Current-controlling circuit

Abstract: A current-controlling circuit for producing either a constant current, independent of supply potential or a current which decreases with increasing supply potential and vice-versa. Three devices 12, 13, 14 are connected together at a point such that the current in the first device 12 and the current in the third device 14 form the current in the second device 13. The current flowing in first device 12 is a mirror of the current flowing in device 11. When the supply potential increases, the increase in current in the first device 12 at least equals the increase in current in the second device 13, so that the current in the third device 14 does not increase. If the current in the third device 14 decreases with increasing supply potential, it may be mirrored into subsequent devices 15,16 which may then pass a constant current. The circuit may include an amplifying current mirror 30, 31, 32, 33 so that any change in current flowing in the first device 12 is an amplified version of the change in current in device 11. The circuit may be implemented in FET technology. The amplitude of the current produced by the circuit is dependent on the potential Vc which is controlled by external means.

Description: Technical Field of the Invention This invention relates to a current-controlling circuit and more particularly to a circuit for generating a controlled current from a dc supply, the potential of which may be subject to small variations. The invention is particularly suited to implementation in field-effect transistor (FET) technology. Background A voltage-current converter integrated circuit is described in GB-A-2118394 in which an input voltage generates a current which produces a voltage drop across a reference resistor. The circuit is so arranged that the accuracy of the voltage-current conversion is substantially independent of the spread in width of the resistance track of the reference resistor with which it was jointly manufacturing. In the circuit of GB-A-2118394, a second current is generated in a circuit branch, which is arranged in parallel with the reference resistor, producing a voltage drop across a second resistor having the same length and resistance per unit area as the reference resistor. The difference between the currents through the two resistors forms the circuit's output current. Summary of Invention This invention provides a circuit for generating a controlled current from a dc supply which may be subject to voltage variations. The particular devices in the circuit may be selected for example to give a controlled current which reduces in value with increasing supply potential or alternatively is invariant with increasing supply potential. The controlled current is derived by the circuit as the difference between two other currents, themselves generated from the supply. Each of these other currents varies with variations in the dc supply potential but the extent to which each varies depends on the characteristics of the transistors employed. By judicious selection of device characteristics, in accordance with the present invention, the rate of increase of one current with the increasing supply potential (say) can be made equal to, greater than or less than the rate of increase of the other current. These currents themselves are generated from the supply, so that the controlled current, generated as the difference between the two other currents, may be made to increase, stay the same or decrease as required. In practice, a current which increases with increasing supply potential is readily obtainable by conventional techniques so the most useful implementations of this invention are in producing a current which either reduces the increasing supply potential or is invariant with increasing supply potential. Accordingly, the present invention provides a current controlling circuit for producing a current defined by an input control voltage (Vc) comprising a dc supply having first and second supply rails defining an electrical potential (Vdd) therebetween, first active device means connected to the first rail for controlling a first current flowing to or from said first rail, the value of which is determined by said input control voltage, second active device means connected to the second rail for controlling a second current flowing from or to said second rail, the value of which is also determined by said input control voltage but is of a different value to that of said first current, third active device means connected to the first rail for passing a third current flowing to or from said first rail, wherein the three active device means are connected to each other such that the third current is the difference between the first and second currents, characterized in that the circuit is arranged and the active device means are selected according to their physical and electrical characteristics such that an increase in the dc supply potential causes an increase in the first current which equals or exceeds any increase caused in the second current, whereby the third current is either unchanged or reduced. Preferably, an increase in the dc supply potential causes an increase in the value of said first current which equals any increase in the value of said second current, whereby the value of said third current remains constant. This represents the simplest embodiment of the invention. Alternatively, the third current reduces in response to an increase in dc supply potential and a fourth active device means arranged to pass a fourth current, is connected in a mirror arrangement with the third active device means, this fourth current being invariant with the supply potential by virtue of the fact that the effect on the fourth current of the reduction in the third current is balanced by the effect on the fourth current of the increase in the supply potential. This can provide the benefit that the output current is now this fourth current, which does not represent part of the second current and so may be replicated if required without upsetting the operation of the circuitry controlling the first, second and third currents. Preferably, said first active device means comprises first, second and third active devices in combination, with an input connection to said first device for application thereto of said input control voltage, whereby an input current is generated in said first device of a value determined by said input control voltage and said second device is connected to said first device and to said third device so as to mirror said input current into said third device as said first current. This facilitates control of the output current by the control voltage since the control voltage may be connected to a control input of the first active device without any buffering or level translation. Alternatively, the first active device means comprises first, second and third active devices and an input connection to said first device for application thereto of the input control voltage and further comprises additional devices, the devices in combination forming a plurality of amplifying current mirrors, whereby an input current is generated in the first device of a value determined by the input control voltage, which input current is amplified by the amplifying current mirrors to form said first current, and whereby a small increase in the input current produces a larger increase in said first current. By this technique the first current is not controlled directly by a single electronic device but is instead dependent on a smaller, input current. Since this input current is smaller, the device controlling it (preferably a FET) can have larger physical dimensions. Due to the production variations inherent in the processing of these devices, a larger device can be made more accurately (as a proportion of its nominal size) than a smaller device can. Hence the current passed can be more accurately controlled to its desired value. Introduction to the Drawings Fig.1 is a schematic circuit diagram of a first embodiment of the invention. Fig.2 is a schematic graph showing electrical characteristics of some of the devices in the first embodiment of the invention. Fig.3 is a schematic circuit diagram of a second embodiment of the invention. Fig.4 is a schematic graph showing electrical characteristics of some of the devices in the second embodiment of the invention. Fig.5 is a schematic graph showing electrical characteristics of some of the devices in the second embodiment of the invention. Fig.6 is a schematic circuit diagram of a third embodiment of the invention. Detailed Description of the Invention Fig.1 shows one simple embodiment of the invention. P-channel Field-Effect Transistors (PFETs) 10,12,14 all have their sources connected to the more positive supply rail of the dc supply Vdd. N-channel Field-Effect Transistors (NFETs) 11,13 have their sources connected to the less positive supply rail of the dc supply which for convenience is grounded. Vdd is nominally at a level of 5 volts. FETs 10 and 11 are connected in series, as are FETs 12 and 13. FETs 10 and 12 have a common gate connection, as have FETs 11 and 13. An input terminal (i/p) is connected to the common gate connection of FETs 11 and 13. FET 14 is connected between the positive supply, as stated, and a node 22 between FETs 12 and 13. PFETs 10 and 14 both have their gates connected to their drains so that each functions effectively as a diode. Due to the diode effect, the potential at node 20 between FETs 10 and 11 is almost a constant voltage below Vdd. The value of the voltage dropped across PFET 10 depends on the physical characteristics of the device, ie its width, length and dopant densities (It should be noted that, in this art, the term "length" refers to the physical distance from the source to the drain and the term "width"′ refers to the other dimension of the source measured in the plane of the substrate on which the device is formed. Devices generally have a greater width than length). In this instance the physical parameters are selected to give a voltage drop of about 1.5 volts so that node 20 is at roughly 3.5 volts above ground. This potential will vary slightly around this nominal value when the current passed through FETs 10,11 varies. The value of the current passed by NFET 11, and hence series connected PFET 10, is controlled by an input voltage Vc applied to input terminal i/p. As the control voltage is increased, the current I1 passed by FETs 10,11 increases. The potential of node 20 is substantially constant but in actuality falls very slightly. The potential of node 22 is likewise controlled primarily by the voltage drop across the equivalent diode provided by PFET 14. PFETs 10,12,14 are all selected to have near-identical physical and electrical characteristics. Consequently, since FETs 10 and 14 are in diode configuration, the potential of node 22 is very close to that of node 20. This similarity in characteristics of the three PFETs is readily achievable since the circuit shown is processed on a single substrate so all three devices will be subject to similar processing variations. If the circuit were to be constructed from discrete devices it would be necessary to ensure device similarity by sampling or other techniques. Since the potential of node 22 is similar to that of node 20 and PFET 12 is physically similar to PFET 10, the current passed by PFET 12 is similar to that passed by PFET 10. The current I2 passed through NFET 13 is determined by its physical and electrical characteristics, its gate-source potential Vgs and its drain-source potential Vds. Its Vgs potential is equal to the Vgs potential of NFET 11, that is the applied control voltage Vc. Its Vds potential is approximately equal to that of NFET 11 since the potentials of nodes 20 and 22 are similar. However NFET 13 is tailored to have significantly different electrical characteristics from NFET 11 by careful selection of its physical dimensions. In this particular embodiment, NFET 13 has a greater width and a greater length than NFET 11 and also NFET 13 has a greater width-to-length ratio. This selection of relative dimensions is such that the characteristics of NFET 11 and NFET 13 are as shown in fig.2. In the upper regions of the curves (ie in the "saturation regions"), the two devices have similar slopes but the curve for NFET 13 is at a substantially higher current level than that of NFET 11. Consequently, as can be seen from fig.2, at any value of supply voltage Vdd between Vmin and the voltage at which device breakdown occurs (very much higher and consequently not shown) the current I2 taken by NFET 13 exceeds the current I1 taken by NFET 11 by a constant amount. However, it has already been shown that the current passed by NFET 11 is approximately equal to that passed by PFET 12. The difference between the current I2 in NFET 13 and the current I1 in PFET 12 is supplied by PFET 14. In this case, with the linear portion of the device characteristics of NFET 11 and NFET 13 arranged to be parallel, the current I3 supplied by PFET 14 is a constant value (I2-I1), independent of supply voltage variations. The characteristics of PFET 14 in this circuit arrangement are shown in fig. 2. This particular embodiment of the invention provides, therefore, an output current I3 in response to an applied control voltage Vc, the value of the current I3 being determined by the value of the control voltage but with the important advantage of being independent of supply voltage variations. The embodiment described above produces only one controlled current output. In certain instances, however, it may be desirable to have many constant current sources. Further, the current produced by the embodiment above may be difficult to incorporate into a circuit since it must flow into node 22. These two problems are solved by the alternative embodiment of fig. 3. In the embodiment of fig. 3, PFET 14 is not used directly to supply the output current. Instead, additional PFETs 15 and 16 are provided, connected across the supply potential Vdd with their gates connected to node 22 to operate as current mirrors. By this means, the current I3 through PFET 14 is replicated in the outputs of the PFETs 15 and 16. Clearly, this technique may be extended to any number of additional devices, not limited to two, in order to replicate the output current as necessary to suit design requirements. However, this raises a problem: If the current through PFET 14 remains constant in response to an increase in supply potential (as in the embodiment of fig. 1), it follows that the potential at node 22 increases by exactly the same extent as the supply potential. Accordingly, the currents passed by PFETs 15 and 16 would increase, since they are subject to an increased source-drain voltage and an unchanged source-gate voltage. Therefore, in order that the conventional current mirroring techniques can be employed to give a constant current through PFETs 15 and 16, it is necessary to modify the circuit so that an increase in supply potential causes a predetermined additional increase in potential at node 22, that is an increase in potential of node 22 over and above that which simply follows any increase in the supply potential. This is achieved by tailoring the circuit so that the current through PFET 14 falls by a controlled amount in response to an increase in supply potential. To achieve this, the devices used as NFETs 11 and 13 are processed slightly differently from those in the embodiment of fig.1, in order to give the characteristics shown in fig.4. This involves fabricating NFET 11 with reduced width and reduced length, causing the slope of the saturation region to be increased to that shown. This in turn produces the characteristic shown for PFET 14 of a falling current with increasing supply potential, since the current in PFET 14 is still constrained to be equal to the current in NFET 13 minus that in PFET 12 (equal to that in NFET 11). Since the current in PFET 14 falls with increasing supply potential, it follows that the source-drain voltage of PFET 14 must fall slightly with increasing supply potential, ie the potential of node 22 increases slightly in excess of any increase in the supply potential (and, conversely, decreases to a slightly greater degree than any decrease in the supply potential). If the potential of node 22 were to change by exactly the same amount as the supply potential then the currents passed by PFETs 15 and 16 would increase with increasing supply potential due to the increase in their source-drain potentials, as previously mentioned. However, since the potential of node 22 changes by slightly more than the change in the supply potential, the effective resistance of PFETs 15,16 is altered such that, as the supply potential increases, their collector currents can be maintained constant as the effect of the rising drain-source voltage is compensated by the falling gate-source voltage. This is further explained by fig.5 which shows characteristics of PFET 15 or 16 and in particular shows source-drain current Isd as a function of the supply potential Vdd for four different values of source-gate potential Vsg (ie four different values of the potential from the supply to node 22). It can be seen that, with Vsg fixed, an increase in Vdd leads to an increase in Isd; however, if Vsg is reduced slightly as Vdd increases, this can give a constant Isd since, as shown by fig.5, Isd reduces with reducing Vsg. There is still one problem with the embodiment of fig.3. To produce a device 11 with characteristics as shown in fig.2 or, more particularly fig.4, requires a very short length in the device NFET 11, this length being of the order of 1 micron. While it is possible to produce a device with a length in this region, production variations mean that it is difficult to control accurately the length produced. Variations in this length will cause undesirable variations in the device characteristics so that circuits produced would have to be individually sampled to ensure that an acceptable device had been produced. This procedure would be expensive and wasteful. An alternative approach is that shown in fig.6. This reproduces the circuit of fig.3 and includes further FETs 30-33 which function as amplifying current mirrors as follows: PFET 10 is arranged in diode configuration, ie with the gate connected to the drain, as in the previous embodiments. It therefore has a voltage drop from source to drain which is almost independent of current. The device is arranged, by judicious selection of its width, length and dopant densities, to have this voltage drop (roughly 1.5 volts) equal to substantially less than half of the nominal supply potential Vdd (5 volts). By similar means, the potential across NFET 31, which is also connected in diode configuration, is also arranged to be less than half the nominal value of Vdd. Considering PFETs 10 and 30, they have exactly the same source-gate voltages, determined by the saturation voltage of PFET 10 and equal to less than half Vdd. However, the current in 30 will be greater than that in 10 since the source-drain voltage of 30 is greater than that of 10 (since it is more than half Vdd compared to less than half Vdd). If Vdd is now increased, the potential across 10 will not increase significantly but the potential across 30 will increase almost by the increase in Vdd. Consequently the current in 30 will increase relative to that in 10. However, the current in 10 will increase since its current is controlled by 11 which has experienced an increase in drain-source voltage. This will cause a small increase in the source-gate voltage of 10 and this increase will be reflected in 30 since both devices 10 and 30 experience the same source-gate voltage. Combined with the effect mentioned above, the overall effect is that an increase in Vdd causes an increase in current in 10 and 11 and a larger increase in current in 30 and 31. The combination of devices 11,10 with devices 30,31 represents an amplifying current mirror since the current in 10 is amplified and mirrored as the current in 30. Similarly, 30,31 with 33,32 represent a further amplifying current mirror. This principle is repeated when the current in 30 and 31 is reflected and amplified by a similar mechanism into NFET 33 and PFET 32. The current in PFET 32 is then reflected into PFET 12 and subsequent operation is as in the embodiment of fig.3. The use of amplifying current mirrors means that the initial current in PFET 11, which affects the operation of the entire circuit, is smaller in magnitude than would otherwise be needed, so PFET 11 may have a greater length and hence be more accurately reproducible. In the embodiment of fig. 6, the various devices have the following widths and lengths, in microns: It can be seen from this table that device 13 is much larger than the others in the circuit, giving it the characteristic shown in Fig. 2. In each of the embodiments the amplitude of the controlled current is dependent on the value of Vc applied to the gates of devices 11 and 13, since the rate of change of the current in 11 with varying Vc is less than that of 13. However, the way the controlled current varies with Vdd will not be affected by variations in Vc. The mechanism for controlling Vc is not shown but any suitable technique known to those skilled in the art may be employed. This invention is primarily directed towards producing a current which is dependent on the value of a control voltage Vc but independent of supply potential Vdd. However, it is quite possible, using any of the three embodiments shown, to produce a circuit to give a current which reduces with increasing supply potential by selecting devices with suitable widths, lengths and dopant densities, if such a characteristic is deemed desirable. Details of design methods for calculating the physical characteristics required for the devices to be employed are not given here but any technique familiar to a person skilled in the art, such as mathematical modelling or simulation, may be used.

Claims: A current-controlling circuit for producing a current defined by an input control voltage (Vc) comprising a dc supply having first (40) and second (41) supply rails defining an electrical potential (Vdd) therebetween, first active device means (10,11,12,30-33) connected to the first rail for controlling a first current (I₁) flowing to or from said first rail (40), the value of which is determined by said input control voltage (Vc), second active device means (13) connected to the second rail (41) for controlling a second current (I₂) flowing from or to said second rail (41), the value of which is also determined by said input control voltage (Vc) but is of a different value to that of said first current (I₁), third active device means (14) connected to the first rail (40) for passing a third current (I₃) flowing to or from said first rail (40), wherein the three active device means are connected to each other (22) such that the third current (I₃) is the difference between the first current (I₁) and the second current (I₂); characterised in that the circuit is arranged and the active device means are selected according to their physical and electrical characteristics such that an increase in the dc supply potential causes an increase in the first current (I₁) which equals or exceeds any increase caused in the second current (I₂), whereby the third current (I₃) is either unchanged or reduced. A circuit as claimed in claim 1 wherein an increase in the dc supply potential causes an increase in the value of said first current (I₁) which equals any increase in the value of said second current (I₂), whereby the value of said third current (I₃) remains constant. A circuit as claimed in claim 1 or claim 2 in which the third current (I₃) reduces in response to an increase in dc supply potential and a fourth active device means (15 or 16), arranged to pass a fourth current (I₄), is connected in a mirror arrangement with the third active device means (14), this fourth current (I₄) being invariant with the supply potential (Vdd) by virtue of the fact that the effect on the fourth current (I₄) of the reduction in the third current (I₃) is balanced by the effect on the fourth current (I₄) of the increase in the supply potential (Vdd). A circuit as claimed in any preceding claim in which said first active device means comprises first (11), second (10) and third (12) active devices in combination, with an input connection (42) to said first device (11) for application thereto of said input control voltage (Vc), whereby an input current is generated in said first device (11) of a value determined by said input control voltage (Vc) and said second device (10) is connected to said first device (11) and to said third device (12) so as to mirror said input current into said third device (12) as said first current (I₁). A circuit as claimed in any of claims 1 to 3 in which said first active device means comprises first (11), second (10) and third (12) active devices and an input connection (42) to said first device (11) for application thereto of the input control voltage and further comprises additional active devices (30,31,32,33), the devices in combination forming a plurality of amplifying current mirrors, whereby an input current is generated in the first device of a value determined by the input control voltage (Vc), which input current is amplified by the amplifying current mirrors to form said first current (I₁), and whereby a small increase in the input current produces a larger increase in said first current (I₁). A circuit as claimed in claim 4 or 5 in which said second active device means comprises a fourth active device (13) and said third active device means comprises a fifth active device (14), said input connection being further connected to said fourth device (13), whereby said second current (I₂) is generated in response to application thereto of said input control voltage (Vc), the current through the device being formed as the combination of the current through the third device (12) and the fifth device (14) whereby the current through the fifth device (14) is said third current (I₃) as aforesaid. A circuit as claimed in claim 6 in which said active devices are each provided by an individual FET and in which the FETs comprising the second (10), third (12) and fifth (14) devices are identical or substantially identical to each other but the active region of the FET comprising the fourth (13) device has a greater width than that of the FET comprising the first (11) device. A circuit as claimed in claim 7 in which the active region of the fourth FET (13) is both longer and has a greater width-to-length ratio than the active region of the first FET (11).

Inventors: GARDNER PETER ALAN, GARDNER, PETER ALAN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1988
Main classification: G05F3
IPC codes: G05F3