Abstract:
A current source with low temperature dependence includes a reference current source and a current mirror for copying the reference source current to at least one output branch. The reference current source and the current mirror may have opposite coefficients of temperature dependence and the current mirror may be a weighted mirror. The present 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 with a low coefficient of temperature dependence.  
         BACKGROUND OF THE INVENTION  
         [0002]    A coefficient of temperature dependence is a parameter which, for an electronic device, relates the variations in the device&#39;s output characteristics (i.e., its output current) to the variations in its operating temperature. The operating temperature may be especially influenced by ambient temperature. The temperature dependence coefficient may be defined both for a device in its entirety and for its constituent parts.  
           [0003]    The present invention finds applications, for example, in the manufacture of electronic integrated circuits and in circuits including a current source. In particular, the invention may be useful for the manufacture of integrated circuits or circuit components requiring a current source having very little sensitivity to variations in temperature, such as oscillators, for example. Oscillators may be used in portable transceivers that are powered by battery and may be used at highly variable temperatures, for example.  
           [0004]    A prior art current source with low temperature dependence is shown in FIG. 1. The current source of FIG. 1 includes a so-called reference current source  10 , a bandgap type reference voltage generator  16  that receives a reference current from the reference current source, and a transconductor  18  for converting the reference voltage of the generator  16  into an output current. The current source  10  has two branches  12 ,  14 . These branches provide a reference current which is copied to the reference voltage generator  16  by a double cascoded current mirror  20 .  
           [0005]    The reference voltage generator  16  includes a resistor  22  connected in series with a bipolar transistor  24  (PNP). The base of this transistor is connected to the collector and to a terminal  26  with a reference potential (e.g., ground). Its emitter is connected to the resistor  22 . The voltage V bg  of the generator  16 , which is measured between a terminal  25  and the terminal  26 , may be expressed in the form V bg =V EB +R 1 I. In this expression, V EB  is the emitter-base voltage of the transistor  24 , R 1  is the value of the resistor  22 , and I is the value of the current copied by the mirror  20  from the reference current source to the reference voltage generator  16 .  
           [0006]    The transducer  18  includes an amplifier  27  and of a transistor  28  of the metal-oxide semiconductor (MOS) type. It delivers a current I out  in a load resistor  29  having a value R 2  such that I out =V bg /R 2 . Thus, for a bipolar transistor such as the transistor  24 , the base-emitter voltage is a negative temperature function (i.e., a negative temperature dependence coefficient). On the other hand, the values R 1  and R 2  of the resistors  22 ,  29 , as well as the current I copied from the reference generator  10 , evolve positively with the temperature.  
           [0007]    By appropriately choosing the values of R 1  and I and summing the terms V EB  and R 1 I it is possible to obtain, at the terminal  26 , a reference voltage generator with a temperature dependence coefficient able to compensate for the temperature drifts of the load resistor  29  and of the transconductor  18 . Thus, the output current I out  may be rendered substantially insensitive to temperature. A more comprehensive description of the output source of FIG. 1 may be found in Analysis and Design of Analog Integrated Circuits, Paul R. Gray/Robert G. Meyer, 3 rd  edition, p. 345 (FIG. 4. 50 ).  
           [0008]    The current source of FIG. 1 provides very good temperature stability. Yet, it includes a large number of components and has a high power consumption. These characteristics do not lend themselves to integration of the current source in a high density integrated circuit or reduced circuit cost. Indeed, the chip surface required for such a current source integration is too great for many applications.  
           [0009]    Another current source according to the prior art having a smaller number of components is illustrated in FIG. 2. The current source of FIG. 2 combines two individual current sources having opposite thermal behavior. The first individual source  30  is a current source with two branches coupled together by a current mirror. Such a source is known per se and delivers a current that varies in proportion to the temperature. More precisely, the current I a  is such that:  
         Ia   =         kT     qR   a          ln                     S   2       S   1         =       Δ                   V   BE         R   a           ,                         
 
           [0010]    where k, T, q, R a , S 1  and S 2  respectively represent the Boltzmann constant, the temperature, the electron charge, the value of a source current fixing resistor  34 , and the surfaces of emitters of bipolar transistors  31 ,  32 ,  33  and  35  (being respectively in two branches of the source). The term ΔV BE  represents a magnitude such that ΔV BE =(V BE33 +V BE32 )−(V BE34 +V BE31 ), where V BE33 , V BE32 , V BE34  and V BE31  respectively indicate the base-emitter voltages of the transistors mentioned above.  
           [0011]    The second individual source  40  includes a bipolar transistor  42  connected in series with a current fixing resistor  44  having a value R b . It is further connected in parallel to the first current source  30 . A current I b  delivered by the second source is such that I b = 
           I   b     =       V   BE       R   b         ,                         
 
           [0012]    where V BE  is the base-emitter voltage of the bipolar transistor  42 . The current I b  is inversely proportional to the temperature, i.e., to  
         1   T     .                         
 
           [0013]    Transistors  51 ,  52 , combined with resistors  53 ,  54 , connect the two sources  30 ,  40  to a first supply terminal  56 , connected to a first potential (V cc ), and to a second supply terminal  58 , connected to a second potential (V ee ). The transistors  51 ,  52  have their bases respectively connected to biasing lines  61 ,  62  which may be used to copy the current of the sources  30 ,  40  to loads (not shown). That is, they are current mirror control transistors, also not shown.  
           [0014]    By adjusting the values R a  and R b  of the current fixing resistors of the two individual sources  30 ,  40  (and possibly the surfaces of the transistors  31 ,  32 ,  33 ,  35  and  42 ), it is possible to set the amount of current each current source contributes to the total current passing through the control transistors  51  and  52 . It is also possible to set the amount of current each individual source contributes to the thermal drift of the overall source combining the two sources.  
           [0015]    Thus, the thermal drifts of the individual sources  30 ,  40  are respectively proportional to the temperature (positive coefficient) and inversely proportional to the temperature (negative coefficient). As discussed previously, this is due to the fact that one of the sources is of the  
         Δ                   V   BE       R                         
 
           [0016]    type and the other source is of the  
         V   BE     R                         
 
           [0017]    type. It is therefore possible to obtain at least a partial compensation for the drifts of the two sources, and therefore an overall source with a low temperature dependence coefficient. A more comprehensive discussion of the current source of FIG. 2 may be found in Evolution of High-Speed Operational Amplifier Architectures by Doug Smith et al., IEEE J. of SSC., Oct. 1994, vol. 29, no. 10.  
           [0018]    [0018]FIGS. 3, 4 and  5  respectively show the temperature behavior of the first and second individual sources  30 ,  40  and the overall source resulting from their combination. These figures respectively show, in graphical form, the current (shown on the ordinate) as a function of the temperature (shown on the abscissa). The evolution of the current is given for two different values of the supply voltage (2.7 and 5.5 V) measured between the supply terminals. On each graph, the letters A and B respectively show the curves obtained at 2.7 and 5.5 Volts. The currents are expressed as 10 −4  A and the temperatures are expressed in ° C.  
           [0019]    It can be seen in FIG. 3 that the curves A and B have a positive slope. This is characteristic of a positive temperature dependence coefficient for the first individual source  30 , i.e., the  
         Δ                   V   BE       R                         
 
           [0020]    source. On the other hand, FIG. 4 shows a negative temperature dependence of the individual source  40 , i.e., the  
         V   BE     R                         
 
           [0021]    source. Temperature drifts of the sources are generally considered to be between −55° C. and +125° C. compared with an ambient temperature of +27° C. Thus, for the first individual source  30 , the drift is +33% between −55 and +27° C. and +20% between +27° C. and +125° C., i.e., an overall drift of 53% for a biasing at 2.7 volts.  
           [0022]    For the second individual source (FIG. 4), the overall (negative) drift between −55° C. and +125° C. is −44%, again for a biasing at 2.7 volts. Furthermore, the variation in current at a fixed temperature for a biasing running from 2.7 V to 5.5 V is respectively +30% and +9% for the two individual sources.  
           [0023]    In FIG. 5, which gives the temperature behavior for the overall source including the combination of the two individual sources, it may be seen that a bell-shaped evolution of the current as a function of the temperature for a biasing at 2.7 volts is obtained. The overall drift is 24% maximum, i.e., 16% between −55° C. and +27° C. and −21% between +27° C. and +125° C. On the other hand, for a supply voltage of 5.5 volts, the bell-shaped behavior disappears and a temperature dependence with a negative coefficient is present. The drift of the overall source is, however, reduced to −36% (−12% from −55° C. to +27° C. and −24% from 27° C. to +125° C.).  
           [0024]    Compared with the current source of FIG. 1, the current source of FIG. 2 has a smaller number of components and a lower power consumption. On the other hand, its temperature dependence is greater and the quiescent current (at 27° C.), just like the temperature dependence coefficient, is very sensitive to the supply voltage.  
         SUMMARY OF THE INVENTION  
         [0025]    An object of the invention is to provide a current source having a low temperature dependence while alleviating the limitations of the sources described above.  
           [0026]    Another object of the invention is to provide a current source that requires a relatively smaller number of components and is therefore able to occupy a small chip surface when it is part of an integrated circuit.  
           [0027]    Still another object of the invention is to provide a current source having a low power consumption and which is less sensitive to variations in its supply voltage.  
           [0028]    These and other objects, features, and advantages in accordance with the invention are provided by a current source with low temperature dependence including a reference current source and at least one current mirror to copy the reference current to at least one output branch. The current mirror may be a weighted mirror, and the reference current source and the weighted current mirror may respectively have opposite temperature dependence coefficients. As used herein, a weighted mirror is a mirror which makes it possible to copy in the slave branches (i.e., the output branches) a current which is different and preferably greater than that in the master branch.  
           [0029]    As the temperature dependence of the current mirror is opposite that of the reference current source, the temperature dependence coefficient of the overall source (reference+mirror) may be lower than that of the reference current source taken in isolation. Adjusting the characteristics of the reference source and of the mirror thus makes it possible to obtain a very low temperature dependence.  
           [0030]    According to the invention, various embodiments may be used for making the reference current source. It may be, for example, a source of the type with a base-emitter voltage reference  
         (       V   BE     R     )     .                         
 
           [0031]    . Such reference current sources are known in the art and are described, for example, in Analysis and Design of Analog Integrated Circuits, Paul R. Gray/Robert G. Meyer, 3 rd  edition, p. 324 (FIG. 4. 9 . a ).  
           [0032]    In one embodiment of the current source of the invention, a reference source with a negative temperature dependence and a current mirror with positive dependence may be selected. In this case, the positive drift of the current mirror compensates for the negative drift of the reference source when the temperature increases and vice-versa when the temperature decreases. The current mirror may include a first mirror transistor in a master branch connected to the reference current source and at least one second mirror transistor connected in each output branch. The the first transistor may further be connected in series with a weighting resistor.  
           [0033]    The current source may include several output branches for the supply of several loads and possibly, as indicated below, to supply the reference current source itself. Indeed, to reduce still further the temperature dependence of the current source, it is possible to supply the reference current source with a supply current substantially insensitive to variations in temperature. Such a current may be provided, for example, by one of the output branches of the current mirror. Such a branch may include a transistor, known as a supply transistor, as one of the second transistors and which forms a current mirror with the first transistor of the master branch.  
           [0034]    The weighting resistor makes it possible to obtain a weighted mirror and, in particular, a mirror capable of copying in the output branch (or branches) a current greater than the current provided by the reference current source. A weighted mirror may also be obtained by selecting in the output branch a second transistor with an emitter surface greater than that of the first transistor. By adjusting the value of the weighting resistor or the supply transistor surface, compensation may be made (by way of the mirror) for the variations in source temperature. This is expressed in practice by a mirror copy coefficient greater than 1. A current is therefore available with low sensitivity to temperature and that may be used as discussed above to supply the source via the supply transistor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    Other characteristics and advantages of the invention will become apparent from the following description, with reference to the appended drawings, given by way of non-limitative example, in which:  
         [0036]    [0036]FIG. 1 (previously described) is a schematic diagram of a first current source according to the prior art;  
         [0037]    [0037]FIG. 2 (previously described) is a schematic diagram of a second current source (a composite) according to the prior art;  
         [0038]    [0038]FIGS. 3, 4 and  5  (previously described) are graphs showing the temperature behaviors of the current source of FIG. 2 and its main constituent parts;  
         [0039]    [0039]FIG. 6 is a schematic diagram of a current source according to the invention; and  
         [0040]    [0040]FIG. 7 is a graph showing the temperature behavior of the current source of FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    Turning now to FIG. 6, a current source according to the invention includes a current source  102  (i.e., a reference current source) which has no particular requirement in terms of temperature dependence. As shown, the current source  102  is a source having a negative temperature dependence coefficient. In other words, the current I R  delivered by the reference source  102  decreases when the temperature increases.  
         [0042]    The current source  102  is connected to a current mirror  104  that copies the reference current I R  to one or more output branches  106 ,  108 . A first output branch  106  provides a supply current to the reference source  102  and a second output branch  108  supplies a load  110 . Although illustrated in FIG. 6, the first output branch  106  may be omitted by providing another supply for the reference current source, as will be appreciated by those of skill in the art.  
         [0043]    The current mirror  104  (i.e., the current mirror formed with the second output branch  108 ) is a mirror having a positive temperature dependence coefficient. Indeed, the output branch delivers a current which, for a fixed value of the reference current I R , would increase with the temperature. This tendency towards temperature drift is therefore inverse to that of the reference current source  102 .  
         [0044]    The reference current source  102  includes a first bipolar transistor  120  having its collector connected to the current mirror  104  and its emitter connected to a supply terminal  122  by a resistor  124 . The supply terminal  122  may be ground, for example. The base of the first transistor  120  is connected to the base of a second diode biased transistor  126  connected in series in the first output branch  106  to a third transistor  128 . That is, the third transistor  128  is connected to the emitter of the second transistor  126  by its base and by its collector. The third transistor  128  connects the second transistor  126  to the ground terminal  122 .  
         [0045]    For simplification, assuming the first and second transistors  120 ,  126  have approximately the same base-emitter voltages, the current I R  of the reference source is:  
           I   R     =       V   BE128       R   124         ,                         
 
         [0046]    where V BE128  is the base-emitter voltage of the third transistor and R 124  is the value of the resistor  124  in series with the first transistor  120 . As will be recalled from the above description of the prior art current source of FIG. 2, the current I R  is inversely proportional to the temperature.  
         [0047]    The current mirror  104  includes in the master branch a fourth transistor  130  connected by its base and its collector to the reference current source  102 . The fourth transistor  130  also is connected by its base to the base of the transistors of the output branches, and by its emitter to the (positive) supply terminal  134 . More specifically, the emitter of the fourth transistor  130  is connected to the supply terminal  134 , positive in the example shown, by a resistor  136  (a weighting resistor).  
         [0048]    Fifth and sixth bipolar transistors (PNP)  146 ,  148  of the current mirror  104  are connected in series respectively in the first and second output branches  106 ,  108 . They are connected by their emitters to the positive supply terminal  134 . Their bases are connected to the bases of the fourth transistor  130 , as discussed above.  
         [0049]    If the fourth, fifth and sixth transistors are identical and have approximately the same emitter surfaces, the weighting resistor  136  allows currents to be fixed in the output branches that are stronger than those in the master branch to compensate for variations in temperature of the source. Indeed, V EB130 +R p I R =V EB146 =V EB148 , where V EB130 , V EB146 , V EB148  are respectively the emitter-base voltages of the transistor  130  of the master branch and of the transistors  146 ,  148  of the output branches and R p  is the value of the weighting resistor. The transistors of the output branches may also have emitter surfaces greater than that of the transistor of the master branch of the current mirror for increasing the output current.  
         [0050]    Adjustment of the output current by the choice of transistors (i.e., emitter surface) and of the value of the weighting resistor allows the positive temperature drift of the current mirror to be fixed. This drift may thus be adjusted to compensate, at least partly, for the drift (i.e., negative) of the reference current source. Preferably, the drift is adjusted to be minimal. Furthermore, in one embodiment, only the second output branch  108  would form a weighted mirror. In this particular case, the emitter surfaces of the transistor  130  of the master branch and of the transistor  146  of the first output branch would be selected to be identical. Further, a resistor having a value identical to that of the weighting resistor would be connected in series with the transistor of the first output branch.  
         [0051]    [0051]FIG. 7 shows the temperature behavior of the source of FIG. 6. The curves A and B represent the current delivered as a function of the temperature for supply voltages of 2.7 and 5.5 volts, respectively. It may be seen that, whatever the supply voltage, a substantially bell-shaped behavior is obtained. The maximum overall drift of the current with a temperature varying between −55° C. and +27° C. and between +27° C. and +125° C. is 20% as an absolute value. It is more precisely +16% between −55° C. and +27° C. and −20% between +27° C. and +125° C.  
         [0052]    Compared with the known prior art current sources described above, the overall temperature drift of the current source of the invention is lower and the extent thereof is substantially unaffected by the supply voltage. Furthermore, the value of the quiescent current at 27° C. (i.e., at a fixed temperature) varies only by about 10% for a supply voltage running from 2.7 to 5.5 volts. The curves in FIG. 7 are obtained by using transistors of the current mirror that are identical to each other and by using a weighting resistor value of 60 kΩ.