Abstract:
A mixer including a stage for inputting a voltage signal to be shifted and a shift and output stage for providing frequency-shifted signals, a biasing network of the output stage including, between a high supply and a biasing node, a constant current source in parallel with an output element of a current mirror, an input element of which receives a bias order from the input stage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the biasing of a mixer. More specifically, the present invention relates to the biasing of switches of a mixer having a variable quiescent current. 
     2. Description of the Related Art 
     FIG. 1 illustrates the simplified diagram of a mixer intended for shifting the frequency of an oscillating voltage signal IF. The mixer receives signal IF in a first stage or input stage  10 . Input stage  10  has a transconductance function. It converts input voltage signal IF into a current signal IF′ and enables setting the quiescent current of the mixer. Current signal IF′ is provided by input stage  10  to a second stage  20 . 
     Second stage  20  is a frequency shift stage or switch stage. It has the function of mixing, that is, of multiplying, current signal IF′ by a shift order LO. Shift order LO is a periodic voltage signal. Output signal RF has a frequency which is shifted with respect to input signal IF having the frequency of order LO. 
     For example, in telecommunication applications, and more specifically in the transmit portion of a mobile phone, signal IF is a signal having a frequency on the order of 200 MHz while shift order LO is a signal having a frequency on the order of 2 GHz. Output signal RF is for example the signal transmitted by a mobile handset to a fixed station. 
     It is further considered that, in the type of mixer shown in FIG. 1, the quiescent current is negligible. To modify the quiescent current of the mixer then enables modifying its gain, its linearity, and its electric consumption. 
     FIG. 2 illustrates the conventional forming of a mixer of the type previously described in relation with FIG.  1 . 
     Input stage  10  includes an NPN bipolar transistor T 1 . Input signal IF is applied to the base of transistor T 1  via a capacitor CF. The collector of transistor T 1  provides output stage  20  with current signal IF′ (output signal of input stage  10 ). The biasing of transistor T 1  is ensured by the connection of the base of transistor T 1  to a block formed of a current source  11  and of a comparator  12 , the operation of which is described hereafter. The emitter of transistor T 1  is connected to a reference low supply rail (ground) via a resistor RE. The current control is made possible by a feedback loop including a comparator  12 . The output of comparator  12  is connected to the control terminal of current source  11 . An inverting input (−) of comparator  12  is connected to a terminal of a voltage source (VG)  13 . The second terminal of voltage source  13  is connected to the circuit ground. The non-inverting input (+) of comparator  12  is connected to the emitter of transistor T 1 . Current source  11  typically is a P-channel MOS transistor P 1 . The source and the substrate of transistor P 1  are connected to a high supply rail Vdd of the circuit. The drain of transistor P 1  forms the output of current source  11  and is connected to the base of transistor T 1 . The gate of transistor P 1  forms the control terminal of source  11  and is connected to the output of comparator  12 . Then, the current of the mixer is determined by the copying of value VG of current source  13  across resistor RE. The value of this current thus is iE=(VG/RE). The value of voltage VG is determined by a control circuit not shown. For example, in mobile telephony applications, the choice of value VG depends on the level of the RF signal received by a fixed station (not shown). 
     Output stage  20  is a differential stage including a pair of NPN-type bipolar transistors, TN and TP. Transistors TN and TP will be placed close to each other to exhibit similar operating characteristics. The emitters of transistors TN and TP receive output signal IF′ of input stage  10 . The emitters of transistors TN and TP are interconnected to the collector of transistor T 1 . A first shift order LON is sent to the base of transistor TN via a capacitor CN. A second shift order LOP, corresponding to first order LON with a 180° phase shift, is sent to the base of transistor TP via a capacitor CP. The collectors of transistors TN and TP form the outputs of stage  20  and provide differential current signals RFN and RFP. 
     Transistors TN and TP are biased by the connection to a same biasing node  14  via respective biasing resistors RN and RP of same value. The value of the voltage at biasing node  14  is determined by a biasing network  15 . Network  15  is formed by the series connection, between high power supply Vdd and the ground, of a constant current source S, of a resistor R 0  and of two diodes D 1  and D 2 . Diodes D 1  and D 2  are connected to enable flowing of a current from source S to the ground. Node  14  is connected to the intermediary node between source S and resistor R 0 . Diodes D 1  and D 2  have the object of reproducing in biasing network  15  the base/emitter voltage drops of transistors T 1  and TN (in parallel with identical transistor TP). Resistor R 0  has the object of reproducing in biasing network  15  the voltage drops across biasing resistors RN and RP and across resistor RE. 
     A disadvantage of the mixer of FIG. 1 is the impossibility of precisely knowing and controlling the level of the D.C. biasing of the bases of transistors TN and TP of output stage  20 . The biasing level, that is, the voltage on the bases of transistors TN and TP, will vary on the one hand from one circuit to another and, on the other hand, during operation. Indeed, the biasing level varies according to the current level and variations. 
     On the one hand, the required biasing currents vary from one circuit to another due to the variation of parameter β, that is, of the ratio between the current transmitted by a bipolar transistor and the control current, due to technological manufacturing dispersions. However, the three transistors TN, TP, and T 1  of the circuit are formed simultaneously and substantially have the same parameter (that is, β=iE/iB=iEN/iN iEP/iP, with the notations of FIG.  2 ). Typically, parameter β varies by a factor four in a range from 50 to 200. 
     On the other hand, for a given circuit, the current varies during operation according to the value VG selected by source  13  of input stage  10 . 
     Whatever the origin of the current variation, it affects biasing currents iN and iP of transistors TN and TP of output stage  20 . This variation causes a current variation in network  15 . This current variation then causes a variation of the voltage across resistor R 0  and thus of the voltage at biasing node  14 . This results in a possible saturation of one or the other of stages  10  or  20 . 
     In practice, the biasing of stage  20  must be known and controlled with a great precision to guarantee, independently from current variations: 
     a sufficiently low biasing for the two transistors TN and TP of the differential pair of output stage  20  not to saturate; and 
     a sufficiently high biasing to avoid saturation of transistor T 1  of input stage  10  due to too low a collector/emitter voltage. 
     These problems are particularly enhanced and saturation phenomena are particularly frequent in the case of cells with a low supply Vdd, for example, on the order of 2.7 V. 
     In practice, the state of the art, to guarantee that transistor T 1  of input stage  10  does not saturate, uses a current source S, the provided current of which has a high value. This value will be chosen according to the worst possible case in terms of current, that is, the case in which parameter β is very small while current iE (set by voltage VG) is maximum. For example for an emitter current iE of transistor T 1  of input stage  10  having a value ranging between 1 and 5 mA, source S will have to provide a current (iEmax)/(βmin)=5*10 −3 /50=100 μA. As previously discussed, this causes risks of saturation of output stage  20 . For example, for a parameter β of average value and for a current iE having a value on the order of 1 mA, it would be enough for source S to provide a 10-μA current. 
     SUMMARY OF THE INVENTION 
     The present invention thus aims at providing a mixer having a controllable biasing. 
     The present invention aims at providing such a mixer, the biasing level of which remains steady during mixer operation. 
     The present invention also aims at providing such a mixer having a biasing level which is steady from one circuit to another, that is, whatever the parameter β of its transistors. 
     To achieve these objects, the present invention provides a mixer including a stage for inputting a voltage signal to be shifted and a shift and output stage for providing frequency-shifted signals, wherein a biasing network of the output stage includes, between a high supply and a biasing node, a constant current source in parallel with an output element of a current mirror, an input element of which receives a bias order from the input stage. 
     According to an embodiment of the present invention, the input and output elements of the current mirror of the biasing network of the output stage are P-type MOS transistors. 
     According to an embodiment of the present invention, the stage of input of the signal to be shifted is a stage with a variable quiescent current. 
     According to an embodiment of the present invention, the biasing network includes between the biasing node and a circuit ground the series connection of a resistor and of two diodes, the diodes being connected to enable flowing of a current from the biasing node to the circuit ground. 
     According to an embodiment of the present invention, the constant current source includes a circuit of bandgap type adapted to ensuring a constant voltage across the resistor of the biasing network. 
     According to an embodiment of the present invention, the high supply of the mixer is a low-voltage supply. 
     The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows the type of mixer considered in the present description; 
     FIG. 2 schematically shows a conventional mixer; and 
     FIG. 3 illustrates an embodiment of a mixer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For clarity, same elements have been designated by the same references in the different drawings. 
     The present invention provides modifying the biasing of input and output stages  10  and  20  of a mixer as detailed hereafter. 
     In FIG. 3, biasing network  35  according to the present invention replaces constant current source S of prior art with the association of a current mirror M and of a constant current source  10  having a value much smaller than that of source S. 
     Current mirror M includes two P-type MOS transistors P 2  and P 3 . The sources and substrate of transistors P 2  and P 3  are interconnected to high supply Vdd. A gate node G common to transistors P 2  and P 3  is connected to the drain of transistor P 2 . Node G is also connected to the source of transistor P 1  of input stage  10 . The drain of transistor P 3  is connected to biasing node  14  of output stage  20 . The substrate of transistor P 1  is connected to high supply Vdd. The amplification ratio of mirror M between its input element P 2  and its output element P 3  is greater than one and will be designated hereafter as A. 
     The other circuit elements are connected as previously described in relation with FIG.  2 . 
     Mirror M enables automatically indexing the biasing currents of output stage  20  without modifying the biasing level of the bases of transistors TN and TP. Mirror M is an asymmetrical mirror. Mirror M copies at node  14  the biasing current of bipolar transistor T 1  of input stage  10  amplified by a given coefficient A greater than 1. With the notations of FIG. 3, voltage V on the bases of transistors TN and TP is the following: 
     
       
           V=VD+R   0 *( I   0 +( A− 1) *iB )− RN*iB /2, or 
       
     
     
       
           V=VD+R   0 * I   0 +( R   0 *( A− 1)− RN /2) *iB;   
       
     
     where: 
     VD is the voltage across diodes D 1  and D 2  at node  36  of FIG. 3; 
     I 0  is the current provided by constant current source I 0  of block  35 ; and 
     iB is the base current of transistor T 1  of input stage  10 . 
     As will be understood by those skilled in the art, to guarantee that the voltage on the bases of transistors TN and TP is independent from the variations of biasing current iB, amplification ratio A of mirror M must be such that R 0 *(A−1)=RN/2, or else A=+(RN/2R 0 ). Thereby, the voltage on the bases of transistors TN and TP becomes: 
     
       
           V=VD+R   0 * I   0 . 
       
     
     Indeed, voltage V then no longer depends either on current control voltage VG or on the value of parameter β. The operating point of output stage  20  is stable. 
     Value I 0  provided by current source I 0  is chosen to obtain the desired bias voltage V on the bases of transistors TN and TP. R 0 *I 0  takes into account the voltage drop across resistor RE of input stage  10  as well as a margin adapted to avoiding saturation of bipolar transistor T 1  of input stage  10 . 
     According to a specific embodiment, a current source I 0  of bandgap type is used. Such a source provides a current proportional to the inverse of the value of a given resistance Rbg (I 0 =Vbg/Rbg, where Vbg is a constant). It is possible to form such a current source I 0  from a given resistor Rbg, having a value proportional to the value of resistor R 0 , the proportionality coefficient α (Rbg=α*R 0 ) being known and accurately controllable. Voltage V on the bases of transistors TN and TP of shift stage  20  then becomes V=VD+α*Vbg. This then ensures a biasing level V of transistors TN and TP which is perfectly defined, controllable and stable independently from possible variations of the value of resistance R 0  due to uncontrollable fluctuations of the manufacturing process. 
     The presence of mirror M enables using a source I 0  having a current depending on the sole biasing level desired on the bases of transistors TN and TP. The value of the current provided by source I 0  will be on the order of a few tens of microamperes. 
     As a non-limiting example, the values of the different currents and components are the following: 
     current provided by source I 0 : 20 μA 
     RN=RP=1 kΩ; 
     RE=3 kΩ; 
     R 0 =5 kΩ; 
     A=1.1. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the previously-indicated numerical examples are linked to a specific application. Those skilled in the art will know how to choose and adapt the values of the different components according to the specific operating conditions and to the desired performances. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and limited only as defined in the following claims and their equivalents. The present invention is limited thereto.