Abstract:
A current source includes a master branch including a branch current fixing resistor, at least one slave branch, and a current mirror including a mirror transistor in each of the master and slave branches, respectively, to couple the branches. The current source may additionally include at least one of a first circuit for injecting in the current fixing resistor a current proportional to the master branch current and a second circuit for injecting in a degeneration resistor of the mirror transistor of the slave branch a current proportional to a current of the slave branch. The invention is particularly applicable to the manufacture of integrated circuits.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of electronic circuits, and, more particularly, to a current source which may be supplied by a very low supply voltage (e.g., about 1.1 Volt) and which has reduced sensitivity to variations in supply voltage.  
         BACKGROUND OF THE INVENTION  
         [0002]    Current sources are found in most integrated circuits. They are used for the biasing the various constituent parts of circuits. Integrated circuits are generally designed to be supplied by a wide range of supply voltages. By way of example, certain operational amplifiers may be supplied by a voltage between 2.7 Volts and 12 Volts. For such integrated circuits, it is important for their current sources to deliver currents that have little variance with respect to the supply voltage so that the operation of the integrated circuit is not influenced by the available supply voltage.  
           [0003]    Furthermore, it is desirable for current sources to operate from a low supply voltage to reduce electrical consumption and to make the best use of the available power. This is particularly the case with devices powered by a battery, for example. The invention finds applications generally in the manufacture of electronic circuits, particularly integrated circuits, such as circuits intended for portable equipment.  
           [0004]    One current source according to the prior art exhibiting substantial independence from the voltage supply includes a voltage generator delivering a regulated voltage and supplying a conventional current source at a constant voltage. Such generators, commonly referred to as bandgap generators, are described, for example, in Analysis and Design of ANALOG INTEGRATED CIRCUITS by Paul R. Gray, Robert G. Meyer, Third Edition, Ch. 4, A 4.3.2, pp. 345-346. These generators deliver a constant voltage of about 1.2 Volts and, therefore, require a supply voltage above this value. The minimum supply voltage required by bandgap generators is at least 1.3 to 1.5 Volts.  
           [0005]    Another known current source may be seen in FIG. 1. This is a so-called crossed source. The crossed source is constructed around four source transistors  10 ,  12 , and  25 ,  26 , connected in a master branch  14  and a slave branch  16 , respectively. A current fixing resistor  18  of a value R is connected in series with the first transistor  10  of the master branch. The base of each of the source transistors  10  and  12  of a given branch is connected respectively to the source transistor collector of the other branch. A current mirror  20  allows the current I circulating in the master branch to be copied to the slave branch. The current mirror  20  is constructed around two transistors  21  and  22  connected in the master branch and the slave branch, respectively. An output current for a load can be copied in an output branch (not shown) either from the master branch or from the slave branch.  
           [0006]    The current I circulating in the master branch  14  is equal to  
       I   =       Δ                   V   BE       R                           
 
           [0007]    where ΔV BE  is such that ΔV BE =(V BE26 +V BE12 )−(V BE25 +V BE10 ). In this expression, V BE26 , V BE12 , V BE25 , and V BE10  represent the base-emitter voltages of the transistors  26 ,  12 ,  25  and  10 , respectively.  
           [0008]    One peculiarity of the current source of FIG. 1 is that the current of the branches  14 ,  16  evolves as a decreasing function of the supply voltage applied between the supply terminals  24 ,  26  of the source. In other words, the source current tends to increase when the supply voltage falls. This characteristic is particularly advantageous when the current source is combined with other elements whose outputs evolve positively, i.e., as a growing function with the supply voltage.  
           [0009]    To allow the operation of a current source such as that shown in FIG. 1, it is necessary to have available between the supply terminals  24  and  26  a voltage V comin  equal to at least twice the base-emitter voltage V be  of a bipolar transistor (source transistor and cascode stage transistor). To this the collector-emitter saturation voltage V cesat  of a third transistor (current mirror) is added. In other words, V comin =2V be +V cesat . For typical bipolar silicon transistors such as those represented in FIG. 1, the minimum supply voltage is about 1.8 Volts. This voltage is comparable with that required by the source using the bandgap type generator.  
           [0010]    A third example of a current source according to the prior art is shown in FIG. 2. This is a simple cascoded source. To simplify the description, different elements of this current source, comparable with those of the current source in FIG. 1, are identified with the same numerical references. Reference may be made, for these elements, to the above description. Unlike the current source of FIG. 1, it may be seen that the bases of the source transistors  10  and  12  are connected to each other. The transistors  25  and  26  which are connected to the source transistors form a cascode stage. An output branch  30  includes a load  32  to be supplied by the output current and a copy transistor  34  controlled by the common bases of the transistors of the mirror stage  20 . The use of a cascode stage  25 ,  26  makes it possible to obtain a high output impedance for the source and therefore a relatively low variation in output current.  
           [0011]    By analogy with the current source of FIG. 1, it may be seen that the minimum supply voltage is still such that V comin =2V be +V cesat?  1.8 Volts. With the current source of FIG. 2, in which an emitter surface ratio of source transistors is equal to 10, and in which the current fixing resistor has a value of 5 k ω, a master branch current sensitivity as low as 1.6% per volt can be obtained (the current sensitivity in the slave branch is then about 5.2% per volt).  
           [0012]    A fourth prior art current source may be seen in FIG. 3. This current source is commonly referred to as a emitter degeneration source and is further described, for example, in Analysis and Design of ANALOG INTEGRATED CIRCUITS by Paul R. Gray, Robert G. Meyer, Third Edition, Ch. 4, A 4.2.1, p. 276. The current source of FIG. 3 still includes two branches  14  and  16  coupled by a current mirror  20 . The master branch  14  includes a first source transistor  10  in series with a current fixing resistor  18 . The slave branch includes a second source transistor  12  connected to the first transistor by its base.  
           [0013]    Unlike the current sources described in the previous figures, the cascode stage has been eliminated from the current source of that of FIG. 3. The source transistors are in fact connected directly to those of the current mirror  20 . On the other hand, the emitters of the bipolar transistors  21 ,  22  used to form the current mirror  20  are connected to the upper supply terminal  24  by so-called degeneration resistors  41 ,  42 , respectively. The values of these resistors will be referred to as R 3  and R 4 , respectively, hereafter. The minimum supply voltage now becomes, for example, V comin =V be12 +V cesat22 +R 4 I 2 . In this expression, V be12  is the base emitter voltage of the source transistor of the slave branch  14 , V cesat22  is the collector-emitter saturation voltage of the mirror transistor  22 , and I 2  is the current circulating in the slave branch  16 . The current circulating in the master branch is I 1 .  
           [0014]    For a current source comparable with that of FIG. 3, the choice of low degeneration resistor values makes it possible to reduce the minimum supply voltage required for the operation of the source. On the other hand, these low values of the degeneration resistors increase the sensitivity of the output current to the supply voltage. This aspect will emerge more clearly in the following description.  
         SUMMARY OF THE INVENTION  
         [0015]    An object of the invention is to provide a current source supplying an output current that is substantially independent of the supply voltage.  
           [0016]    Another object of the invention is to provide such a current source that may be powered at a low supply voltage.  
           [0017]    These and other objects, features, and advantages according to the invention are provided by a current source including a master branch including a branch current fixing resistor, at least one slave branch, and a current mirror including a mirror transistor in each of the master and slave branches, respectively, to couple the branches. The current source may additionally include at least one of a first circuit or means for injecting in the current fixing resistor a current proportional to the master branch current and a second circuit or means for injecting in a mirror transistor degeneration resistor of the slave branch a current proportional to a current of the slave branch. The injection means make it possible to reduce at the same time the minimum value of the supply voltage and the sensitivity of the source current to this voltage.  
           [0018]    An output current can be copied in an output branch by a transistor controlled either by the common bases of so-called source transistors or by the common bases of the mirror transistors. As used herein, “source transistors” are those transistors intended to set the source current value. They may be in series with the mirror transistors, for example.  
           [0019]    More specifically, the first current injection means may include a first injection transistor connected to the current fixing resistor and forming a current mirror with the mirror transistor of the master branch. The current fixing resistor thus passes not only the master branch current but also the current supplied to it by the first injection transistor. The injection transistor is preferably controlled by the mirror transistor to form with it a weighted current mirror. More precisely, the weighted current mirror may be obtained by combining a degeneration resistor with the mirror transistor of the master branch.  
           [0020]    Further, the weighted current mirror may be obtained by using a first injection transistor having an emitter surface that is greater than that of the mirror transistor of the master branch. Also, the second current injection means may include a second injection transistor connected to the degeneration resistor and forming a current mirror with a source transistor connected in series with the mirror transistor of the slave branch. If both the first and second current injection means are used, the master branch and the slave branch may each include a degeneration resistor, for example. The second injection transistor may also be chosen to have an emitter surface greater than that of the branch source transistor to form therewith a weighted mirror. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Other characteristics and advantages of the present invention will become apparent from the following description, with reference to the appended drawings, given by way of non-limitative example, in which:  
         [0022]    [0022]FIG. 1 (previously described) is a schematic circuit diagram of a first current source according to the prior art;  
         [0023]    [0023]FIG. 2 (previously described) is a schematic circuit diagram of a second current source according to the prior art;  
         [0024]    [0024]FIG. 3 (previously described) is a schematic circuit diagram of a third current source according to the prior art; and  
         [0025]    [0025]FIG. 4 is schematic circuit diagram of a current source according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Turning now to FIG. 4, a current source according to the invention includes essentially two branches  114 ,  116  combined by a current mirror  120 . The source branches  114 ,  116  are connected between a first supply terminal  124  with a positive potential (V cc ) and a second supply terminal  126  connected to ground, for example.  
         [0027]    The first branch  114  is a master branch. It includes, in order from the first supply terminal, a first degeneration resistor  141  of a value R 2 , a first mirror transistor  121 , a first source transistor  110 , and a current fixing resistor  118 . The first mirror transistor (shown as a PNP type) is connected to the degeneration resistor  141  by its emitter and is connected by its collector to the collector of the source transistor  110 . The collector of the mirror transistor is also connected to the base of this transistor. The source transistor  110  of the master branch (shown as an NPN type) is connected to the current fixing resistor by its emitter.  
         [0028]    The second branch  116  of the current source is a slave branch. It includes, in order from the first supply terminal, a second degeneration resistor  142  of a value R3, a second PNP mirror transistor  122  connected by its emitter to the degeneration resistor  142 , a second source transistor  112  (NPN) connected by its collector to that of the mirror transistor and to the ground terminal by its emitter. The collector of the second source transistor is connected to its base and to the base of the source transistor  110  of the master branch. In the same way, the bases of the mirror transistors of the two branches are connected to each other.  
         [0029]    A first current injection transistor  151  (PNP) is connected by its emitter to the first supply terminal  124  and by its collector to a node  154  located between the emitter of the first source transistor and the current fixing resistor. The base of the first current injection transistor  151  is connected to the bases of the mirror transistors to be controlled by the mirror transistor of the master branch  114 .  
         [0030]    A second current injection transistor  152  of the NPN type is connected by its collector to a node  156  located between the degeneration resistor  142  of the slave branch  116  and the emitter of the mirror transistor  122  of this same branch. The emitter of the second current injection transistor is connected to the ground terminal  126 . Operation of the two current injection transistors  151 ,  152  is independent. However, each injection transistor contributes to the constancy of the current supplied by the source.  
         [0031]    The first current injection transistor  151  forms a weighted current mirror with the mirror transistor  121  of the master branch. The weighted character of the mirror stems from the degeneration resistor  141 . Indeed, we may write V be151 =V be121 +R 2 I 3 , where V be121 , V be151  and I 3  represent respectively the base-emitter voltage of the mirror transistor of the master branch, the base-emitter voltage of the first current injection transistor, and the current circulating in the master branch. In other words, the base-emitter voltage of the current injection transistor is greater than that of the mirror transistor of the master branch. The current injection transistor therefore makes it possible to inject in the current fixing resistor  118  a current greater than that of the current that it receives from the master branch.  
         [0032]    As the supply voltage Vcc applied between the supply terminals  124  and  126  tends to increase, the current I 3  circulating in the master branch  114  also tends to increase by the Early effect on the source transistor  110  of the master branch. As the current of the master branch is copied in the current fixing resistor  118  by the first current injection transistor  151 , the voltage at the terminals of this resistor tends also to increase.  
         [0033]    Furthermore, as the current in the master branch is also copied in the slave branch by the current mirror  120  formed by the mirror transistors  121 ,  122 , an increase in the current I 3  of the master branch entails an increase in the current I 4  of the slave branch. This results from the mirror effect, to which is added the Early effect of the mirror transistor  122  of the slave branch. The current I 4  therefore increases more rapidly. Also, when the current I 4  of the slave branch tends to increase, the same is true with the base-emitter voltage of the second source transistor  112 .  
         [0034]    The current injection in the current fixing resistor makes it is possible to obtain a variation in the voltage at the terminals of this resistor. This variation is greater than that in the base-emitter voltage of the source transistor  112  of the slave branch  116 . Further, when the voltage at the terminals of the current fixing resistor  118  increases more than the base voltage of the source transistor  112  of the slave branch  116 , the current I 3  circulating in the master branch tends to decrease. This is because the base-emitter voltage of the source transistor  110  of the master branch tends to decrease. This phenomenon compensates for the tendency to increase of the same current in response to an increase in the supply voltage. Additionally, the current of the master branch, just like that of the slave branch, remains substantially stable and independent of variations in the supply voltage.  
         [0035]    The second current injection transistor  152  forms a current mirror with the source transistor  112  of the slave branch  116 . This current mirror makes it possible to copy in the degeneration resistor  142  of the slave branch a current proportional to the current I 4  circulating in this branch. In other words, the degeneration resistor  142  passes not only the current of the slave branch, as does the source transistor, but also the current of the second injection transistor.  
         [0036]    As the supply voltage Vcc applied between the supply terminals  124  and  126  tends to increase, the same is true with the currents I 3  and I 4  circulating in the master and slave branches. This point has been discussed above (i.e., the Early effect on transistors  110  (source transistor) and  122  (mirror)). As the current of the slave branch increases, the current delivered by the current injection transistor  152  also increases. The voltage at the terminals of the second degeneration resistor, which passes the sum of these currents, tends therefore a priori to increase with the supply voltage. However, the voltage at the terminals of the second degeneration resistor  142  (slave branch) tends to increase more than the voltage at the terminals of the first degeneration resistor  141  (master branch). This is due to the fact that the current supplied by the second current injection transistor is injected only in the second degeneration resistor and not in the first.  
         [0037]    As a result, the base voltage of the mirror transistor  122  of the slave branch  116  tends to fall and entails a drop in the current I 4  of the slave branch, and therefore of the master branch. This drop therefore compensates for the tendency of the same current to increase that is caused by the increase in the supply voltage. In this case again, a variation in the supply voltage leaves the current of the current source approximately unchanged.  
         [0038]    To supply an electrical load from the current source, it is possible to copy the current from one of the branches  114 ,  116  in an output branch. Although not being directly part of the current source, FIG. 4 shows, in a dashed line, such output branches. In these branches  160   a  and  160   b,  the electrical loads are identified by the reference  162   a  and  162   b  and copy transistors, combined with the loads, are identified by the references  164   a  and  164   b.  The transistor  164   a  of the first output branch may be of the PNP type and is connected by its emitter to the first supply terminal  124 . Its collector is connected to the electrical load and its base is connected to the base of the mirror transistor  121  of the master branch  114 . The current supplied to the electrical load is therefore proportional to the current I 3  circulating in the master branch.  
         [0039]    The transistor  164   b  of the second output branch  160   b  may be of the NPN type and is connected to the ground terminal by its emitter. Its collector is connected to the first supply terminal by the electrical load. Also, its base is connected to that of the source transistor of the slave branch to be controlled thereby.  
         [0040]    Table 1 below makes it possible to compare the behavior of the prior art current source of FIG. 3 and the current source according to the invention (FIG. 4). For different characteristics of the sources, the table shows the following values: the currents I 2 , I 4  circulating in the slave branch for a supply voltage of 2.7 Volts; the current variation of the slave branch in percent per volt; the current variation of the master branch in percent per volt; the total current passing through the source branches; and the minimum supply voltage necessary for the operation of the source.  
         [0041]    The columns of Table 1 respectively show the following cases. Case A1 represents the current source of FIG. 3 with R 2 =R 3 =0 and R 1 =5.5 K ω. Case A2 represents the current source of FIG. 3 with R 2 =R 3 =1.4 K ω and R 1 =5.5 K ω. Case A3 represents the current source of FIG. 3 with R 2 =R 3 =50 K ω and R 1 =5.5 K ω. Case I1 represents the current source of FIG. 4 with R 2 =R 3 =1.4 KW, with transistor  151 , and without transistor  152 . Case I2 represents the current source of FIG. 4 with R 2 =R 3 =1.4 K ω, without transistor  151 , and with transistor  152 . Case I3 represents the current source of FIG. 4 with R 2 =R 3 =1.4 K ω and with transistors  151  and  152 . The current variations are shown in percent per volt of V cc .  
                                       TABLE 1                       Case   A1   A2   A3   I1   I2   I3                   I 2     10 μA   10 μA   10 μA                   I 4                 10 μA   10 μA   10 μA       ΔI 2     5.8%/V   3.8%/V   1.1%/V       ΔI 4                 1.9%/V   2.3%/V   0.74%/V       ΔI 1     2.2%/V   1.7%/V   0.9%/V       ΔI 3                 −0.25%/V   1.4%/V   −0.4%/V       ΔI cc     21 μA   21 μA   &lt;μA   55 μA   32 μA   66 μA       V ccmin     1.1 V   1.1 V   1.6 V   1.1 V   1.1 V   1.1 V                  
 
         [0042]    It may be seen in Table 1 that the current variations in the source branches according to the invention (FIG. 4) are almost always smaller than those of the emitter degeneration source (FIG. 3). The variation is particularly small in the master branch. Only a very large source emitter degeneration of FIG. 3 makes it possible to obtain high current insensitivity to the supply voltage. However, this is at the cost of a higher value of the minimum supply voltage (1.6 Volts instead of 1.1 Volts).