Low voltage reference current generating circuit

Abstract: An integrated circuit current generator for operation at low power supply voltages is disclosed. The circuit utilises two transistors (Q3,Q4) connected as a current mirror with two further cascode transistors (Q5,Q6). The invention provides an additional transistor (Q9) connected to limit the voltage across one of the current mirror transistors. In this way, fluctuations in the reference currents is reduced even where the supply voltage fluctuates.

Description: This invention relates to a low voltage reference current generating circuit, capable of providing either a current source or current sink of a reference current which is defined by a current setting resistor. Current generating circuits are well known in the art and in their simplest form consist of a pair of matched current mirror transistors, each having a controllable path and a control node for controlling conduction of the controllable path. In bipolar technology, the control node is the base and the controllable path is from collector to emitter. In MOS technology, the control node is the gate and the controllable path is the source/drain channel. The present invention is concerned particularly but not exclusively with bipolar technology. One of the transistors has a current setting resistor connected in its controllable path and the other transistor has its control node connected to the control node of the one transistor and also into its own controllable path. When a current flows through the current setting resistor, the same current is caused to flow in the controllable path of the other transistor and can be used to drive a suitable output transistor to sink or source a reference current related to that current through the area ratio of the output transistor and the current mirror transistors. In practical terms, the basic current mirror circuit has many limitations. One of these is that its impedance is too low for it to act as a perfect current source or sink when connected to other circuitry. To increase the impedance, it is common to include a pair of matched cascode transistors connected respectively to the current mirror transistors. Such a circuit is shown in Figure 1a. In Figure 1a, references Q3 and Q4 denote a first set of matched transistors. Their bases are connected together at the connection point denoted as node 41. In addition, the base of the transistor Q3 is connected to its collector. Reference numerals Q5 and Q6 denote a second set of matched transistors. The transistor Q5 has its collector connected to the emitter of transistor Q3 and its emitter connected to ground. Its base is connected to its own collector at node 42 and to the base of transistor Q6. The transistor Q6 has its collector connected to the emitter of transistor Q4 at node 43 and its emitter connected via a current setting resistor R to ground. Reference numerals Q8 and Q7 denote output transistors connected in cascode for sinking the reference current Ir. Each output transistor has its base connected to receive the base current being injected into the transistor of the associated set (Q8 for Q3 and Q7 for Q5). The circuit is such that the reference current Ir is intended to match a current I flowing through the current setting resistor R. Reference numerals Q1 and Q2 denote bias transistors which have their bases connected together and their emitters connected to the supply voltage Vdd. In addition, the base of the transistor Q2 is connected to its collector. The collectors of bias transistors Q1 and Q2 are connected respectively to the collectors of the first matched transistors Q3 and Q4, the latter connection being denoted as node 44. Other known current generating circuits are illustrated for example in EP-A-155720 in the name of Philips which illustrates a cascode current source arrangement having a current mirror circuit with two current paths comprising transistors and resistors. Reference is also made to DE-C-3335379 which describes an integrated low voltage constant current source with a transistor for amplifying differential current and controlling a pair of bias transistors. The present invention seeks to provide particularly a current source or current sink circuit which can operate down to a relatively low voltage (down to about 1.4 volts) and which has a high DC PSRR (power supply rejection ratio). The DC PSRR is defined as the ratio of the change in current source/sink reference current to the change in DC power supply. According to the present invention there is provided a circuit for providing a reference current comprising: first and second matched transistors each having a control node and a controllable path and connected so that with a current setting resistor in the controllable path of the second transistor, the current set in that controllable path is related to the difference in voltage characteristics between the first and second transistors and to the value of the current setting resistor; third and fourth matched transistors each having a controllable path connected respectively to the controllable paths of the first and second transistors and their control electrodes connected together; and a set of output transistors connected in the circuit to be driven to supply said reference current in dependence on the set current, characterised in that said circuit further comprises: a fifth transistor connected in the circuit with its controllable path between a bias node related to a first supply voltage level and a node set at one voltage characteristic relative to a second supply voltage level so as to maintain the voltage across one of the third and fourth transistors at a value which is independent of the first supply voltage level thereby to reduce the magnitude of changes in the reference current as a function of the first supply voltage. In a first embodiment the transistors are bipolar n-p-n transistors; the first supply voltage level is a positive value Vdd and the second supply voltage level is ground. The bases of the first and second transistors are connected together and the base of the first transistor is connected to its collector. The emitters of the third and fourth transistors are connected respectively to the collectors of the first and second transistors and the collector and base of the fourth transistor are connected together. With this arrangement, the base of the fifth transistor is connected to the collector of the third transistor so as to maintain the collector emitter voltage of the third transistor at a value which is independent of the supply voltage. The collector of the fifth transistor is connected to the bias node of the circuit and the emitter of the fifth transistor is connected to the bases of the first and second transistors, which are at a voltage level of one base-emitter voltage Vbe above the second supply voltage level (ground). The collector emitter voltage of the third transistor is thus held at 2Vbe above ground and this reduces the so-called "early effect", described later. In a second embodiment, the base of the first transistor is connected to the collector of the second transistor while the base of the second transistor is connected to the collector of the first transistor so that the first and second transistors are cross-coupled. In this embodiment, the emitter of the fifth transistor is connected to the base of the first transistor. Also, the collector of the fourth transistor is connected to its base. In these arrangements, the early effect of the collector-emitter voltage of the fourth transistor is reduced since its collector is now connected to a point which is held at 2Vbe above ground (Vbe of the first transistor and Vbe of the third transistor). In the described embodiment, the bias node for the fifth transistor is provided by two bias transistors each being of opposite type to the first to fifth transistors i.e. p-n-p where the first to fifth transistors are n-p-n and having their emitters connected to the first supply voltage level and their collectors connected respectively to the collectors of the third and fourth transistors. The bases of the bias transistors are connected together to provide the bias node for the fifth transistor. It will be appreciated that the term "matched transistors" used herein denotes transistors whose collector currents are substantially the same in the same conditions. Other characteristics of the transistor may vary, in particular the base emitter voltages where the transistors are bipolar transistors. For a better understanding of the present invention, and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings, in which: Figure 1a is a circuit diagram of a known current source; Figure 1b is a circuit diagram of the current source of Figure 1a modified by having cross-coupled current source transistors; Figure 2 is a circuit diagram of a preferred embodiment of the present invention, with a starting circuit and a capacitor for frequency compensation; Figure 3 is a graph of the change in reference current Ir with power supply voltage Vdd for the circuit of Figure 2; Figure 4 is a graph of the mismatch between the set current I and the reference current Ir in the circuit of Figure 2; Figure 5 is a graph similar to that of Figure 3 for the circuit of Figure 1a; Figure 6 is a graph similar to that of Figure 3 for the circuit of Figure 1b; Figure 7 is a circuit diagram of an embodiment of the invention in which the cascode transistors are not cross-coupled; Figure 8 is a graph similar to that of Figure 3 for the circuit of Figure 7; Figures 9a and 9b are circuit diagrams of embodiments of the present invention similar to that of Figure 2 for sourcing reference current; and Figure 10 shows the usual I-V characteristics of a bipolar transistor. Figure 1a shows a conventional current mirror circuit which has already been described above with reference to the prior art. Figure 5 is a graph which shows the variation in reference current with power supply for such a circuit. As can be seen from Figure 5, the DC PSRR at 10µA nominal reference current is 10.564µA - 8.344µA 7V - 1.7V = 418nA/V. Figure 1b is a circuit diagram of a circuit which is similar to that of Figure 1a except that the first and second transistors are cross-coupled. That is, the base of the transistor Q5 is connected to the collector of the transistor Q6 and the base of the transistor Q6 is connected to the collector of transistor Q5. With this arrangement, the voltages at nodes 42 and 43 are fixed at 1Vbe above ground, where Vbe is the normal base emitter voltage of a bipolar transistor, typically 0.7V. The cross coupling of the transistors Q5,Q6 also minimises the mismatch between the reference current Ir and the set current I as will be described in more detail hereinafter. There follows an analysis of the circuit of Figure 1b. The collector emitter voltage across the current mirror transistor Q3, VceQ3 is equal to the supply voltage Vdd less the voltage drops in the path through Q1,Q6 and the current setting resistor R. That is, the collector emitter voltage across the transistor Q3 is given by the following equation: VceQ3 = Vdd - VbeQ1 - VbeQ6 - IR where I is the current flowing through the resistor R; and VbeQ1 and VbeQ6 in each case designates the base emitter voltage of the respective transistors Q1 and Q6. VbeQ1 is normally equal to 1Vbe drop, that is normally .7V. Due to the cross coupling arrangement of the transistors Q5,Q6, closed loop analysis gives VbeQ5 = VbeQ6+IR. Taking VbeQ5 = 7V, then VceQ3 = Vdd - .7 - .7 = Vdd - 1.4. Consider now Figure 10 which shows the normal I-V characteristic of a bipolar transistor. That is, Figure 10 shows the variation of collector current Ic with the collector emitter voltage Vce for three different values of base current IB1, IB2 and IB3. By extrapolating backwards the linear portions of the curve, they are seen to meet at a point on the voltage axis at a value Va which is called the early voltage, where Va = Ic dIc/ dVce and a typical value for Va is 50 to 100V. The variation of Ic with Vce is called the early effect. The influence of the early effect can be expressed by the following equation: where Ic is the collector current, Is is the saturation current, Vce is the collector emitter voltage, Va is the early voltage, Vbe is the base emitter voltage and V T is the thermal voltage. For normal design work, the equation is normally shortened to its first order form of However, the present inventors have now discovered that the term 1+ Vce / Va which qualifies Is can have an adverse effect. Inserting the results of the analysis of the circuit of Figure 1b into Equation 2 gives a value for the collector current of Q3, IcQ3 as follows: Thus, in practice the current flowing through Q3 and thus the current through Q5 and the output transistors Q7 and Q8 is modulated by the collector emitter voltage of the transistor Q3 which is, through Eqn. 1a, very much related to the supply voltage Vdd. Figure 6 shows the variation of the reference current with power supply for the circuit of Figure 1b. As can be seen from Figure 6, the DC PSRR at 10µA nominal is 9.445µA - 10.653µA 7V - 1.7V = 228nA/V. Thus, although there is an improvement over the circuit of Figure 1a, the DC PSRR is still of an unacceptable order of magnitude. Figure 2 shows a circuit according to a preferred embodiment of the present invention. In this circuit, like numerals designate like parts as in Figures 1a and 1b. That is, there is a first pair of cross-coupled matched transistors Q5,Q6, a second pair of matched transistors Q3,Q4 and a set of two output transistors Q7,Q8. These are connected as described above with reference to Figure 1b. The circuit also comprises bias transistors Q1,Q2 each having their emitter connected to a supply voltage Vdd and their collectors connected to respective collectors of the second pair of matched transistors Q3,Q4. In the circuit of Figure 2 there is a further transistor Q9 having its base connected at node 44 to the collector of one of the second pair of transistors Q3 and having its own collector connected to the bias node 40 provided by the bias transistors Q1,Q2 where their bases are connected together. In the circuit of Figure 2, the emitter of the transistor Q9 is connected at the junction of the base of one of the first pair of transistors Q5 and the collector of the other of the first pair of transistors Q6. The addition of the transistor Q9 eliminates the so-called early effect by fixing the collector voltage of the transistor Q3 at node 44 to a value which is 2Vbe above ground, VbeQ5+VbeQ9. This effectively fixes the collector emitter voltage of the transistor Q3 at 2Vbe, and thus renders it independent of the supply voltage Vdd. Thus, the collector current of the transistor Q3 is now independent of variations in the supply voltage Vdd. Thus, as the collector current IceQ3 flows through the transistor Q5 and is reproduced at the output transistor Q7, it will remain independent of changes to the supply voltage Vdd. The reproduction at the output transistor Q7 will of course depend on the ratio of areas between Q7 and Q5, as described later. The variation of the reference current Ir with the set current I can be seen from the following analysis of the current in the loop Q3, Q6 and R of Figure 2. In this loop, VbeQ3 + VbeQ6 + IR = VbeQ4 + VbeQ5. Thus, IR = VbeQ4 + VbeQ5 - (VbeQ3 + VbeQ6). As explained above VceQ6 is set at 1Vbe above ground (by Q5) and VceQ4 is set at 2Vbe above ground (by tying the collector of Q4 to its base and thus to the base of Q3). The fixing of Vce of Q3 by Q9 has been explained. Thus, the equation can be used in its shortened, supply voltage independent form. Also, the transistors are selected so that VbeQ4 = VbeQ5 and VbeQ3 = VbeQ6 while VbeQ5 ≠ VbeQ6. Therefore where Is1 and Is2 are the saturation currents of Q4 and Q3 respectively. Thus, where A1 is the area ratio between Q3 and Q4 or Q5 and Q6. In one example, A1 = 4. Thus, the reference current generation can be controlled by altering R or A1 depending on requirements. The reference current output by the circuit follows the set current, in dependence on the area ratio between Q7 and Q5, A2. Namely, Ir = A2I. In the example described herein A2 = 1. In practice, a simple current analysis of the circuit of Figure 2 shows that the reference current differs from the load current I by a small value equivalent to the base current Ib of a transistor, typically 20nA. Thus, although there is a mismatch between set and reference currents this is minimised by the cross coupling of the transistors Q5,Q6. This analysis is shown in Figure 2 where current values are marked aside the collector emitter and base currents of each transistor. In the circuit of Figure 2, a starting circuit is shown indicated by a broken line defining block S. This starting circuit comprises a transistor Q10 having its emitter connected to the supply voltage Vdd, its base connected to the junction of the bases of the bias transistors Q1,Q2 and its collector connected to the base of a further transistor Q12. The further transistor Q12 has its emitter connected to ground and its collector connected via a resistor R2 to the supply voltage Vdd. A start up transistor Q11 has its base connected downstream of the resistor R2, its collector connected to its base and its emitter connected to drive the base of the further transistor Q9 of the current source circuit. Figure 3 also shows a capacitor CC for frequency stabilisation purposes between the base and emitter of the transistor Q9. Figure 3 is a graph showing the variation in reference current with supply voltage for the circuit of Figure 2. As can be seen from Figure 3, the DC PSRR at 10µA nominal is 10.045µA - 10.003µA 7V - 1.7V = 7.9nA/V. This is a significant improvement over the equivalent values for the circuits of Figures 1a and 1b. Figure 4 shows the current mismatch between the load current I and the reference current Iref as being 20nA at 2V supply. Figure 7 is a diagram of a circuit according to another embodiment of the present invention which is the same as that of Figure 2 except that the start up circuit is not illustrated and except that the transistors Q5 and Q6 are not cross-coupled but instead are arranged as in the prior art circuit of Figure 1a. This circuit nevertheless represents a significant improvement in the DC PSRR as illustrated by Figure 8 from which it can be seen that the DC PSRR is 10.272µA - 10.334µA 7V - 1.7V = -11.7nA/V. Figures 9a and 9b are circuit diagrams of circuits arranged to act as a current source of a reference current. Like numerals designate like parts as in Figure 2 and the circuits function in an analagous way and have the same advantages as described above with reference to Figure 2. It will be appreciated that the main function of the transistor Q9 is to hold the collector voltage of Q3 independent of the supply voltage. The transistor Q9 could achieve this function with its emitter connected to any of the nodes in the circuit which are set at 1Vbe above ground, particularly node 45 between the output transistors Q7,Q8. The only problems which can arise with other connections of Q9 are those of starting up the circuit but these could be overcome with more start up circuitry. An additional advantage of the circuit is that the collector of Q4 is likewise held independent of the supply voltage through its connection to the base of Q3. Thus, the "early effect" of both Q3 and Q4 are overcome, rendering the reference current largely independent of supply voltage. Furthermore, the circuit can function down to a supply voltage level of 2Vbe+1Vce, i.e. normally 1.7V. However, if different transistors are used having lower Vbe, this would be as low as 1.4V. Although the explanation given above relates only to bipolar transistors, it will be appreciated that a similar concept would be utilised in a CMOS current generating circuit.

Claims: A circuit for providing a reference current comprising: first (Q5) and second (Q6) matched transistors each having a control node and a controllable path and connected so that with a current setting resistor (R) in the controllable path of the second transistor (Q6), the current set in that controllable path is related to the difference in voltage characteristics between the first (Q5) and second (Q6) transistors and to the value of the current setting resistor (R); third (Q3) and fourth (Q4) matched transistors each having a controllable path connected respectively to the controllable paths of the first (Q5) and second (Q6) transistors and their control electrodes connected together; and a set of output transistors (Q7,Q8) connected in the circuit to be driven to supply said reference current in dependence on the set current, characterised in that said circuit further comprises: a fifth transistor (Q9) connected in the circuit with its controllable path between a bias node related to a first supply voltage level (Vdd) and a node set at one voltage characteristic relative to a second supply voltage level (GND) so as to maintain the voltage across one of the third (Q3) and fourth (Q4) transistors at a value which is independent of the first supply voltage level (Vdd) thereby to reduce the magnitude of changes in the reference current as a function of the first supply voltage. A circuit as claimed in claim 1 wherein each of the first to fifth transistors (Q5,Q6,Q3,Q4,Q9) and the output transistors are bipolar transistors and wherein each voltage characteristic is the base-emitter voltage of a transistor. A circuit as claimed in claim 2 wherein the transistors (Q5,Q6,Q3,Q4,Q9) are n-p-n bipolar transistors and wherein the first supply voltage level is a positive supply voltage Vdd and the second supply voltage level is ground. A circuit as claimed in claim 2 wherein the transistors (Q5,Q6,Q3,Q4,Q9) are p-n-p bipolar transistors and wherein the first voltage supply level is a negative voltage and the second voltage supply level is ground. A circuit as claimed in claim 2 wherein the transistors (Q5,Q6,Q3,Q4,Q9) are p-n-p bipolar transistors and wherein the first voltage supply level is ground and the second voltage supply level is a positive voltage supply Vdd. A circuit as claimed in claim 3 wherein the first (Q5) and second (Q6) transistors have their bases connected together and the base of the first transistor (Q5) is connected to its collector, wherein the base of the fifth transistor (Q9) is connected to the collector of the third (Q3) transistor, the emitter of the fifth transistor (Q9) is connected to the bases of the first (Q5) and second (Q6) transistors, the emitter of the first transistor (Q5) is connected to ground and the emitter of the second transistor (Q6) is connected to ground via said resistor (R). A circuit as claimed in claim 3, 4 or 5 wherein the base of the first transistor (Q5) is connected to the collector of the second transistor (Q6) and the base of the second transistor (Q6) is connected to the collector of the first transistor (Q5), the base of the fifth transistor (Q9) is connected to the collector of the third transistor (Q3), the emitter of the fifth transistor (Q9) is connected to the base of the first transistor (Q5), the emitter of the first transistor (Q5) is connected to the second voltage supply level (GND) and the emitter of the second transistor (Q6) is connected, via the resistor (R), to the second voltage supply level (GND). A circuit as claimed in any of the claims 3 to 7 wherein the bias node is provided by two bipolar transistors (Q1,Q2) of opposite polarity to said transistors (Q5,Q6,Q3,Q4,Q9), having their bases connected together at said bias node, their emitters connected to the first supply voltage level (Vdd) and their collectors connected respectively to the collectors of the third (Q3) and fourth (Q4) transistors. A circuit as claimed in any of claims 2 to 6 wherein the base of the fourth transistor (Q4) is connected to its collector.

Inventors: LOH GEE HENG, NG SOLOMON KENGLONG, LOH, GEE HENG, NG, SOLOMON KENGLONG
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS PTE LTD.
Application year: 1993
Main classification: G05F3
IPC codes: G05F3