antenna ANT 2  is to be used. For example, a transformation ratio specific to PRIM 1  or to PRIM 2  is selected. 
     The arrangement of the inductors IND 1 , IND 2 , and IND is now described with reference to  FIG. 3 . 
     The three inductors IND 1 , IND 2 , IND are represented in this Figure. These inductors are substantially flat inductors (in the sense indicated above), each comprising a given number of octagonal turns. 
     These inductors are created using integrated circuit etching processes known to a person skilled in the art, for example by photolithography. 
     In the example represented, the inductor IND comprised in the secondary circuit of the transformer is located between the coils IND 1  and IND 2  of the primary circuits PRIM 1  and PRIM 2 . In addition, the inductors are concentrically arranged to ensure satisfactory magnetic coupling. 
     The inductor IND 1  comprises four turns while the inductor IND 2  comprises three. The circuits PRIM 1  and PRIM 2  therefore have different transformation ratios. The transformation ratio is the ratio of the number of turns in the secondary to the number of turns in the primary. 
     The turns of each of the inductors are mostly coplanar. These coplanar parts can therefore be in the same circuit layer. The inductors also comprise certain parts which form bridges between different turns. These bridge-forming parts may be comprised in layers adjacent to those which comprise the substantially coplanar turns. 
     In the embodiment illustrated by  FIG. 4 , the circuit MATCH is controlled by means of a set of transistors. 
     These transistors are arranged so that they operate as switches on the branches of the circuits PRIM 1 , PRIM 2  and SECOND. The transistors on the circuit PRIM 1  and the transistor on the branch of the circuit SECOND coupled to the amplifier AMPLI 2  are controlled by a signal Cmd. The other transistors are controlled by a signal Cmd_not which is the complement of the signal Cmd. 
     Thus, when the transistors controlled by the signal Cmd are passing current, the transistors controlled by the signal Cmd_not are not passing current. 
     The transistors coupled to the secondary circuit have their drain connected to the secondary and their source to the ground. 
     In this manner, it is possible to alternate between using the circuit PRIM 1  to connect the circuit CIRC 1  and the amplifier AMPL 1 , and using the circuit PRIM 2  to connect the circuit CIRC 2  with the amplifier AMPL 2 . 
     To connect CIRC 1  to AMPLI 1 , the signal Cmd is activated which enables the transistors controlled by this signal to pass current. In this case the circuit PRIM 1  closes, the secondary branch connected to AMPLI 2  goes to the ground, and the transistor on the secondary branch connected to AMPLI 1  does not pass current. 
     Symmetrically, to connect CIRC 2  to AMPLI 2 , the signal Cmd is deactivated which enables the transistors controlled by the signal Cmd_not to pass current. In this case the circuit PRIM 1  opens and the circuit CIRC 2  closes, the secondary branch connected to AMPLI 1  goes to the ground, and the transistor on the secondary branch connected to AMPLI 2  does not pass current. 
     In the embodiment illustrated in  FIG. 5 , the transistors on the circuits PRIM 1  and PRIM 2  have been replaced by two variable impedance circuits IMPED 1  and IMPED 2 . 
     Each of these circuits is controlled by a current source, which itself is controlled by the signal Cmd for the current source controlling the circuit IMPED 1 , and the signal Cmd_not for the current source controlling the circuit IMPED 2 . 
     For example, the circuits IMPED 1  and IMPED 2  are operational amplifiers. The current sources are coupled to the amplifiers to obtain an output impendence of the operational amplifiers which is zero or infinite (theoretically). 
     As will be apparent to a person skilled in the art, the operation of this control circuit is inferred from that of the circuit described with reference to  FIG. 4 . 
     A communication terminal according to an embodiment of the invention is now described with reference to  FIG. 6 . 
     The terminal TERM comprises a processing unit PROC for controlling the terminal. The terminal additionally comprises a communication unit COM and an antenna ANT for communication purposes. The terminal additionally comprises a matching circuit MATCH comprising a circuit of the invention. 
     Of course, the invention is not limited to the embodiments described above. It extends to all equivalent variations.