Temperature-dependent Current Supplier

Abstract

A temperature-dependent current supplier specially suitable for application to an integrated circuit is characterized as follows. The base of a first transistor is connected to a connecting point between two resistors which are connected in series to each other in a collector circuit of said transistor, the voltage between the base and the collector of said transistor being selected at about kT/q where the charge quantity is q, the Boltzmann's constant is k and the absolute temperature is T; and the base of a second transistor is connected to the collector of said first transistor, respective temperature coefficients of the base-emitter voltages of said first and second transistors being selected to differ from each other; and also current-outputs of the quantity proportional to the absolute temperature is taken out from the collector circuit of said second transistor.

Description

BACKGROUND OF THE INVENTION

This invention relates to a temperature-dependent current supply circuit capable of stably supplying a current output having a magnitude proportional to the absolute temperature, irrespective of fluctuations in the source voltage.

In electric circuits employing transistors, unlike those employing vacuum tubes, it is always necessary to provide a circuit for compensating for the fluctuations of the transistor characteristic due to temperature changes. It is generally known that if the characteristic of such a compensating circuit is such as to maintain a current which is constant in spite of fluctuations of the source voltage, but variable in proportion to the absolute temperature, the circuit construction will be simplified and effective for providing the desired circuit temperature compensation. Such temperature compensation has been attained in practice by utilizing thermistor elements or other thermally variable resistance elements of copper wire, etc. However, in the case where monolithic integrated circuits are employed for the circuit, the above-mentioned elements are not usable, and hence, the special characteristic of a thermally variable nature of a transistor or diode must be utilized. For such purpose, a constant current circuit as shown in FIG. 1 is widely used. In such a conventional constant current circuit, however, if there happens to be a fluctuation in the source voltage E, there will occur a change, though small, in the voltage between the terminals of the diode D as well as in the base potential of the transistor Q, and it will cause a change in the current output I0 flowing in the load L, thus failing to attain fully the desired object of obtaining the constant current condition. Moreover, although such a circuit configuration has a temperature-dependent characteristic, it is impossible to stably obtain therefrom a current output proportional to the absolute temperature.

SUMMARY OF THE INVENTION

This invention has as a main objective to overcome the foregoing drawbacks.

A temperature-dependent current supply circuit is constituted in such a manner that the base of a first NPN transistor (Q1) is connected to a connecting point (P) between two resistors (R1 and R2) which are connected in series to the collector of said transistor (Q1), the voltage between the base and the collector of said transistor (Q1) being selected at about kT/q, where the charge quantity is q, the Boltzmann's constant is k and the absolute temperature is T. The base of a second NPN transistor (Q2) is connected to the collector of said first transistor (Q1), respective temperature coefficients of the base-emitter voltage of said first and second transistors (Q1 and Q2) being selected to differ from each other, and also the current outputs of the quantity proportional to the absolute temperature is taken out from the collector circuit of said second transistor (Q2).

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a schematic circuit diagram of a conventional current supply circuit;

FIG. 2 is a schematic circuit diagram of an example of the present invention;

FIG. 3 is a graph of general characteristics of the transistor;

FIG. 4 is a graph of characteristic curves for explanation of an apparatus by this invention; and

FIGS. 5 and 6 are schematic circuit diagrams of other examples of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The circuit of the temperature-dependent current supply circuit embodying the present invention, as shown in FIG. 2, comprises a pair of similar type, for instance, NPN transistors Q1 and Q2. In this case, when the source voltage E decreases (or increases), a current I1 flowing through resistors R1 and R2 and the first transistor Q1 decreases (or increases). By the decrease (or increase) of this current I1, the base-emitter voltage VBE(Q1) of the transistor Q1 decreases (or increases). Also by the decrease (or increase) of said current, the voltage across the resistor R2 decreases (or increases). Therefore, by selecting the value of the resistor R2 to cause the decreased (or increased) portion of said voltage VBE(Q1) and the decreased (or increased) portion of said voltage across the resistor R2 to compensate each other, the base potential of the transistor Q2 can be maintained constant. Consequently, a current I2 flowing through the load L can be maintained constant, irrespective of fluctuations in the source voltage.

Assuming the amplification factors of both transistors Q1 and Q2 to be adequately high, and defining the saturation currents of the transistors Q1 and Q2 as Is1 and Is2, respectively, the charge quantity as q, the Boltzmann's constant as k and the absolute temperature as T, then from the relations between the base-emitter voltage VBE(Q1) of the transistor Q1 and the base-emitter voltage VBE(Q2) of the transistor Q2:

Vbe(q1) = k.sup.. T/q .sup.. ln(I1/Is1) = R2.sup.. I1 + VBE(Q2) + R3.sup.. I2

= r2.sup.. i1 + kT/q .sup.. ln(I2/Is2) + R3.sup.. I2 (1)

from the preceding equation:

I2 = kT/q.sup.. R3 (ln[I1/I2] + ln[Is2/Is1]) - (R2/R3) .sup.. I1 (2)

the condition to maintain the current I2 constant, against the change of the current I1 due to the change of the source voltage is that

.delta. I2/.delta. I1 = kT/q.sup.. R3 .sup.. 1/I1 - (R2/R3) = 0

that is,

R2.sup.. i1 = kT/q (3)

Accordingly, by arranging the value of resistor R2 to satisfy the condition of the above equation, the current I2 can be maintained constant irrespective of fluctuations of the current I1. Taking a temperature of 25.degree.C. for instance, the value of resistor R2 may be arranged to produce about 26 mV across both terminals of the resistor R2 to fulfill the condition of the above equation.

Relations between the absolute temperature in this circuit and the collector current of the transistor Q2 will be described hereunder. Generally, the following relations exist between the base-emitter voltage VBE of a transistor and the emitter current Ie:

Vbe = (kT/q) ln ([Ie/Is] - 1) .apprxeq. (kT/q) ln Ie - (kT/q) ln Is (4)

By differentiating both sides of the equation with respect to the absolute temperature T, the temperature coefficient of VBE can be obtained as follows:

dVBE/dT = (k/q) ln Ie - (k/q) ln Is - (kT/q) .sup.. d ln Is/dT

= vbe/t - (kT/q) .sup.. d ln Is/dT (5)

as is shown in the curve of FIG. 3 indicating the preceding characteristic, the temperature coefficient of VBE proves that the larger VBE is, the smaller the absolute value of the temperature coefficient dVBE/dT is. Now in FIG. 2, a difference is created in the current density in each emitter layer of both transistors Q1 and Q2, by regulating the values of the currents I1 and I2 through properly selecting the value of each of the resistors R1 and R2, or by creating a difference in the area of each emitter region of the transistors Q1 and Q2. By creating such a difference in current densities, each base-emitter voltage VBE will show a value always different from each other depending on the fluctuations of each absolute temperature. For an example, the transistors Q1 and Q2 having sufficiently large amplification factors and the same characteristics can be selected, and each value of the resistors R1, R2 and R3 set so that at the temperature 25.degree.C., the current I1 becomes 160 .mu.A, the voltage across the terminals of the resistor R2 becomes 26 mV and the current I2 becomes 10 .mu.A. Also set the voltage VBE of the transistor Q1 to be larger than that of Q2 by about 71.2 mV, and the voltage across the resistor R3 at about 45.2 mV. As a result of these settings, the voltage VBE of the transistor Q2 is smaller than that of the transistor Q1, and hence, as shown in FIG. 3, the transistor Q2 has a larger absolute value of temperature coefficient. Due to such a difference in the temperature coefficients corresponding to the respective base-emitter voltages VBE of the transistors Q1 and Q2, the difference in the voltages VBE being 71.2 mV, the currents I1 and I2 differ considerably. As a characteristic obtainable, in the case of R3 = 0, that is, in case a voltage difference corresponding to a voltage across the terminals of the resistor R2 appears between each voltage VBE of the transistors Q1 and Q2, the relations between the current I2 and the absolute temperature show a near-proportional characteristic, as indicated by the broken line, contrasted with the full line showing exact proportionate characteristic in FIG. 4, the largest variation being about 15 percent seen at 243.degree.K.

In the preceding examples of numeral values, if the difference between each voltage VBE of the transistors Q1 and Q2 is set at several tens of millivolts and the voltage across the terminals of the resistor R3 is set at about 30 mV, the variation becomes about 1 percent at 243.degree.K, and thus, in the usual temperature range of from -20.degree. to +60.degree.C., a characteristic of high precision can be obtained. Especially when the above-mentioned difference of voltage VBE is set at about 100 mV and the voltage across the terminals of the resistor R3 is set at about 75 mV, the collector current of the transistor Q2 can be maintained proportional to the absolute temperature with a wide range of from about -30.degree.C. to about 60.degree.C.

The above explanation describes an example of the use of NPN transistors. However, also in case of PNP transistors applied to this invention, a current output proportional to the absolute temperature can be stably obtained irrespective of fluctuations of the source voltage. An example of a circuit of this invention applying PNP transistors in a monolithic integrated circuit (hereinafter called monolithic IC) will be explained in detail in the following.

In general, with PNP transistors in the monolithic IC, the current amplification factors are very low, being usually only in the order of 2 to 5. Therefore, if a circuit corresponding to that of FIG. 2 is constituted only with such elements, it cannot perform its function sufficiently. Accordingly, in the examples of FIGS. 5 and 6, circuits are provided wherein the emitter and the collector of a PNP transistor on a monolithic IC is connected to the collector and the base of another NPN transistor, respectively, both PNP and NPN transistors being provided closely to each other on the same monolithic IC. Since this circuit configuration performs an equivalent function to the function of a PNP transistor of high performance, the circuits of FIGS. 5 and 6 are constituted considering this composition as an equivalent PNP transistor.

In FIG. 5, parts corresponding to those in FIG. 2 are designated with the same symbols. To the collector of the equivalent PNP transistor Q3 (namely, the emitter of the constituent-element NPN transistor Q14) is connected the collector of the transistor Q2 shown in FIG. 2. On the other hand, the emitter of the equivalent PNP transistor Q3 (namely, the point commonly connecting the emitter of the constituent-element PNP transistor Q13 and the collector of the element NPN transistor Q14) is connected to a positive power source terminal E through a resistor R4.

A PNP transistor Q16 and an NPN transistor Q17, provided near the PNP transistor Q13 and the NPN transistor Q14, respectively on the same monolithic IC, constitute a second constituent circuit, namely, a second equivalent PNP transistor Q4. Both bases of the equivalent PNP transistors Q13 and Q16 are connected in common to the emitter of a PNP transistor Q5 also on the same monolithic IC, and the base of this transistor Q5 is connected to the collector of said transistor Q2.

The emitter of the second equivalent transistor Q4 is connected to the source terminal E through a resistor R5, and the collector of the equivalent transistor Q4 (namely, the emitter of an NPN transistor Q17 as a constituent element) is connected to the negative source terminal through the load L. The emitter of the transistor Q17 is also connected to the collector of said PNP transistor Q5.

In said circuit, since the first and second equivalent transistors Q3 and Q4 are similarly constituted by the transistors provided closely to each other on the same monolithic IC, the current amplification factors between both PNP transistors and those between both NPN transistors, respectively, are considered to be equal. Hence, the following relations are formed.

V34b = VBEQ(13) + R4 (I2' + IBQ(13))

= vbeq(16) + r5 (i3' + ibq(16))

therefore,

I3' = (r4/r5) i2' + (r4/r5) ibq(13) - ibq(16) + vbeq(13) - vbeq(16)

wherein IBQ(13) and IBQ(16) are the base current of the transistors Q13 and Q16, IBEQ(13) and IBEQ(16) are the base-emitter voltages of the transistors Q13 and Q16, respectively, and I2' and I3' are collector currents of the equivalent transistors Q3 and Q4, R4 and R5 are resistances of resistors R4 and R5 in emitter circuits of the equivalent transistors Q3 and Q4, respectively.

On the other hand, the base currents of the first and second equivalent PNP transistors Q3 and Q4 flow in the transistor Q5, respectively. However, these currents being very small, the amplification factor of the transistor Q5 amounts to only about 1 to 2. On the other hand, the composite base current of the first and second equivalent PNP transistors Q3 and Q4 is divided about evenly and each bomes a part of the currents I2 of the transistor Q2 and Q3 of the load L, respectively.

Now defining R4 = R5, and if the amplification factor of each transistor of the same type is considered equal, then the operating currents of the transistors Q13 and Q16 are equal. Hence, VBEQ(13) = VBEQ(16), IBQ(13) = IBQ(16) and I2' = I3'.

Also I2 = I2' + (1/2) (IBQ(13) + IBQ(16))

I3 = i3' + (1/2) (ibq(13) + ibq(16))

hence, I3 = I2, and a current output faithfully responding to the current output of the transistor Q2, which is proportional to the absolute temperature, is obtained from the collector circuit of the second equivalent PNP transistor Q4 as a load current which flows through the load L.

In the example of this invention shown in FIG. 6, which is a variation of the example of FIG. 5, the base of the transistor Q5 is connected to the base of the transistor Q2, the emitter of the transistor Q5 is grounded through the resistor R6, the base of the transistor Q14 in the first equivalent PNP transistor Q3 is connected to the base of the NPN transistor Q6, the transistor Q6 is connected by its emitter to the collector of the transistor Q5 and is connected by its collector to the positive source E, and the commonly connected bases of the first and second equivalent PNP transistors Q3 and Q4 are connected to the emitter of the transistor Q6 through diodes D1 and D2, respectively.

According to such circuit constitution as the above, the composite base current of the first and second equivalent PNP transistors Q3 and Q4 becomes a part of the collector current of the transistor Q5 through the diodes D1 and D2. Hence, the difference between the collector current of the transistor Q5 and the base currents of the equivalent transistors Q3 and Q4 flow through the transistor Q6. However, the transistor Q6 being an NPN transistor, its amplifying factor is high and its base current is very small, and consequently, it has almost no effect on the current I2. Moreover, since the base currents of the first and second equivalent PNP transistors Q3 and Q4 flow into the collector of the transistor Q5, they give almost no effect on the currents I2 and I3. Hence, the current I3 fluctuates similarly to the current I2, and therefore, a current output (I3) which is proportional to the absolute temperature can be supplied to the load L.

As has been made clear from the above explanations, according to the present invention, a current output proportional to the absolute temperature can be obtained stably irrespective of fluctuations in the source voltage (E), and not only the compensation of a circuit employing, for instance, a transistor can be made very precisely, but also excellent merits can be achieved, especially in the application to an integrated circuit.

Claims

What is claimed is:

1. A temperature dependent current supply circuit comprising first and second transistors, a source of supply voltage, first and second resistors connected in series to said first transistor across said source of supply voltage, the base of said second transistor being connected to the collector of said first transistor, the base of said first transistor being connected to a point between said first and second resistors, a third resistor connected in series with the emitter of said second transistor to one side of said source of supply voltage and a load connected in series with the collector of said second transistor to the other side of said source, said first and second resistors having values such that the base-collector voltage of said first transistor is approximately kT/q, where the charge quantity is q, the Boltzmann's constant is k and the absolute temperature is T.

2. A temperature dependent current supply circuit as defined in claim 1, wherein the areas of the emitter layers of said first and second transistors are different so as to cause a desired difference between the temperature coefficient of the base-emitter junction of said first transistor and the temperature coefficient of the base-emitter junction of said second transistor.

3. A temperature dependent current supply circuit as defined in claim 1, wherein said third resistor has a resistance value sufficient to restrict the current flow through the second transistor so as to cause a desired difference between the temperature coefficient of the base-emitter junction of said first transistor and that of said second transistor.

4. A temperature dependent current supply circuit comprising first and second transistors, a source of supply voltage, first and second resistors connected in series to said first transistor across said source of supply voltage, the base of said second transistor being connected to the collector of said first transistor, the base of said first transistor being connected to a point between said first and second resistors, said first and second resistors having values such that the base-collector voltage of said first transistor is approximately kT/q, where the charge quantity is q, the Boltzmann's constant is k and the absolute temperature is T, a third transistor and third and fourth resistors connected in series with said second transistor across said source of supply voltage, said third resistor being connected between the emitter of said second transistor and one side of said source of supply voltage, a fourth transistor connected in series with a load and a fifth resistor across said source of supply voltage, the bases of said third and fourth transistors being connected together, and a fifth transistor having its base connected to the collector of said second transistor, its emitter being connected to the bases of said third and fourth transistors and its collector being connected to the emitter of said fourth transistor.

5. A temperature dependent current supply circuit as defined in claim 4, wherein said third and fourth transistors are each formed as composite transistors comprising interconnected NPN and PNP transistors.

6. A temperature dependent current supply circuit as defined in claim 4, wherein said third resistor has a resistance value sufficient to restrict the current flow through the second transistor so as to cause a desired difference between the temperature coefficient of the base-emitter junction of said first transistor and that of said second transistor.

7. A temperature dependent current supply circuit as defined in claim 4, wherein the areas of the emitter layers of said first and second transistors are different so as to cause a desired difference between the temperature coefficient of the base-emitter junction of said first transistor and the temperature coefficient of the base-emitter junction of said second transistor.

8. A temperature dependent current supply circuit comprising first and second transistors, a source of supply voltage, first and second resistors connected in series to said first transistor across said source of supply voltage, the base of said second transistor being connected to the collector of said first transistor, the base of said first transistor being connected to a point between said first and second resistors, said first and second resistors having values such that the base-collector voltage of said first transistor is approximately kT/q, where the charge quantity is q, the Boltzmann's constant is k and the absolute temperature is T, a third resistor and third and fourth resistors connected in series with said second transistor across said source of supply voltage, said third resistor being connected between the emitter of said second transistor and one side of said source of supply voltage, a fourth transistor connected in series with a load and a fifth resistor across said source of supply voltage, the bases of said third and fourth transistors being connected together, fifth and sixth transistors connected together through a sixth resistor in series across said source of supply voltage, a pair of diodes connecting the bases of said third and fourth transistors to a point between said fifth and sixth transistors, the base of said fifth transistor being connected to the base of said second transistor and the base of said sixth transistor being connected to said third transistor.

9. A temperature dependent current supply circuit as defined in claim 8, wherein said third and fourth transistors are each formed as composite transistors comprising interconnected NPN and PNP transistors.

10. A temperature dependent current supply circuit as defined in claim 8, wherein said third resistor has a resistance value sufficient to restrict the current flow through the second transistor so as to cause a desired difference between the temperature coefficient of the base-emitter junction of said first transistor and that of said second transistor.

11. A temperature dependent current supply circuit as defined in claim 8, wherein the areas of the emitter layers of said first and second transistors are different so as to cause a desired difference between the temperature coefficient of the base-emitter junction of said first transistor and the temperature coefficient of the base-emitter junction of said second transistor.