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.
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.
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.
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.
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.
The current I circulating in the master branch 14 is equal to   I  =            Δ      ⁢              xe2x80x83            ⁢              V        BE              R  
where xcex94VBE is such that xcex94VBE=(VBE26+VBE12)xe2x88x92(VBE25+VBE10). In this expression, VBE26, VBE12, VBE25, and VBE10 represent the base-emitter voltages of the transistors 26, 12, 25 and 10, respectively.
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.
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 Vcomin equal to at least twice the base-emitter voltage Vbe of a bipolar transistor (source transistor and cascode stage transistor). To this the collector-emitter saturation voltage Vcesat of a third transistor (current mirror) is added. In other words, Vcomin=2Vbe+Vcesat. 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.
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.
By analogy with the current source of FIG. 1, it may be seen that the minimum supply voltage is still such that Vcomin=2Vbe+Vcesat? 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 xcfx89, 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).
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.
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 R3 and R4, respectively, hereafter. The minimum supply voltage now becomes, for example, Vcomin=Vbe12+Vcesat22+R4I2. In this expression, Vbe12 is the base emitter voltage of the source transistor of the slave branch 14, Vcesat22 is the collector-emitter saturation voltage of the mirror transistor 22, and I2 is the current circulating in the slave branch 16. The current circulating in the master branch is I1.
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.
An object of the invention is to provide a current source supplying an output current that is substantially independent of the supply voltage.
Another object of the invention is to provide such a current source that may be powered at a low supply voltage.
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.
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, xe2x80x9csource transistorsxe2x80x9d are those transistors intended to set the source current value. They may be in series with the mirror transistors, for example.
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.
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.