05/385271

05/27/1975

08/03/1973

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RCA Corporation (New York, NY)

323/315

327/513, 323/907, 327/535, 323/901

G05F3/26; G05F3/08; G05F1/58

323/4,9,19,22T,68,69 307/270,297,310

3648153	REFERENCE VOLTAGE SOURCE	March 1972	Graf
3683270	INTEGRATED CIRCUIT BILATERAL CURRENT SOURCE	August 1972	Mattis
3753079	FOLDBACK CURRENT LIMITER	August 1973	Trilling
3777251	CONSTANT CURRENT REGULATING CIRCUIT	December 1973	Cecil et al.
3794861	REFERENCE VOLTAGE GENERATOR CIRCUIT	February 1974	Bernacchi
3825778	TEMPERATURE-SENSITIVE CONTROL CIRCUIT	July 1974	Ahmed
3831040	TEMPERATURE-DEPENDENT CURRENT SUPPLIER	August 1974	Nanba et al.

Goldberg, Gerald

Whitacre, Eugene Rasmussen Paul M. J.

What is claimed is

1. A current regulator which supplies a substantially constant current over a substantial range of operating temperatures, comprising:

2. A current regulator according to claim 1 wherein:

3. A current regulator according to claim 1 wherein:

4. A current regulator according to claim 1 wherein:

5. A current regulator which supplies a substantially constant current over a substantial range of operating temperature comprising:

6. A current regulator which supplies a substantially constant current over a substantial range of operating temperatures according to claim 5 wherein:

7. A current regulator which supplies a substantially constant current over a substantial range of operating temperatures according to claim 6 wherein:

8. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures comprising:

9. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 8 wherein:

10. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 9 wherein:

11. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 9 wherein:

12. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 9 wherein:

13. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 9 wherein said constant current ratio amplifier comprises:

14. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures comprising:

15. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 14 wherein said constant current ratio amplifier comprises:

16. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 15 wherein:

17. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 16 wherein:

18. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 16 wherein:

19. A circuit for temperature compensation of a substantially constant current amplifier over a substantial range of operating temperatures according to claim 16 wherein:

BACKGROUND OF THE INVENTION

This invention relates to constant current sources and more specifically to temperature independent constant current sources.

An aspect of some concern relating to the use of transistor integrated circuits in many applications is the temperature dependence of many of the operating parameters of the transistor device employed such as, for example, its collector current. The problem of temperature variation of operating parameters becomes more acute when such integrated circuits are subjected to the relatively broad temperature variations encountered in hostile environments or in uses in which large numbers of integrated circuits operate in relatively close proximity to one another prohibiting effective cooling of some of the devices on the integrated circuits. Under such circumstances, integrated circuit arrangements which provided temperature compensation to insure relatively temperature independent characteristics are of great aid to the designer.

SUMMARY OF THE INVENTION

In accordance with the present invention a current regulator which supplies a substantially constant current comprises a source of temperature dependent voltage, wherein the temperature dependent voltage is proportional to the voltage established across a semiconductor P-N junction structure in the conductive state, first resistance means coupled in parallel with the source of temperature dependent voltage for drawing current equal to the voltage applied thereacross divided by the resistance of the first resistance means, and serially coupled P-N junction structure and second resistance means coupled in parallel with the source of temperature dependent voltage and the first resistance means such that the potential across the serially coupled P-N junction structure and second resistance means equals the potential across the source of temperature dependent voltage, the current in the serially coupled P-N junction structure and second resistance means thereby having a temperature coefficient which is proportional to the negative of the temperature coefficient of current in the first resistance means for substantially compensating for the temperature coefficient of current in the first resistance means over a substantial range of operating temperature.

The invention may best be understood by reference to the following explanation and drawings of which:

FIG. 1 illustrates a partly block and partly schematic circuit diagram of the first embodiment of a temperature compensated current regulator in accordance with the invention:

FIG. 2 illustrates a schematic diagram of a second embodiment of a temperature compensated current regulator in accordance with the invention; and

FIG. 3 illustrates a schematic diagram of a third embodiment of a temperature compensated current regulator in accordance with the invention .

In a first embodiment of the invention illustrated in FIG. 1, a source of voltage equal to the forward biased base-emitter voltage V _BE of a semiconductor junction 8 is connected with its positive terminal connected through a resistor 29 to point G. The negative terminal of V _BE voltage source 8 is connected to point G. A diode 26 has its anode connected to the positive terminal of V _BE voltage supply 8 and its cathode connected to one terminal of a resistor 35. The other terminal of resistor 35 is connected to point G. A load circuit 60 which requires a substantially temperature invariant current over a substantial range of operating temperatures is connected serially between a source of supply voltage B+ and the junction of the anode of diode 26, the positive terminal of V _BE voltage supply 8 and a terminal of resistor 29.

In the embodiment of the invention illustrated in FIG. 1, V _BE supply 8 is characterized by its forward biased base-emitter junction voltage which decreases with increasing temperature according to the relation, ##EQU1## in which V _BE is the base-emitter junction voltage drop, V _go is the junction drop at absolute zero, V _BEO is the junction drop at the reference temperature T _O , n is a constant relating to the construction of the device (typically 1.5 for a double diffused silicon transistor), k/q is a physical constant with a value of approximately 8.66 × 10 ^{- 5} V/°C, T is the temperature of the junction, I _c is the collector current at temperature T (in degrees Kelvin), I _co is the collector current at temperature T _O (in degrees Kelvin) and base-emitter voltage V _BEO and ln indicates the natural or Napierian logarithm of the succeeding parenthetic expression. The formula, its derivation and applications are explained in some detail in, for example, R. J. Widlar, "An Exact Expression for the Thermal Variation of the Emitter-Base Voltage of Bipolar Transistors," Proc. IEEE (Letters), Vol. 55, pp. 96-97, January, 1967, and J. S. Brugler, "Silicon Transistor Biasing for Linear Collector Current Temperature Dependence," IEEE Journal of Solid State Circuits (correspondence), Vol. SC-2, No. 2, pp. 57-58, June, 1967, and references cited therein.

As the voltage across V _BE supply 8 varies, the current supplied to resistor 29 must vary so that the voltage across V _BE supply 8 equals the resistance of resistor 29 multiplied by the current flowing through resistor 29. Hence, it can be seen from an examination of the foregoing expression that the current through resistor 29 will vary directly with the voltage across V _BE supply 8.

It may also be seen from the foregoing relation that the voltage across V _BE supply 8 varies inversely with the temperature of the base-emitter junction (the latter two terms of the equation are relatively small compared to the V _go and V _BEO terms of the equation as shown in the first of the references hereinbefore cited).

Since the voltage across the anode-cathode junction of diode 26 is governed by the same equation as the voltage across V _BE supply 8, it may be seen that as temperature increases, the anode-cathode junction voltage drop of diode 26 and hence the effective resistance of the series combination of diode 26 and resistor 35 decreases with increasing temperature.

For a reasonably broad range of operating temperatures it is desired to make the current flowing through load impedance 60 substantially invariant with respect to temperature. Therefore, it is desired to make the change in the current drawn by the series combination of diode 26 and resistor 35 with respect to temperature equal the negative of the change in the current flowing through resistor 29 with respect to temperature.

From a consideration of the derivative of the foregoing V _BE relation with respect to temperature, the fact that the voltage across V _BE supply 8 approximately equals the current through load impedance 60 less the current through diode 26, divided by the value of resistor 29, the temperature about which compensation is desired, (for purposes of this discussion room temperature of 25° C or 298° K), the diode equation relating the junction voltage drop V _BE of a forward biased semiconductor P-N junction to the saturation current I _s of that junction ##EQU2## (wherein all other symbols are as defined in the foregoing V _BE equation), and the fact that it is desired to make the derivative of the current flowing through load impedance 60 with respect to temperature equal to zero, a relation involving the ratio of resistors 29 and 35 may be achieved.

For example, at room temperature the first V _BE relation can be approximated by 0.7 volt for silicon double diffused junction devices. The derivative with respect to temperature of the base-emitter junction voltage, V _BE , is approximately -2 millivolts per Centigrade (or Kelvin) degree for silicon junction devices.

It can be seen from the equation relating saturation current to forward biased base-emitter junction voltage that the voltage across resistor 35 may be written as (assuming equal saturation currents for V _BE voltage supply 8 and diode 26), ##EQU3## where I _S8 is the effective diode current of V _BE source 8.

The derivative of this expression with respect to temperature is ##EQU4## Choosing a value for I _S8 /I _D26 at room temperature of 4 (the designer may, of course, choose some other value for this current ratio at room temperature according to particular circuit considerations) makes δV _R35 /δT approximately 1.2 × 10 ^{- 4} volts/C°. Since the value of resistor 35 divided into the voltage change with respect to temperature across resistor 35 must equal the negative of the value of resistor 29 divided into the voltage change across the V _BE voltage supply 8, which was determined above to be approximately -2 millivolts per Centigrade degree, a ratio of the value of resistor 29 to the value of resistor 35 of approximately 16.6 results. Other circuit considerations, such as the voltage applied between terminals B+ and G and the desired current in load impedance 60 set the maximum and minimum values of the resistors themselves. From these considerations, values for the resistors can be chosen which yield good results over the selected range of temperatures.

In a second embodiment of the invention illustrated in FIG. 2, direct current operating voltage is supplied from a positive voltage source B+ through point C to the junction of one terminal of each of three resistors 10, 11, and 12.

The other terminal of resistor 10 is connected to the collector and base electrodes of a transistor 14. The other terminal of resistor 11 is connected to the collector and base electrodes of a transistor 15. The other terminal of resistor 12 is connected to the emitter of a transistor 23. The emitters of transistors 14 and 15 are both coupled to the base of transistor 23 and to the emitter of a transistor 21. It should be noted that the described connections of transistors 14 and 15 comprise one of many configurations representing a diode with its anode analogous to the collector-base connection of the transistor and its cathode represented by the emitter of the transistor. It may be noted that transistors 14 and 15 may be of either conductivity type since they will function only as diodes. When they are of the other conductivity type, their emitters will be the anodes of the diodes and their collector-base terminals will be the cathodes.

The base of transistor 21 is coupled to the collector of transistor 23 and this connection represents an input terminal, point D, of a current regulator. The collector of transistor 21, point A, is an output terminal of the current regulator.

Point D is connected to the collector of a transistor 24. Point A is connected through a terminal A' to the base of transistor 24 and to the collector of a transistor 22. The emitter of transistor 24 is connected to the base of transistor 22 and to one terminal of a current monitoring resistor 29. The emitter of transistor 22 and the other terminal of resistor 29 are coupled to point G. Transistors 22 and 24 and resistor 29 comprise a V _BE source which provides drive current to the current regulator and a path for its output current through point G.

The collector of a transistor 25 is connected to the junction of the base of transistor 22, the emitter of transistor 24 and resistor 29. The base of transistor 25 is also connected to the junction of resistor 29, the emitter of transistor 24 and the base of transistor 22. The emitter of transistor 25 is connected through a resistor 35 to point G. It can be seen that transistor 25 as connected in this configuration performs the same function as diode 26 in the embodiment illustrated in FIG. 1.

Transistor 25 and resistor 35 in this configuration are the elements which supply the positive temperature coefficient drive current for the current regulator which, when added to the negative temperature coefficient drive current supplied by the V _BE source by virtue of the decreasing V _BE of transistor 22 with increasing temperature, yields a substantially temperature independent input current to the current regulator.

A starting transistor 50 is connected with its collector at the direct current supply voltage and its emitter at point A'. It is not required to connect the base of transistor 50 in the circuit since its only function is to provide collector to emitter leakage current to start the circuit.

After transistor 24 has initially been placed in conduction by appropriate current supplied by the collector to emitter leakage current of starting transistor 50 (or by leakage currents in the circuit which may be sufficient to induce starting depending upon the transistors used, in which case transistor 50 may be omitted), collector current in transistor 24 will vary approximately as the value of V _BE of transistor 22 divided by the value of resistor 29.

In the circuit illustrated in FIG. 2 the elements numbered 10, 11, 12, 14, 15, 21, and 23 comprise a variation of a type of current regulator known as a current repeater. A current repeater is a device in which the ratio of the input current supplied to an input terminal (point D) to the resulting output current supplied to an output terminal (point A) is substantially constant, creating at the output terminal current equal to a constant ratio times the current supplied at the input terminal. It should be noted that the ratio may be altered simply by adding to or subtracting from the number of parallel connected diode and resistor series combinations coupled between the direct current operating voltage and the base of transistor 23. The ratio may also be changed by changing the ratio of the base-emitter areas of the one or more transistors which are being used as diodes (elements 14 and 15 in this circuit) in relation to the base-emitter area of the transistor 23 with whose base-emitter circuit they are coupled in parallel.

In the current repeater illustrated, the ratio of current flowing from the emitter of transistor 23, the input current, to that flowing from the collector of transistor 21, the output terminal of the current repeater, from the direct current voltage supply B+ through resistors 10 and 11 and diode connected transistors 14 and 15 is one to two assuming equal base-emitter areas and diffusion profiles of transistors 14, 15, and 23. This results from the fact that in the steady state the forward biased emitter-base junction of transistor 23 and the forward biased diode connected transistors 14 and 15, which are virtually identical to transistor 23, and resistors 10, 11, and 12, which are all of equal value, represent substantially equal impedance paths for the direct current voltage supply B+. Resistors 10, 11, and 12 are added to decrease the effect of any differences which might occur in the fabrication of the devices 14, 15, and 23 and may be used or eliminated in this embodiment of the invention. Substantially equal currents flow in these three parallel paths from the supply, the currents flowing in diodes 14 and 15 flowing through the collector of transistor 21 and the emitter of transistor 23 supplying substantially all of a current equal in magnitude to the contribution of either diode 14 or 15 to the collector of transistor 23. For purposes of this discussion base currents have been ignored since the base currents are orders of magnitude smaller than the collector currents under discussion.

Transistors 22 and 24 and resistor 29 in FIG. 2 comprise a familiar V _BE voltage source. The operation of this circuit is in accordance with well-known principles but it is discussed here to aid in understanding the invention.

Transistor 22 is arranged in a common emitter configuration. When energizing potential B+ is supplied at point C, an equilibrium point is established at which an average one V _BE voltage drop is developed across its base-emitter junction in the forward diode direction. The collector current conducted by transistor 22 by virtue of this average drop (which is approximately 0.7 volt for a silicon device at room temperature of 25° C) sets the conductivity of transistor 24 which is supplying current to resistor 29 to sustain this V _BE drop.

As voltage across resistor 29 tends to decrease below this V _BE value as, for example, when the collector voltage of transistor 24 decreases, transistor 22 tends to become less conductive, raising the voltage applied to the base, point A', of transistor 24, and hence increases the conductivity of transistor 24 so that more voltage is supplied at the base of transistor 22 to sustain the V _BE drop. Should the voltage on the base of transistor 22 tend to rise substantially above the equilibrium value, transistor 22 becomes more conductive, lowering the base voltage of transistor 24 and hence its conductivity which in turn tends to bring the base voltage of transistor 22 back toward its equilibrium value.

This action tends to maintain a substantially constant current flow through resistor 29 and thus through the collector-emitter circuits of both transistors 22 and 24 even with substantial changes in the supply voltage B+, provided temperature remains substantially constant. However, variations in temperature have adverse effects on the performance of the circuit unless compensation is provided.

As the temperature of transistor 22 varies from some arbitrary reference temperature T _O , its base-emitter forward voltage drop changes according to the relation for V _BE stated previously.

As the base-emitter voltage of transistor 22 varies, the current supplied to resistor 29 must also vary. Since all of this current flows in the collector of transistor 24, transistor 22 has the effect of varying the collector current of transistor 24 inversely with temperature.

In order to compensate for this effect and to insure that constant current is drawn toward ground through point D, the input terminal of the current repeater, and thus through point A, the output terminal of the current repeater, transistor 25 is connected with its base to the sensing resistor 29 of the V _BE voltage source. Its collector is connected to its base forming a diode configuration as previously explained. The positive and negative temperature coefficient currents are summed at the emitter of transistor 24. Its emitter current is approximately equal to its collector current (again the base currents of these devices have been assumed throughout the analysis to be much smaller than their respective collector currents).

In a similar manner to that outlined in the discussion directed toward FIG. 1 then, the change in the collector current drawn by transistor 25 with respect to temperature can be made to compensate for the change in the current flowing through resistor 29 with respect to temperature.

From considerations of the aforestated V _BE equations, the fact that V _BE of transistor 22 approximately equals the collector current of transistor 24 divided by the value of resistor 29, the temperature about which compensation is required and the fact that it is desired to make the derivative of the current flowing from point D or point D' with respect to temperature equal to zero, relations involving the ratio of resistors 29 and 35 may again be achieved which determine the ratio as previously explained.

Other circuit considerations again determine the values for the resistors themselves. From these considerations, values for the resistors can be chosen which yield good results over the selected range of temperatures. A system constructed with silicon resistors 29 and 35, a value of resistor 29 of 4300 ohms and a value of resistor 35 of 250 ohms (for a ratio of 17.2) exhibited approximately a 1 percent current variation between 25° and 110° C with a current repeater output current of approximately 1.02 milliamperes at 25° C.

Over a chosen temperature range then, the current through points A, D, C, and G is held substantially constant with changing temperature by virtue of the compensation circuit in accordance with the invention. The input current to the current repeater flows through point D (note that in the illustrated circuit the "input" current actually flows out of the current repeater toward reference potential since the transistors of the repeater are P-N-P types), the "image" current of the repeater flows through point A, and the sum of the two, which is therefore also substantially temperature independent over a range of operating temperatures, flows from the direct current voltage supply B+ through point C and out of the circuit through point G. Therefore, load circuits which require constant currents can be connected between points A and A', C and C', D and D', and G and reference potential.

In another embodiment of the invention illustrated in FIG. 3 transistor 25 has an internal impedance that being the output impedance of transistor 25, in parallel with its collector to emitter connection.

In recognition of this fact, a third embodiment of the invention illustrated in FIG. 3 disconnects the collector of transistor 25 from its base and connects the collector of a transistor 30 to point D. The base of transistor 30 is connected to the base of transistor 25. The emitter of transistor 30 is coupled to the collector of transistor 25. All other points, elements, and connections remain as described in the discussion of FIG. 2 and the functions there described are unchanged for purposes of the discussion of FIG. 3.

To decrease the effect of collector to emitter output resistance of transistor 25 on the temperature compensation component of current under discussion, the collector voltage of transistor 25 may be controlled at some low direct current voltage. This is done by connecting transistor 30 in a cascode configuration with transistor 24 as shown in FIG. 3. In this manner the collector voltage of transistor 25 is thereby controlled at one base-emitter voltage drop. In FIG. 3 the voltage drop will be the sum of the base-emitter voltage drops across either transistor 22 or 25 and transistor 24 minus the base-emitter voltage drop of transistor 30. Since the collector voltage of transistor 25 is not allowed to "float" at some higher direct current voltage level but is constrained to be one base-emitter voltage drop, the effect of its output resistance is minimized.

The difference between the circuit of FIG. 3 and that shown in FIG. 2 is that in FIG. 3 the positive and negative temperature coefficient currents are summed at point D' rather than at the emitter of transistor 24. In all other respects, the operation of th two circuits is substantially the same.

It should also be noted that this positive temperature coefficient compensating arrangement may be used to control constant currents drawn by a number of transistors connected in cascode configurations with transistors 21, 22, 23, or 24 of FIGS. 2 and 3 since the collector currents of all of these transistors will be temperature compensated. This effectively divides the output current of the current repeater through the collector of each cascoded device by the number of cascoded devices. The cascoded transistors could also have different base-emitter junction areas providing non-integral current division.