HIGH EFFICIENCY BASE CURRENT HELPER

Abstract: High efficiency base current helper to improve the accuracy of current mirrors particularly useful for current mirror utilizing relatively low beta bipolar junction transistors. The high efficiency base current helper utilizes two feedback loops, the first attempting to force the bias rail voltage to a mirror reference voltage and the second sensing excess current in the first loop and forcing it to match a reference level. This causes the first loop to be biased with no more excess current than desired for any process or temperature condition yielding dramatic reductions in wasted bias current. A specific exemplary embodiment is disclosed.

Description: HIGH EFFICIENCY BASE CURRENT HELPER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to base current helpers, and more particularly to base current helpers as are used in such circuits as current mirrors to help overcome the effects of base current in a collector circuit of the mirroring device. 2. Prior Art Current mirrors are commonly used in many circuits to provide one or more currents equal to or proportional to a reference current for biasing as well as various other purposes . A commonly used current mirror is shown in Figure 1. The current mirror shown consists of diode connected transistor Ql, the base and collector of which are connected to the base of transistor Q2. To the extent that the transistors are high gain transistors so that base currents can be ignored and transistors Ql and Q2 are substantially identical, the collector current in transistor Q2 will equal the collector current in transistor Ql, namely the reference current II. In fact, however, the base current for both transistors Ql and Q2 flows in the collector circuit of transistor Ql, so that considering base currents, the mirrored current in the collector of transistor Q2 will be equal to II - 2IB, where II is the reference current in the collector circuit of transistor Ql and IB is the base current in each of transistors Ql and Q2. To the extent the gain of the transistors is limited, the mirrored current will be in error. For instance, a simple current mirror of this type suffers a 10% error in a 1-to-l current mirror with betas (beta is the ratio of collector current to base current of a transistor) of 20. Also, it is frequently desired to make transistor Q2 n times as large as transistor Ql so that for the same base-emitter voltage as transistor Ql, transistor Q2 would conduct approximately n times the reference current II. However, with limited beta transistors, the simple current mirror will not produce ratios significantly above 1-to-l effectively. Also, current mirrors are frequently used to mirror a reference current on a 1-to-l or other basis to a plurality of transistors rather than the single transistor Q2 of Figure 1, increasing the error of the current mirror because of the increased number of base current components in the collector circuit of transistor Ql . The foregoing base current induced errors are not limited to bipolar junction transistors, but are particularly severe in the case of lateral PNP bipolar junction transistors because of the finite beta of such devices. As for many present day processes, these lateral devices have betas that may fall into the single digits . To prevent accuracy problems in current sources, base current helpers are typically employed. These buffers absorb the excess base current at the cost of biasing the additional buffer. Unfortunately, the present state of the art biases these helpers in class A, meaning that the standing current in the helpers must exceed the worst case possible demands to keep the current sources alive. This can be several times greater than the nominal required, and can require in excess of 10% of the current source value. A typical prior art current mirror with base current helper may be seen in Figure 2. Here, the current for the bases of transistors Q3 and Q4 is set by transistor Q5 responding to the collector voltage of transistor Q3. Thus the current in the collector of transistor Q4 is equal to 12 minus the base current of transistor Q5. Because of the isolation of the base current of transistor Q4 from the collector circuit of transistor Q3 , this circuit is much more tolerant to the use of a transistor to which the current is mirrored (Q4) which is n times larger than the mirroring transistor (Q3). However, even this mirror with helper, while improved, will produce errors in excess of 5% for a 10-to-l current ratio, assuming the same exemplary betas of 20. Figure 3 is a circuit diagram for another prior art base current helper. The circuit of Figure 3 has the advantage of using NPN devices for the feedback loop. Such devices typically have significantly higher betas, and even if they didn't, their currents can be set independent of the currents in the mirror devices Q13 and Q14, reducing the error when compared to the mirror with helper of Figure 2. Current ratios in excess of 10-to-l are possible with the circuit of Figure 3 at accuracies at around 1%. In this circuit, the reference current 16 is fed to the collector of transistor Q13. Transistor Q16 acts to force the collector of transistor Q13 to bias at a potential relative to the base of transistor Q13 determined by the voltage V2. Thus, the base to collector voltage of transistor Q13 is forced to a known potential difference. When this occurs, the collector current through transistor Q13 roughly matches the reference current, and with the exception of Early voltage effects, the current through the collector of transistor Q14 will equal n times the reference current 16 (transistor Q14 being n times as large as transistor Q13) . The NPN base current helper of Figure 3 may be found in a December 1993 IEEE Journal of Solid-State Circuits, Vol. 28, No. 12, pp. 1246-1253. The right half of Figure 8 on p. 1250 of the Journal corresponds to Figure 3 of this disclosure. This circuit is the preferred existing method for building high ratio, high accuracy PNP current mirrors in processes with lateral PNPs . V2 can be made to be any reference, but the most common practices are to replace it with NPN diodes, Schottky diodes, or a ground referenced voltage to make the voltage across current source 16 supply-voltage independent. The major disadvantage to this solution (and all other prior art the inventor has found) lies in the fact that the structure composed of Q16 and whatever implementation of voltage V2 is used must be biased by fixed current source 18. This current source must be set at a level that is determined by the absolute worst case base current of Q13 and Q14, which varies dramatically over processing and temperature for most processes . The excess current required by the circuit of Figure 3 is primarily the collector current of transistor Q16, which is basically wasted except for the fact that it keeps transistor Q16 active. As will be seen in the detailed description of the invention herein, the present invention comprises a class of circuits that can be employed to eliminate this wasted excess current while still assuring that transistor Q16 remains active. SUMMARY OF THE INVENTION High efficiency base current helper to improve the accuracy of current mirrors particularly useful for current mirror utilizing relatively low beta bipolar junction transistors are disclosed. The high efficiency base current helper utilizes two feedback loops, the first attempting to force the bias rail voltage to a mirror reference voltage and the second sensing excess current in the first loop and forcing it to match a reference level. This causes the first loop to be biased with no more excess current than desired for any process or temperature condition yielding dramatic reductions in wasted bias current. A specific exemplary embodiment is disclosed. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a circuit diagram illustrating a simple, commonly used current mirror. Figure 2 is a circuit diagram for a typical prior art current mirror with base current helper Figure 3 is a circuit diagram for an additional prior art base current helper. Figure 4 is a circuit diagram which illustrates an exemplary high efficiency base current helper in accordance with the present invention DETAI ED DESCRIPTION OF THE PRESENT INVENTION Now referring to Figure 4, an exemplary high efficiency base current helper in accordance with the present invention may be seen. In this circuit, transistor Q6 is the mirroring transistor, mirroring a current approximately n times the reference current 14 to transistor Q7 , which is n times larger than transistor Q6. The transistor Q7 is shown operating into a load R3 , though frequently the current or currents produced are used to bias or drive other transistor circuits. In that regard, the n-to-1 current ratio might alternatively represent mirroring the current in transistor Q6 to n transistors, each the same size as transistor Q6, or to some different number of transistors to provide current sources totaling n times the current in transistor Q6. In the circuit shown in Figure 4, transistor Q9 corresponds generally to transistor Q16 of Figure 3, and the combination of transistor Q8 and voltage source V3 is an alternative to the voltage source V2 of Figure 3. The voltage source V3 sets the base voltage of transistor Q8 which, because of the common emitter connection between transistors Q8 and Q9, and assuming transistor Q9 is active, sets the base voltage of transistor Q9 substantially equal to V3 also. The voltage V2 of Figure 3 and the alternative of voltage V3 and transistor Q8 of Figure 4 provide a level shift between the collectors of transistor Q13 and transistor Q6, respectively, and the bases of those transistors, respectively. Without this level shift, provided as shown or by some further alternate way, transistor Q13 and transistor Q6 would saturate. The base current for the mirrored transistors Q6 and Q7 is provided by transistor Q8 , resistor R4 and transistor Q12. In particular, if the collector voltage of transistor Q6 begins to rise, indicating that transistor Q6 is conducting more than the sum of the base current of transistor Q9 and reference current 14, transistor Q9 will turn on harder, raising the voltage of the emitter of transistor Q8 to turn the same off a little to decrease the base current of transistor Q6 to reduce the current flow therethrough to the desired operating point . However, rather than to allow the current in transistor Q9 to vary as required, and particularly to be high enough to allow shifting of an adequate amount of current from transistor Q9 to transistor Q8 to provide the worst case base currents to transistors Q6 and Q7 considering the worst case process and temperature variations, the current through transistor Q9 is monitored by the voltage drop across resistor Rl . This voltage drop is compared to a reference voltage drop across resistor R2 , the reference voltage drop being provided as a result of the reference current 15 through diode connected transistor Qll . When the voltage drop across resistor Rl is less than the voltage drop across resistor R2 , transistor Q10 will be turned on, supplying base current to transistor Q12 to turn the same on further. When the voltage drop across resistor Rl is approximately equal to the voltage drop across resistor R2 , transistor Q10 will provide limited current therethrough to limit the base current to transistor Q12. The net effect is that the conductive state of transistor Q12 will be varied as required so that the current through transistor Q9 will be relatively fixed, and preferably fixed at a relatively low value, substantially equal to the reference current 15 if resistors Rl and R2 are equal. This means that the current through transistor Q12 need not always be adequate to provide the base current for transistors Q6 and Q7 under the worst temperature and process conditions, but rather that the current in transistor Q12 is made to be only that required for the present temperature and process conditions for that specific integrated circuit. This is to be compared with the reference current source 18 in Figure 3, wherein that reference current must be set to provide the worst case base currents through transistors Q13 and Q14, even though the worst process variations are not frequently encountered and even though the circuit is not operating, or for that matter may never actually operate, at the worst case temperature specification for the part. Thus, while the circuit of Figure 3 provides a feedback loop to control the base currents for transistors Q13 and Q14, the circuit of Figure 4 has a second feedback loop to control the current in the first feedback loop, and more particularly to limit the current in the first feedback loop to only that required under the then present conditions to supply the required base currents through transistor Q8 plus a little more current to keep transistor Q9 active. For current mirrors running at a few milliamps, the savings through the use of the adaptive biasing of Figure 4 can measure in the hundreds of mircoamps when compared against the prior art . Although the second feedback loop typically requires stabilization, this is not usually a penalty when compared to the prior art. The second feedback loop is fairly easy to stabilize with the addition of resistor R4. This resistor permits the use of the Miller effect to multiply the loading the capacitor Cl on the collector of transistor Q10. Also, it may be noted that the present invention is normally used as part of a larger integrated circuit. Therefore, it is not usually necessary to actually separately provide the reference current source 15, transistor Qll and resistor R2 as most bias generators have a suitable supply reference voltage already available. Another benefit of the high efficiency base current helper of the present invention is the flexibility it gives the designer in controlling turn-on transients. Through careful selection of loop components, the designer can tailor the speed of the turn-on ramp and determine whether an underdamped or overdamped response is delivered. This can benefit those who need to limit spurious transmissions emitted when their devices are first activated, or in situations where the devices are powered down between transmission slots as a matter of protocol, as in TDMA. While the present invention is particularly beneficial in circuits using modest or low beta transistors such as lateral PNP transistors such as is typical of most high frequency RF bipolar processes in use today, most other biasing structures could also benefit by incorporation of the invention. Further, current mirrors with high mirroring ratios could also benefit from the efficiency improvements. Of course, the efficiency gains are particularly important in portable battery powered and wireless communication devices where they may reduce current consumption and extend battery life. The embodiment of the present invention shown in Figure 4 is shown in conjunction with the supply of base current for PNP devices Q6 and Q7. In that regard, note that the phrases "current source" and "source of current" are used herein and in the claims in a most general sense, as is common in the technology, to include both current sources and current sinks. Thus, for instance, while 14 and 15 and Figure 4 are referred to as current sources, the same actually provide a current sinking function in the specific embodiment shown. Similarly, it will be understood that statements such as the base currents for transistors Q6 and Q7 are supplied by transistor Q8, actually relate to the flow of current from the bases through transistor Q8 for the embodiment shown. Of course, the circuit of Figure 4 can be reversed by changing the direction of the current sources and the conductivity types of each of the transistors and of course reversing the polarity of the applied voltage to the circuit, so as to provide base current to what now would be NPN mirror transistors corresponding to transistors Q6 and Q7. In the foregoing description, the reference current 14 to be mirrored has been described as if the same is a fixed or predetermined current. While a constant reference current 14 represents one typical application of the present invention, it should be noted that the present invention is not limited to such applications. By way of example, current source 14 may be a trimmable source based on characteristics of other circuits, so that the actual value of the current 14 under any given condition may vary significantly circuit to circuit. Further, the current source 14 may in fact even be variable in some applications. Similarly, while the circuit of Figure 4 is shown as an n-to-1 ratioing circuit, whether by a single transistor Q7 n times larger than transistor Q6 , or a plurality of transistors making up the equivalent of a single transistor Q7 n times the size of transistor Q6 , the effective value of n may be variable either with time or circuit to circuit. By way of example, the current 14 may be mirrored to a number of individual circuits, one or more of which may have a power down or standby mode wherein the device or devices in that circuit to which the current is mirrored may at times be inactive. By way of a further example, one or more of the transistors in a circuit to which the current 14 is mirrored may be disconnectable for circuit trimming purposes, such as during the manufacturing process. While the present invention has been disclosed and described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.

Claims: C AIMS What is claimed is : 1. A base current helper comprising: a first transistor having an emitter, a base and a collector, the collector being coupled to a first source of current; a first feedback circuit coupled to the base and the collector of the first transistor and to a second source of current, the first feedback circuit directing a part of the current in the second source of current from the base of the first transistor to maintain the voltage on the collector of the first transistor; a second feedback circuit coupled to the first feedback circuit and to the second source of current, the second feedback circuit controlling the amount of current provided by the second source of current responsive to the base current requirements of the first transistor. 2. The base current helper of claim 1 wherein the first feedback circuit comprises a second transistor responsive to the voltage on the collector of the first • transistor to divert some of the current of the second source of current away from the base of the first transistor. 3. The base current helper of claim 2 wherein the second feedback circuit controls the amount of current provided by the second source of current responsive to the base current requirements of the first transistor by controlling the current in the second transistor. 4. A base current helper comprising: first and second sources of current, the second source of current being controllable; first and second transistors of opposite conductivity type, each having an emitter, a base and a collector, the collector of the first transistor being coupled to the first source of current and to the base of the second transistor; the second source of current being coupled to the base of the first transistor and to the emitter of the second transistor; 5. The base current helper of claim 4 wherein the feedback circuit comprises a resistor in series with the collector of the second transistor and a comparison circuit controlling the second source of current to maintain the voltage drop across the resistor equal to a predetermined voltage. 6. The base current helper of claim 5 wherein the predetermined voltage is a voltage generated by a third source of current passing through a second resistor. 7. The base current helper of claim 5 wherein the second source of current is a third transistor having an emitter, a base and a collector, the comparison circuit controlling the base of the third transistor. 8. The base current helper of claim 5 wherein the second source of current is coupled to the base of the first transistor through a voltage level shifting circuit. 9. The base current helper of claim 7 wherein the collector of the third transistor is coupled to the base of the first transistor and to the emitter of the second transistor. 10. A current mirror for mirroring current to at least one additional transistor comprising: a first transistor having an emitter, a base and a collector, the collector being coupled to a first source of current, the emitter and base of the first transistor being coupled to the emitter and base of at least one additional transistor to which the current in the first transistor is mirrored; a first feedback circuit coupled to the base and the collector of the first transistor and to a second source of current, the first feedback circuit directing a part of the current in the second source of current away from the base of the first transistor to maintain the voltage on the collector of the first transistor; a second feedback circuit coupled to the first feedback circuit and to the second source of current, the second feedback circuit controlling the amount of current provided by the second source of current responsive to the base current requirements of the first transistor. 11. The current mirror of claim 10 wherein the first feedback circuit comprises a second transistor responsive to the voltage on the collector of the first transistor to divert some of the current of the second source of current away from the base of the first transistor. 12. The current mirror of claim 11 wherein the second feedback circuit controls the amount of current provided by the second source of current responsive to the base current requirements of the first transistor by controlling the current in the second transistor. 13. A current mirror for mirroring current to at least one additional transistor comprising: first and second power supply terminals; first and second sources of current coupled to the second power supply terminal, the second source of current being controllable; first and second transistors of opposite conductivity type, each having an emitter, a base and a collector, the collector of the first transistor being coupled to the first source of current and to the base of the second transistor, the emitter of the first transistor being coupled to the first power supply terminal, the emitter and base of the first transistor being coupled to the emitter and base of at least one additional transistor to which the current in the first transistor is mirrored; the second source of current being coupled to the base of the first transistor and to the emitter of the second transistor; a feedback circuit coupled to the collector of the second transistor and to the second current source controlling the second source of current to maintain a substantially constant current in the second transistor. 14. The current mirror of claim 13 wherein the feedback circuit comprises a resistor coupled between the collector of the second transistor and the first power supply terminal, and a comparison circuit controlling the second source of current to maintain the voltage drop across the resistor equal to a predetermined voltage. 15. The current mirror of claim 14 wherein the predetermined voltage is a voltage generated by a third source of current passing through a second resistor coupled to the first power supply terminal. 16. The current mirror of claim 14 wherein the second source of current is a third transistor having an emitter, a base and a collector and being of the same conductivity type as the second transistor, the emitter of the third transistor being coupled to the second power supply terminal and the collector of the third transistor being coupled to the emitter of the second transistor and to the base of the first transistor, the comparison circuit controlling the base of the third transistor.

Inventors: CARGILL ROBERT S, CARGILL, ROBERT, S.
Applicants: MAXIM INTEGRATED PRODUCTS, MAXIM INTEGRATED PRODUCTS, INC.
Application year: 1997
Main classification: G05F3
IPC codes: G05F3, H03F3