TECHNICAL FIELD

This invention relates to analog electronic circuits, and more particularly to current sources for supplying controlled current to electronic devices.

BACKGROUND ART In some electronic circuits it is desirable or necessary to have a source of current that is regulated to maintain a constant current output. For example, in analog signal processing integrated circuits, data converter circuits, such as analog to digital converters and digital to analog converters, require a fixed current reference that does not change with changes in load or temperature. Any change in the fixed current reference causes inaccuracies in the data conversion process. One circuit which is currently being used to provide such a constant current source is shown in FIGURE 1 . In FIGURE 1 a conventional bias servo network provides a constant current source by driving a bipolar transistor 10 with an operational amplifier 20 to maintain a voltage across a resistor 30 which is equal to a reference bandgap voltage V _A . It is clear that the resistor 30 must be precisely controlled in order to accurately set the amount of output current. However, the resistors fabricated in integrated circuits can not be controlled to greater than 15-20% accuracy due to variations in the fabrication process. Therefore, in order to accurately set the current output level the resistor 30 must be an external resistor or, alternatively, the resistor 30 must be laser trimmed. External resistors require far greater space and additional labor, since they must be installed in a separate operation. Likewise, trimming resistors is costly and time consuming.

In addition to the requirement for a current source that remains constant with changes in temperature, some circuits require a current source that compensates for changes that occur within the circuit due to temperature. For example, analog signal processing integrated circuits in which bipolar amplifiers are used are typically biased with a current source commonly referred to as a Proportional To Absolute Temperature ("PTAT") current source. PTAT current sources, as the name implies, vary the current output in proportion to changes in temperature. Bipolar amplifiers typically have a gain,

g,„R ₇ ; where R, is the load resistance, g,„ is equal to (q\ _c )/(kT), q= 1 .6 x 10 ^{" |q} coulomb (i.e.. is the electron charge), I _c is the source current, k - 1.38 x 10 ^"23 joule per K (i.e., BoltzmaniTs constant), and Tϊs current temperature in C°. Variations in the performance of component of the amplifier due to changes in the ambient temperature are compensated by the changes which occur in the current supplied by the current source.

FIGURE 2 is an illustration of a conventional PTAT current source. A bandgap voltage reference V, is used to generate a reference voltage that is applied to the base of a bipolar transistor 50. The transistor is biased by a resistor 60 to maintain a current that is proportional to the reference voltage. As is the case with temperature independent current sources such as the source shown in FIGURE 1 , resistor 60 must be a precision resistor in order to have a precision current output.

Furthermore, it is often necessary to provide one or more constant current sources and one or more PTAT current sources within the same integrated circuit. For example, analog signal processing integrated circuits typically may include for separate classes of circuits: ( 1 ) bipolar amplifiers; (2) CMOS amplifiers; (3) Power amplifiers; and (4) data converter circuits. The bipolar amplifiers require a PTΛT current source having a first known relationship between the output current and the ambient temperature. A second PTAT current source having a second known relationship between the output current and the ambient temperature is required for supplying current to CMOS amplifiers. A constant current source is required for the power amplifiers to achieve constant output power. Also, data converter circuits require fixed references that are independent of the temperature, variations in the process, and fluctuations and changes in the voltage supply.

The bandgap reference voltage V _Λ is typically provided by a conventional bandgap reference circuit as shown in FIGURE 3 which includes two pairs oϊ bipolar transistors Q, Q-,, Q ₃ , Q _A . In one of these two pairs Q,, Q\, one bipolar transistor Q _: is preferably substantially larger than the other Q,. The difference in the size of the two transistors Q,. Q\ results in a difference in the current density with equal current flowing within each transistor. The difference in current density with equal current flowing results in a difference in the voltage drop across the base to emitter of each transistor.

V _{/ *} V,„, . A resistor R ₆ coupled between the emitter of the larger transistor Q\ and

ground provides a resistance across which the voltage Δv _A( , is dropped. An additional resistor R _s is coupled to the collector of the Q ₂ . The bandgap reference voltage equals:

V„, = V _Λt + Δv _Ar (R ₅ + R /R,

Therefore, the reference can be designed to be independent of temperature providing the temperature coefficient of V _Λc/ cancels the temperature coefficient of Δv,,,. which can be scaled by setting the value of R ₅ .

Furthermore, the PTAT reference voltage for use in generating a constant current source is typically provided by the bandgap reference circuit using the same two transistors and each of the same resistances. In addition, a third resistor is provided coupled to the emitter of Q ₄ . The PTAT reference voltage _riA , is taken at the emitter to Q ₄ . The PTAT reference voltage is equal to:

-. _/ = V _Ae , + Δv _Λ( , (R ₄ + R ₅ + R ₆ )/R ₆ .

Therefore, by setting each of the resistors R ₄ , R ₅ , R^ to a desired value with respect to each other, the change in V ,, _rA , over temperature can be set to a desired value which will result in a PTAT current source that properly compensates for temperature variations in the circuits to which the PTAT current is supplied.

The use of the constant current source circuit of FIGURE 1 and the PTAT current source circuit of FIGURE 2 together with the reference voltage circuit of FIGURE 3 provides reasonably good current sources. However, if each current source is independent, then a conventional analog signal processing integrated circuit would require at least one external or internal resistance for each current source. Each such resistance must be laser trimmed or otherwise calibrated to set the current level with a sufficient accuracy. External resistors are relatively large with respect to integrated resistors and require additional labor to install. Accordingly, it would be desirable to provide a current source that is capable of providing more than one constant current source, as well as more than one PTAT current

source without the need for more than one external or laser trimmed resistor. The present invention provides such a current source.

DISCLOSURE OF INVENTION

The present invention is a multi-purpose current source which provides both a PTAT and a constant current source and which requires only one precision external or laser trimmed resistance.

In accordance with the present invention, the PTAT constant current circuit includes a differential amplifier having one input coupled to a V,, _//(/ reference voltage and the other input coupled to a V _Λ( , scaling circuit. Alternatively, the other input may be coupled directly to V _Aχ . The tail current for the differential amplifier is held constant at the current level of an associated constant current source based upon V _Λ ,. Therefore, the amount of current output from the PTAT current source will be dependent upon the current of the constant current source and the ratio of V _/1//(/ to V _Λ , rather than upon a resistance value. By setting the scaling circuit appropriately, the current that flows through the output leg of the differential amplifier in the PTAT current source when the ambient temperature is equal to 25° C will be equal to one half the tail current through the differential amplifier, and thus one half the current output from the constant current source. Since the PTAT current source only requires resistors in the scaling circuit and the value of each of these scaling circuit resistors need be controlled only with respect to each other, there is no need for a precision resistance within the PTAT current source.

The details of the preferred embodiment of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.

BRIEF DESCRIPTION OF DRAWING

The objects, advantages and features of this invention will be more readily appreciated from the following detailed description, when read in conjunction with the accompanying drawing, in which:

FIGURE 1 is a conventional constant current source circuit.

FIGURE 2 is an illustration of a conventional PTAT current source.

FIGURE 3 is an illustration of a conventional bandgap voltage reference circuit.

FIGURE 4 is an illustration of a Multi-purpose Current Source Circuit in accordance with one embodiment of the present invention. FIGURE 5 illustrates the relationship between temperature, V _/ , V^.,. and Δv _Λι ,.

FIGURE 6 is an alternative embodiment of a current source in which a current mirror circuit is coupled to the source of an N-Channel FET to provide a current source rather than a current sink as shown in FIGURE 4.

FIGURE 7 is an illustration of an embodiment of the present invention in which an additional resistance is used to generate an additional PTAT Voltage having a different temperature characteristic.

Like reference numbers and designations in the various drawings refer to like elements.

BEST MODE FOR CARRYING OUT THE INVENTION Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than limitations on the present invention.

Overview

The present invention is a current source which is capable of providing one or more temperature independent current sources (hereafter referred to as "Constant" Current Sources), and one or more temperature dependent current sources (hereafter referred to as "PTAT" Current Sources). A single precision resistance is required to precisely set the voltage levels of multiple Constant Current Sources and the PTAT Current Sources.

FIGURE 4 is an illustration of a Multi-purpose Current Source Circuit 100 in accordance with one embodiment of the present invention. The circuit of FIGURE 4 includes a Bandgap Reference Circuit 101 , a Constant Current Control Circuit 103, and a PTAT Current Control Circuit 105. The heart of the invention lies in the coupling of the constant current circuit to the PTAT Current Control Circuit and the architecture of the PTA T Current Control Circuit. The Bandgap Reference Circuit 101 is essentially

conventional and is explained in detail to provide a complete understanding of the operation of the present invention.

The Bandgap Reference Circuit 101 provides a constant current reference voltage or bandgap reference voltage V _hj , to the Constant Current Control Circuit 103 and a PTAT reference voltage V ,, _rA -, to the PTAT ^' Current Control Circuit 105. Both _h) , and

V,, _/ are derived from the sum of a bandgap voltage drop which occurs between a first and second terminal of a three terminal bandgap device, such as the base and the emitter of two bipolar transistors Q, and Q ₂ . Three factors affect the voltage drop that occurs between the base and emitter of a bipolar transistor: ( 1 ) ambient temperature in which the device is operating, (2) the physical dimensions of the transistor, and (3) the amount of current flowing out the emitter. The combination of the physical dimensions of the transistor and the amount of current that flows determine the current density. Transistors with the same current density operating at the same ambient temperature will have an equal voltage drop between base and emitter. The greater the current density, the greater the voltage drop.

In the preferred embodiment of the present invention, Q ₂ is eight times as large as Q, . Therefore, when the same amount of current flows through both Q, and Q ₂ , the current density within the bandgap of Q is one eighth the current density within the bandgap of Q,. This results in a smaller voltage V _At ,, across the base to emitter junction of Q _^ than the voltage V _Λ(>/ across the base to emitter junction of Q, . This difference is used to generate V _Λ and V _{; /1/} in the following manner.

The collectors of Q, and Q ₂ are each coupled to two series coupled resistance devices, such as resistors, R ₈ and R,, and R ₄ and R ₅ , respectively. Each pair of series resistors is coupled to the emitter of another pair of bipolar transistors, Q ₃ and Q ₄ . The transistors Q, and Q,, are base and collector coupled in a current mirror configuration which ensures that the same amount of current flows through both Q, and Q ₄ . Accordingly, the same amount of current will flow through each leg of the current mirror. That is. the same amount of current will flow through the pair of resistors R ₈ and R _C) , and R ₄ and R„ and through the collectors and emitters of Q, and Q.,. A resistor, R is coupled between the emitter of Q, and Q-.. The emitter of Q, is also coupled to ground

(i.e.. the negative port of the power supply). Therefore, any difference Δv _Λι , between the voltages V _A( . _; and V,, _[ will be dropped across R^.

The voltage V, is taken from the point of connection between R ₄ and R ₅ . Therefore:

V _Λ = V _lκ , + Δv _Λι , |(R _S + R ₆ )/R eq. 1

This can be understood by noting that:

V _Λ , = V,,,, + I _bg ( + R ₅ ) eq. 2

where; I _b „ is the current through Q ₂ .

In the preferred embodiment of the present invention, the values of pairs R ₈ and Rq. and R ₄ , and R _s are equal. Therefore, the voltage at the collectors of both Q, and Q ₂ must be equal. Therefore:

V„„ = V _tc , + Δv _Λl . eq. 3

Furthermore, as stated above:

Δv _Al . = V _M - V _hc2 eq. 4

Substituting eq. 4 into eq. 3 to solve for V _e2 :

, - V _At „ eq. 5

Substituting eq. 5 into equation 2:

V,„, = V _Ar , + I _bE (R ₀ + R ₅ ) eq. 6

Also noting:

ι _b8 = ^ΔV Λ, / Re ^e - ⁷

Substituting eq. 7 into eq. 5 results in eq. 1.

In the preferred embodiment of the present invention, the sizes of Q, and Q ₂ are selected such that the temperature effects on V _hc/ are compensated for by the temperature effects on Δv _Λf . FIGURE 5 illustrates the relationship between temperature, V _Λ , V _Λt , ₂ , and Δv _Λc ,. It can be seen that as the temperature rises, both V _Aι ,, and V _Ae , drop. However, V _Λ drops at a lesser rate than V _Ai _,,. Therefore, the change in Δv _Al , is directly proportional to temperature. That is. as temperature increases, Δv _A[ . also increases. Therefore, by properly selecting the dimensions of Q, and Q ₂ , and the relative dimensions of R ₅ and

R ₆ , the affect of temperature on Δv _A( . will exactly offset the affects of temperature on V _Λ( ,,. It should be noted that the factor [(R ₅ + R /R ₆ ] increases the affect that Δv _A( , has on the overall value of V _/)χ . Therefore, even though the affect of temperature on Δv _Λι , is not as great as the affect that temperature has on V _Ac .,, the factor [(R ₅ + R /R provides emphasis to allow the affects to cancel. It should also be noted that the values of each of the resistors R ₄ , R ₅ , and R^ are important only with respect to each other. Therefore, process variations do not affect the accuracy of the present circuit.

As shown in FIGURE 4, a resistor R ₄ is coupled to the resistor R _s to add additional resistance to the load across which V _{n I} is developed. Accordingly, it will be clear that:

\ _riAI = V„ _L , + Δv _Λt , [(R ₄ +R, + R R

The addition of R to the equation increases the influence of Δv _A( , on V _{r/ (/} , and thus makes the influence exerted by Δv _Aι . dominant over the influence of V _/>( .,. Therefore, _HI will be directly proportional to temperature (i.e., will rise with a rise in temperature). The relationship between V and temperature will be a function of the value of R with respect to R _s and R,.

The V _Λ output from the Bandgap Reference Circuit 101 is coupled to the input of the Constant Current Control Circuit 103. The Constant Current Control Circuit 103 includes an input operational amplifier OP, . V _Λf , is coupled to the non-inverting input of OP,. The output from OP, is coupled to the gate of an N-Channel field effect transistor (FET) N _| . The drain of N, is coupled to the drain of a P-Channel FET P, which is coupled to three other P-Channel FETs P ₂ -P ₄ in a current mirror configuration. That is, the gates of P ₂ -P ₄ are coupled together and the sources are coupled together. Thus, the same volume of current that flows through one must flow through all. A load resistance R, is coupled to the source of N,. A resistance R ₂ is coupled to the drain of P ₂ . as is the inverting input to OP,. Thus, OP, attempts to drive the current mirror comprising P ₂ -P to maintain a voltage at the non-inverting input which is equal to V _A (i.e., which is coupled to the non-inverting input). The current that flows through P ₄ is considered the output current from the Constant Current Control Circuit 103. This current may be used as a source for any device which requires a current source that is independent of temperature. It will be apparent to those skilled in the art that by precisely controlling the value of R ₂ , this output current can be precisely controlled. Each of the other resistors need only be controlled with respect to one another. For example, the resistance of R ₄ need only be controlled with respect to the values of R _s and R ₍₎ . Thus, the process variation affects on R ₄ are the same as each of the other resistors. Therefore, the output current is unaffected by process variations which affect the resistance of R ₄ -R ₆ . Those of ordinary skill in the art will understand that relative values of resistance within an integrated circuit may be controlled very precisely. However, the absolute values of resistances is more difficult to control.

As stated above, the heart of the present invention lies in the coupling of the Constant Current Control Circuit 103 to the PTAT Current Control Circuit 105. The current that flows through P ₃ is coupled to the PTAT Current Control Circuit 105 and couples the Constant Current Control Circuit 103 to the P TAT Current Control Circuit 105. The current that flows through an N-Channel device N ₂ . The N-Channel device N-, is one half of a current mirror which sets the tail current for a differential amplifier. For example, in the embodiment of the present invention shown in FIGURE 4, the two N-

Channel FETs N ₄ and N ₅ are configured as a differential amplifier. The sum of the

current through these two FETs is held constant by the current mirror comprising N ₂ and

N ₆ *

Also coupled to the PTAT Current Control Circuit 105 is the ,, _1A , voltage and the V _hi , voltage output from the Bandgap Reference Circuit 101 . The voltage ,, _IΛI is coupled to a first input to the differential amplifier (i.e., the gate of N,) _* The voltage V _{Λ ,} is coupled to a scaling circuit which in one embodiment comprises a second operational amplifier OI\. as shown in FIGURE 4. The output from the scaling circuit is coupled to the second input to the differential amplifier. The scaling circuit provides a means for regulating what portion of the current that flows through N ₆ will flow through N ₄ . and thus through N,.

The voltage V _hg is coupled to the non-inverting input to the operational amplifier OP,. The output of OP ₂ drives an N-Channel FET N ₃ which sets a current through two resistances R, and R ₇ . The point of connection between R ₃ and R ₇ is coupled to the inverting input to OP _: . Thus, the current through R ₃ and R ₇ is held constant by OP ₂ at a level that causes the voltage across R ₇ to remain constant. By setting the relative values of the resistor R, with respect to the resistor R ₇ , the voltage applied to the gate of N ₄ is preferably set to equal the voltage V _IA , which occurs at a particular ambient operating temperature. In the scaling circuit shown in FIGURE 4, the output voltage from the scaling circuit to the gate of N ₄ is greater than the bandgap reference voltage V _A . However, in an alternative embodiment, the voltage applied to the input of the differential amplifier may be any voltage equal to ( 1 + R ₃ /R ₇ )V _/ψ and that provides the desired current output from the differential amplifier. It should be apparent to one of ordinary skill in the art that since the ratio of R ₃ to R ₇ determines the voltage at the gate of N ₄ , as opposed to the absolute value of either R ₃ or R ₇ , process variations will not affect the precision with which the voltage at the gate of N ₄ can be set.

In one embodiment of the present invention, OP ₂ scales V _A to match the V _iA/ at 25 C°. Therefore, at 25 C° approximately half the current that flows through N-, will flow through each of the FETs of the differential pair. In accordance with one embodiment of the present invention, the output of the PTAT Current Control Circuit 105 is taken as a current sink through N,. Alternatively, a current source may be provided by coupling a current mirror circuit to the source of N _s as shown in FIGURE

6. As the temperature increases, V _A remains constant, V _F/A/ increases, and additional tail current is steered through N,. The steering is linear and depends only on the change in V _{/)/ /} and the device characteristics of N ₄ and N It can be seen that the current which flows through the device N ₅ is proportional to absolute temperature and is closely related to the constant current which flows through P

Summary

It will be apparent to those skilled in the art that the present invention provides both a PTAT current and a temperature independent constant current source which require only one precision resistance (i.e., R ₂ , in the embodiment shown in FIGURE 4). Additional PTAT voltages and bandgap voltages may be generated by the Bandgap

Reference Circuit 101 and applied to additional PTAT Current Control Circuits or Constant Current Control Circuits to generate additional current sources. For example, as shown in FIGURE 7, an additional resistance R ₄ . may be used to generate an additional PTAT voltage which has a different temperature characteristic (i.e., relationship between temperature and voltage). Such additional PTAT voltages may be applied to additional PTAT Current Control Circuits which are essentially identical to the circuit shown in FIGURE 4. By varying the ratio of the resistors R ₃ and R ₇ , the relative amount of current that flows through each portion of the differential amplifier may be varied to bias the differential amplifier at any operating temperature independent of any other PTAT current sources. That is, the second input to the differential amplifier may be set such that equal current flows through each leg of the differential amplifier at virtually any operating temperature.

A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the differential amplifier of the present invention may be any differential type amplifier capable of providing an output current that is proportional to the ratio of the voltages applied to each of two inputs, and wherein the total current through the differential amplifier is equal to a regulated current. In addition, the scaling circuit may be any voltage divider circuit which is capable o\ ^' providing a useful range of voltage levels based upon the

bandgap reference voltage V _A Furthermore, while the present invention is described as being implemented using bipolar transistors and field effect transistors, a broad range of active devices may be used in place of these devices. For example, MOSFETs, vacuum tubes, etc. may be used. In addition, any device which provides resistance may be used in place of the resistors illustrated and described above. Still further, the resistors of the present invention may be any resistive element, such as wire wound resistors, carbon composite resistors, carbon film resistors, integrated circuit resistors deposited upon a substrate, etc. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.