Method of making a reference voltage source with a desired temperature coefficient

Abstract

A temperature stability of a high order is obtained by voltage divider trimming in a circuit in which a common current supply feeds a first branch containing one or more Zener diodes and a second branch containing a voltage divider composed of ohmic resistances, from the tap of which the output voltage is taken, by provision of a circuit configuration meeting one design criterion and trimming an output voltage divider to meet, at a single reference temperature, another design criterion and thus to set both the designed output voltage and the designed temperature coefficient, independently of the scatter of the characteristics of the diodes in the circuit.

Description

The invention relates to a method of making source of reference voltage having a desired high stability with respect to temperature or a desired temperature coefficient of voltage.

It is known to make a source of reference voltage with a parallel connection of two current carrying circuit branches the first of which contains a Zener diode and the second a series connection of two ohmic resistances and of two conductively poled diodes connected together in series. The reference voltage is taken from this circuit between the common connection point of the two ohmic resistances and that one of the common points of the two current branches at which the Zener diode is connected with the second conductively operated diodes. The voltage division ratio of the two ohmic resistances is constant, so that the reference voltage source has the exact desired temperature coefficient only for a single value of Zener breakdown voltage. But since the breakdown voltage of Zener diodes is subject to an unavoidable scatter in values as the result of the operation of the processes of manufacture, the above described known circuit provides a reference voltage that has a relatively large scatter of the values of the temperature coefficients of individual units.

It is an object of the invention to provide a method of making source of reference voltage in such a manner that the temperature coefficient of the reference voltage supplied by the source takes on a predetermined value which is independent of the scatter effects caused by manufacture.

SUBJECT MATTER OF THE PRESENT INVENTION

Briefly, the number of Zener diodes used in cascade and the number of conductively operated diodes used in series respectively with the Zener diodes and with the voltage divider resistors is selected to fit a formula found to give superior temperature stability and then the resistances of the voltage divider are trimmed in such a manner that the temperature dependence of the reference voltage meets another criterion further described below.

The first formula, stating a condition to be met according to the invention, is in terms of k, the number of Zener diodes in cascade, which is at least 1; the number l of conductive diodes in series with the Zener diodes, which may be 0; the number m, which may be 0, of conductive diodes connected in series with the voltage divider and connected on that side of the voltage divider where the voltage drop is not included in the output voltage, and the number n of conductive diodes in series with the voltage divider and connected on the other side of the voltage divider (where the voltage drop is included in the output voltage), which number is at least 1. The formula in question is ##EQU1## where B is the voltage dependent portion of the temperature coefficient of the Zener diodes in volts/.degree. K, U.sub.Do is the voltage drop at the reference temperature To for the conductively driven diodes, in volts, A is the constant portion of the temperature coefficient of the Zener diodes in .degree.K.sup.-.sup.1, A.sub.D is the temperature coefficient in .degree.K of the conductively poled diodes when conducting and E is the desired temperature coefficient of the reference voltage U.sub.1, likewise in .degree.K.sup.-.sup.1 and A.sub.D is the temperature coefficient in .degree.K.sup.-.sup.1 of the diodes operated in their conducting state. Their relation with reference to which the voltage divider is balanced by trimming the voltage divider is as follows ##EQU2##

In a further development of the invention, the voltage drop per diode U.sub.Do and the temperature coefficient A.sub.D of the diodes operated in their conducting condition can be chosen in such a manner that the equation first above mentioned is satisfied, not merely approximately, but exactly. The quotient on the right hand side of this equation then becomes a whole number (integer). Particularly in the case of a voltage reference source made on a silicon substrate the particular numbers represented in the formula by l, k, m and n may be chosen to satisfy at least approximately the relation ##EQU3##

In the method of the invention the division ratio of the voltage divider at a predetermined reference temperature To is modified progressively by trimming the resistors of the voltage divider until the reference voltage U.sub.1 at this reference temperature To reaches a fixed predetermined value given by the expression

The invention will be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a circuit having two branches in parallel which is shown to illustrate the definition of the numbers k, l, m, and n;

FIGS. 2 - 12 are circuit diagrams of preferred embodiments of reference voltage sources in accordance with the invention;

FIGS. 13, 14 and 15 show subdivisions of a resistor into elementary resistors for trimming the voltage divider to adjust the circuit in the case of a reference voltage source built as a monolithic integrated circuit, and

FIGS. 16 and 17 are circuit diagrams of combinations of a reference voltage source and a following voltage regulator to constitute voltage stabilizers shown as examples of application of the manufacturing method of the invention.

FIG. 1 shows a diagram in general form of a reference voltage source consisting of two current carrying circuit branches connected in parallel to each other and carrying an aggregate current I. The first circuit branch, shown at the left, contains a series chain of a number l of conductively operated diodes and an number k of cascaded Zener diodes. The second circuit branch contains, in series, a number m of conductively operated diodes connected one behind the other, a voltage divider composed of two ohmic resistances R.sub.1 and R.sub.2 and a number n of conductively operated diodes connected one behind the other. One of the places where the two circuit branches are connected together is the so called foot point of the circuit where the anode of the k-th Zener diode and the cathode of the conductively operated diode which is last in the direction of flow of positive current in the second circuit branch. The output reference voltage U.sub.1 is taken from the circuit between the common connection point of the two ohmic resistances R.sub.1 and R.sub.2 and the connection point constituting the above identified foot point of the circuit. The voltage drop across each of the conductively operated diodes is designated U.sub.D and the breakdown voltage of each of the Zener diodes is designated U.sub.Z. Applying Kirchhoff's law, the reference voltage U.sub.1 is given by the following expression: ##EQU5## The temperature dependence of the breakdown voltage U.sub.Z of the Zener diodes can be expressed with sufficient exactitude by the following equation

(2) U.sub.Z = U.sub.Zo . (1 + A.sub.Z . d)

in which

(3) d = T - To

where T is the absolute temperature, To is the reference temperature, A.sub.Z signifies the temperature coefficient of the Zener diode and U.sub.Zo represents the breakdown voltage at d = 0, that is, when T = To.

Within the tolerance range defined by the unavoidable manufacturing scatter of the values of the breakdown voltage U.sub.Z of the Zener diodes, the temperature coefficient A.sub.Z of these diodes can be described with adequate accuracy by the following equation ##EQU6##

A accordingly represents the constant part of the temperature coefficient of the Zener diodes in reciprocal degrees of the Kelvin scale, that is, in .degree.K.sup.-.sup.1, and B represents the voltage dependent part of that temperature coefficient in volts per degree of the Kelvin scale.

If now the expressions for U.sub.Z and U.sub.D from the equations (2) and (4) are substituted into the equation (1), there is obtained: ##EQU7## or, otherwise stated, ##EQU8##

If now, by analogy to equations (2) and (4) there is formed for the temperature dependence of the reference voltage U.sub.1 (d) the following further expression:

(8) U.sub.1 (d) = U.sub.1o . (1 + E . d)

in which E is the temperature coefficient of the reference voltage U.sub.1 in .degree.K.sup.-.sup.1 and U.sub.1o is the value of the reference voltage U.sub.1 when d = 0, there is then obtained, by comparison of the coefficients of equations (7) and (8) the following expression: ##EQU9##

If equation (9) is solved for R.sub.1 /R.sub.2, there is obtained for the voltage divider ratio: ##EQU10##

If the value for the voltage divider ratio from equation (10) is substituted into equation (7), the following expression is obtained for the reference voltage U.sub.1 : ##EQU11##

From equations (8) and (11) there is then obtained for U.sub.1o : ##EQU12##

If now the value for A.sub.Z from equation (5) is substituted into equation (12), there is obtained for U.sub.1o : ##EQU13##

If U.sub.1o is to be independent of U.sub.Zo, the following relation must hold: ##EQU14##

If the equation just given is solved for (m + n - l) / k, the result is ##EQU15##

If this condition is put into equation (13), there is obtained for U.sub.1o : ##EQU16##

The following conditions hold for the number of diodes in the various groups shown in FIG. 1:

k .gtoreq. 1; m .gtoreq. 0; l .gtoreq. 0; n .gtoreq. 1.

If the reference voltage source is built on a silicon substrate in accordance with monolithic integrated circuit technology, the following values of A and B are obtained for a particular manufacturing technology and diode geometry and a reference temperature To of 300.degree.K:

a = 1.2 . 10.sup.-.sup.3 /.degree.k; b = 5.35 . 10.sup.-.sup.3 volts/.degree.K.

If it is further assumed that the current through the diodes is 1 mA, the following values are obtained for A.sub.D and U.sub.Do :

A.sub.D = 2.86 . 10.sup.-.sup.3 /.degree.K; U.sub.Do = 670 mV.

For this case of a current of 1 mA through the diodes there is then obtained: ##EQU17##

For other technologies and diode geometries on a silicon substrate, values of A, B, A.sub.D and U.sub.Do varying only slightly from the above result are obtained so that in those cases also the numerical value on the right hand side of equation (17) lies close to 2.

Since of course m, n, l and k are integers, equation (15), for a current of 1 mA through the diodes, cannot be fulfilled exactly. But since U.sub.Do is a function of a current through the diode, U.sub.Do can be set at any desired value within certain limits. If the current through the diode is I: ##EQU18##

In this expression K is the Boltzman constant, q is the elementary charge and Io is the reference current, for example 1 mA.

If the value for U.sub.D from equation (4) is substituted in equation (18), there is obtained: ##EQU19##

By setting U.sub.Do (Io) = U.sub.Do and A.sub.D (Io) = A.sub.D, equation (19) becomes: ##EQU20##

In the above equation the following simplifications may be made: ##EQU21## For a particular current through the diodes it can be found that the expression ##EQU22## in equation (15) becomes exactly equal to 2. ##EQU23## is then the determining equation for this current. From it there may be derived by substituting the expression for U.sub.Do.sub.' from equation (22) and A.sub.D.sub.' from equation (23) a solution for the current through the conductively operated diodes as follows: ##EQU24## There is thus obtained, from the values for A, B, A.sub.D and U.sub.Do holding for a particular technology and diode geometry, a value for the current at which the expression ##EQU25## is exactly equal to 2, namely I = 2.27 mA.

Equations (15) and (16) were derived for a temperature coefficient E of the reference voltage U.sub.1 that may have any desired value. That means that for all reference voltage generator circuits that satisfy the condition (15) or -- in the case of greater accuracy requirements -- the condition ##EQU26## the desired temperature coefficient E for the reference voltage U.sub.1 can be obtained by trimming the voltage divider R.sub.1, R.sub.2 at a particular reference temperature To until the reference voltage U.sub.1o at this reference temperature To has reached the fixed value determined by equation (16).

The temperature dependence U.sub.1 (T) of the reference voltage may be expressed in terms of equation (16) in connection with equations (3) and (8) as follows: ##EQU27##

In connection with this trimming or balancing operation it is to be noted that the conditions (15) and (26), which limit the range of combination of Zener and conductively operated diodes, are independent of the desired temperature coefficient E of the reference voltage U.sub.1. The circuits of the present invention thus have the property that by setting the reference voltage U.sub.1o by trimming the voltage divider R.sub.1, R.sub.2 at the temperature To to a value specified by equation (16), the temperature coefficient E of the reference voltage U.sub.1 is simultaneously adjusted to the desired value, so that the temperature coefficient of the reference voltage becomes independent of the scatter of the breakdown voltage values of the Zener diodes inherent in the manufacturing process and for their temperature coefficients dependent on their breakdown voltage.

The circuits that satisfy the equation ##EQU28## can be produced with conventional manufacturing techniques utilizing discrete partly integrated components on printed circuit plates or utilizing thick or thin film techniques on insulating substrates. The monolithic integrated circuit form of construction, however, offers particular advantages because of the good thermal coupling of the elements.

Circuits that satisfy equation (26') are shown in FIGS. 2 through 11.

FIG. 2 shows the simplest embodiment. A single Zener diode Z.sub.1 is provided in the first circuit branch. In parallel to the Zener diode Z.sub.1 there are, in the direction of flow of positive current, the series connection of the voltage divider composed of the two ohmic resistances R.sub.1 and R.sub.2 and then, in succession, two conductively operated diodes D.sub.1 and D.sub.2, all of these forming the second circuit branch. In this arrangement, therefore, the values k = 1 and m = l = 0 have been chosen. From equation (26') it follows that n = 2.

FIG. 3 shows, as a second embodiment, a reference voltage source having its first circuit branch, as enumerated in the direction of the flow of positive current, a conductively operated diode D.sub.4 and a single Zener diode Z.sub.1 in series. The second circuit branch, enumerating the components in the same order, contains the voltage divider composed of the two ohmic resistances R.sub.1 and R.sub.2 and a succession of three conductively operated diodes D.sub.1, D.sub.2 and D.sub.3, all in series. In this case, therefore, k = 1, m = 0 and l = 1. From equation (26') it follows that n = 3.

FIG. 4 shows a modification of the embodiment of FIG. 3. The difference is that in FIG. 4 the two diodes D.sub.3 and D.sub.4 are replaced by a single diode D.sub.3,4 which is common to both circuit branches and connected to the so called foot point of the circuit. The shared diode D.sub.3,4 is so selected that it can operate with the same current density that is present in the other conductively operated diodes D.sub.1 and D.sub.2.

In the embodiment shown in FIG. 5 there is again only a single Zener diode Z.sub.1 in the first circuit branch. In the second circuit branch connected in parallel to this Zener diode is the series connection of a first conductively operated diode D.sub.1, the voltage divider composed of the two ohmic resistances R.sub.1 and R.sub.2 and a second conductively operated diode D.sub.2. In this case, accordingly, k = 1, l = 0 and m = 1. From equation (26') it follows that n = 1.

FIG. 6 shows a modification of the embodiment of FIG. 5. The difference is that in FIG. 6 the first conductively operated diode D.sub.1 of FIG. 5 is constituted by the base-emitter path of aa transistor T.sub.1.

In the embodiment shown in FIG. 7 the first circuit branch contains, enumerating the elements in the direction of the flow of positive current, a conductively operated diode D.sub.4 and a single Zener diode Z.sub.1, in series. In parallel thereto, forming the second circuit branch, is the series connection of a first conductively operated diode D.sub.1, the voltage divider composed of the two ohmic resistances R.sub.1 and R.sub.2 and two further conductively operated diodes D.sub.2 and D.sub.3 in succession. In this case, accordingly, k = 1, l = 1 and m = 1. From equation (26') it follows that n = 2.

FIG. 8 shows a modification of the embodiment of FIG. 7. The difference is that in FIG. 8 two diodes D.sub.3 and D.sub.4 of FIG. 7 are replaced by a single diode D.sub.3,4 common to both circuit branches. The shared diode D.sub.3,4 is so selected that it may operate at the same current density as do the other conductively operated diodes D.sub.1 and D.sub.2.

In the embodiment shown in FIG. 9 the first circuit branch is composed of a series connection of a conductively operated diode D.sub.4 and a single Zener diode Z.sub.1. In parallel to this series connection is the second circuit branch which is composed, enumerating the elements in the order of flow of positive current, of first and second conductively operated diodes D.sub.1 and D.sub.2, the voltage divider composed of the two ohmic resistances R.sub.1 and R.sub.2 and a further conductively operated diode D.sub.3, all in series. In this case, accordingly, k = 1, l = 1 and m = 2. From equation (26') it follows that n = 1.

FIG. 10 shows a modification of the embodiment of FIG. 9. The difference is that in FIG. 10 the two diodes D.sub.3 and D.sub.4 of FIG. 9 are replaced by a single diode D.sub.3,4 common to both circuit branches. The common diode D.sub.3,4 is again so chosen that it may operate at the same current density as do the other conductively operated diodes D.sub.1 and D.sub.2 (k = 1, l = 1, m = 2, n = 1).

In the embodiment shown in FIG. 11 the first circuit branch again is composed of a single Zener diode Z.sub.1. In the second circuit branch there are, enumerated in the direction of positive current flow, the voltage divider composed of the two ohmic resistances R.sub.1 and R.sub.2 and two conductively operated diodes D.sub.1 and D.sub.2, all in series (k = 1, m = l = 0, n = 2). The cathode of the second conductively operated diode D.sub.2 is connected to the anode of the Zener diode Z.sub.1. The cathode of the Zener diode Z.sub.1 is connected to the non-inverting differential input of an operational amplifier 0.sub.1, the output of which is connected both to its inverting differential input and to that end the first ohmic resistance R.sub.1 of the voltage divider which is not connected to the second ohmic resistance R.sub.2.

FIG. 12 shows an embodiment in which two Zener diodes Z.sub.1 and Z.sub.2 are provided in the first circuit branch. In the second circuit branch in parallel to the first there are, in the direction of flow of positive current, in series, the voltage divider composed of the two ohmic resistances R.sub.1 and R.sub.2 and then, in succession, four conductively operated diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4. In this arrangement, therefore, the values K = 2 and m = l = 0 have been chosen. From equation (26') it follows that n = 4. This circuit is analogous to FIG. 2, which is the simplest of the embodiments described above utilizing a single Zener diode. Similarly circuits with two Zener diodes can be developed corresponding to the other preferred circuits described above, and likewise also circuits utilizing three Zener diodes.

In connection with FIGS. 4, 8 and 10, the relation to the other figures was explained by stating that the diode D.sub.3,4 was common to both branches of the circuit and was counted in determining both l and n for the purpose of equations (15) and (26'). Of course, just as there may be more than one of the diodes l and more than one of the diodes n, there may be more than one diode in series instead of the single diode D.sub.3,4. If the number of these diodes is designated p, then the conductively operated diodes remaining in the first circuit branch will number l - p, where is at least as great as p, and the number remaining between the resistor R.sub.2 and the first diode D.sub.3,4 will likewise be n - p, where n is at least as great as p. If we want to rewrite equation (15) to bring in diodes p separately, however, we will find that when l - p is subtracted from the sum of m and n - p, the p's cancel out. Consequently, for circuits like FIGS. 4, 8 and 10, the criterion of equations (15) and (26') can be stated precisely the same way treating the diode D.sub.3,4 as not included either in l or n, as they can be stated treating that diode as included in both l and n. When the diode D.sub.3,4 and anymore like it are treated as not included in either l or n, the language of a statement stating the invention as embodied in FIGS. 4, 8 and 10 is somewhat simpler, although this defined n, for example, in FIG. 4 in the same way as in FIG. 2 and treats FIG. 4 as a case of l = 0, m = 0 and n = 2, with the further fact that p = 1, which is irrelevant to equations (15) and (26'), (since if p is added to both l and n it drops out of the equation).

If in construction of any of the circuits shown in FIGS. 2-12 the monolithic integrated circuit form of construction is used, it would be natural to provide the conductively operated diodes in the form of emitter diodes and the resistances R.sub.1 and R.sub.2 by means of a base diffusion zone, in which case the unavoidable path resistance of the conductively operated diodes will constitute a portion of the resistance R.sub.1 or of the resistance R.sub.2, or both, as the case may be, and these path resistances will all have the same thermal behavior.

In conventional circuits, trimming can be done by means of a potentiometer. A more exact procedure is the trimming of film resistances by means of a sand jet (sandblasting) or a laser beam (easer beam cutting) or by electrochemical etching or oxidation (electrochemical action) of the resistances R.sub.1, R.sub.2 or both.

In monolithic integrated circuits it is convenient to subdivide the resistor R.sub.1, the resistor R.sub.2 or both, for purposes of trimming. The resistor in question may be separated into a number of component resistors and all but one of them short-circuited by conductive bridges. FIG. 13 shows such a development of the resistance R.sub.1. The component resistors are designated R.sub.10, R.sub.11, R.sub.12, R.sub.13, ... R.sub.1n. R.sub.10 represents the smallest possible value for the resistor R.sub.1 and is not short-circuited. It is advantageous to provide the resistance values of the short-circuited component resistors R.sub.11, R.sub.12, R.sub.13, ... in such a way that these values are in the progression 1 : 2 : 4 : 8 : ... . The more accurately the reference voltage U.sub.1o is to be set, the more component resistors are necessary. The trimming process is desirably combined with the now conventional premeasurement procedure applied to the chips at a wafer test station of the manufacturing process. In that case the measuring apparatus of the wafer test equipment measures the actual value of the reference voltage U.sub.1o . A computer coupled to the measuring equipment calculates, from the difference between the measured value and the desired value, what combination of component resistors must be added in circuit to the component resistor R.sub.10 or R.sub.20 as the case may be (the minimum operating resistors), in order to reach the prescribed design value at which equation (16) will be fulfilled. The conductive bridges to be removed, for example B.sub.11 and B.sub.13 in FIG. 13, are then burned away with a current pulse. Although this operation increases the effective resistance in the circuit, it may be referred to properly as "trimming" the voltage divider, since it brings the voltage divider to a desired permanent condition, and the word "trimming" is herein used in a sense broad enough to comprehend the operation just described.

If resistances of relatively low ohmic value are necessary, these can be provided by connecting in parallel two or more resistors R.sub.111, R.sub.112, ... , all of the same geometry, in accordance with FIG. 14.

With the configuration of FIG. 15 it is possible by separating off component resistors of high ohmic value to make very small changes of the effective resistance. If, for example, R.sub.111 has the value of 200 ohms and R.sub.112 1,800 ohms, the only value of this combination is changed from 180 ohms to 200 ohms if the bridge B is cut.

FIG. 16 shows an example of the application of the source of reference voltage according to the present invention as part of a circuit for stabilizing a voltage, that circuit consisting of the source of reference voltage and a voltage regulator. U.sub.E is the unstabilized input voltage and U.sub.A is the stabilized output voltage. The unstabilized input voltage U.sub.E is applied to the supply voltage terminals 1 and 2 of an operational amplifier 0.sub.2. The noninverting input of the operational amplifier 0.sub.2 is connected to the reference voltage U.sub.1 taken from the reference voltage source at the common connection of the resistors R.sub.1 and R.sub.2 of the voltage divider of the reference voltage source. The reference voltage source, in accordance with the invention, has a diode combination in its two circuit branches which satisfies equation (15). The inverting input of the operational amplifier 0.sub.2 is connected to the common connection point of the two ohmic resistances R.sub.3 and R.sub.4, which constitute a second voltage divider. The output 3 of the operational amplifier 0.sub.2 is connected with the free end of the resistor R.sub.3. The free end of the resistor R.sub.4 is connected to the second terminal 2 of the supply voltage and also to the connection point 4 of the two circuit branches of the reference voltage source, where the k-th Zener diode of the first circuit branch is connected to the n-th conductively operated diode of the second circuit branch (this illustrated embodiment is the connection point of the single Zener diode Z.sub.1 with the second conductively operated diode D.sub.2). The stabilized output voltage U.sub. A of the voltage stabilizer appears between the output 3 of the operational amplifier 0.sub.2 and the second terminal 2 of the supply voltage of the operational amplifier.

For further clarification a voltage stabilizer is shown in FIG. 17 in which the operational amplifier 0.sub.2 of FIG. 16 is made up of the differential amplifier formed by the two transistors T.sub.2 and T.sub.3 and the resistor R.sub.5, together with the transistor T.sub.4 driven by the differential amplifier by means of the resistor R.sub.6.

In the case of heretofore known voltage stabilizers composed along the lines of FIG. 16 and FIG. 17 without using reference voltage sources of the present invention, the stabilized output voltage U.sub.A is either not adjusted by trimming to the desired design value or else is adjusted by changing the second voltage divider R.sub.3, R.sub.4.

In a further application of the invention, however, the stabilized output voltage U.sub.A of the circuits shown in FIGS. 16 and 17 is trimmed to bring it to the desired specifications, not in the usual way by trimming the voltage divider R.sub.3, R.sub.4, but rather by trimming of the voltage divider R.sub.1, R.sub.2 of the reference voltage source. This operation proceeds as follows: from the desired temperature coefficient of the stabilized output voltage U.sub.A, which is the same as the temperature coefficient E of the reference voltage U.sub.1, the reference voltage U.sub.1o for the reference temperature T.sub.o is calculated by reference to equation (16). The second voltage divider R.sub.3, R.sub.4 is then so dimensioned that it provides the desired output voltage U.sub.A when the reference voltage has the value U.sub.1o . This is the case when the divider ratio is ##EQU29## If now the stabilized output voltage U.sub.A is adjusted to the desired value by trimming the voltage divider R.sub.1, R.sub.2, the reference voltage U.sub.1o then has the correct value prescribed by equation (16) and the temperature coefficient of reference voltage and of stabilized output voltage likewise has the desired value.

In the circuits above described, the emitter-base path of a transistor can be used as a Zener diode. The conductively operated diodes can be diodes having pn-junctions, diodes formed by metal-to-semiconductor contacts or partly pn-junction diodes and partly metal-to-semiconductor diodes. Each of these diodes can also be provided by the base-emitter path of a transistor connected for conductive operation, that is, poled in the direction of conduction. In this last mentioned case, the collectors of these transistors can in some cases be utilized for further purposes, for example, for generation of a constant current, as indicated in connection with FIG. 6.

Claims

We claim:

1. A method of making a source of reference voltage having a desired temperature coefficient of said reference voltage and containing at least one Zener diode, the number of Zener diodes being designated k, and a number of conductively poled diodes, possibly zero and designated l, connected in series with said Zener diode(s) and comprising also a shunt circuit branch connected in parallel with said Zener diode(s) and the aforesaid conductively poled diodes, composed of a resistive voltage divider having an intermediate tap providing an output terminal for a reference voltage (U.sub.1) and, in series with said voltage divider, a number of conductively poled diodes, possibly zero and designated m, connected between a first pole of a current supply source and said voltage divider and also at least one conductively poled diode, the number thereof being designated n, connected between said voltage divider and a second pole of said current supply means, which method comprises the steps of:

making a source of reference voltage having components as above set forth in such a way as to approximate a design criterion of ##EQU30## without regard to the scatter of Zener breakdown voltages of Zener diodes used, said scatter being within a reasonable manufacturing tolerance, and

progressively modifying the resistance values of the resistances (R.sub.1, R.sub.2) of said voltage divider and thereby trimming said voltage divider at a predetermined reference temperature T.sub.o until the value U.sub.1o that the output reference voltage U.sub.1 has at the aforesaid reference temperature T.sub.o reaches the predetermined value ##EQU31## where A is the constant portion of the temperature coefficient of the Zener diodes in .degree.K.sup.-.sup.1,

A.sub.d is the temperature coefficient of the conductively poled diodes in their conductive state, in .degree.K.sup.-.sup.1,

B is the voltage-dependent portion of the temperature coefficient of the Zener diodes in volts per .degree.K,

U.sub.do is the voltage drop per diode in volts across the conductively poled diodes during conduction at the reference temperature T.sub.o, and

E is said desired temperature coefficient of said reference voltage U.sub.1, in .degree.K.sup.-.sup.1.

2. A method as defined in claim 1 in which the aforesaid step preceding the step of modifying voltage divider resistance values is carried out by providing for the occurrence of a voltage drop U.sub.Do per diode across the conductively poled diodes and for a temperature coefficient A.sub.D of the conductively poled diodes so chosen by reference to the current density that the condition ##EQU32## is exactly satisifed for the particular numbers l, k, m and n of diodes contained in said two parallel circuits.

3. A method as defined in claim 1 in which the aforesaid step preceding the step of modifying voltage divider resistance values is carried out by constituting a circuit in which the number of diodes in the two parallel circuits designated by l, k, m and n satisfies the condition ##EQU33## at least approximately equals 2.

4. A method as defined in claim 1 in which said voltage divider is trimmed progressively by removal of portions of the resistors of said voltage divider at least in part by sandblasting.

5. A method as defined in claim 1 in which a reference voltage source is produced in monolithic integrated circuit form and in which the trimming of said voltage divider (R.sub.1, R.sub.2) is carried out on a monolithic integrated circuit chip.

6. A method as defined in claim 5 in which said voltage divider (R.sub.1, R.sub.2) is trimmed during the premeasurement of the chip on a wafer test equipment.

7. A method as defined in claim 6 in which at least one of the resistors (R.sub.1, R.sub.2) constituting said voltage divider is subdivided into at least two resistor portions for purposes of trimming and that the trimming step is carried out by the interruption of at least one conduction path by which said resistor portions were originally interconnected.

8. A method of manufacture as defined in claim 7 in which the resistance values of said resistor portions utilized in the trimming step are related to each other in the progressive ratio 1 : 2 : 4 : 8 : 16 : 32 : ... .

9. A method as defined in claim 1 in which the step of trimming a voltage divider is performed with a voltage regulator connected with said source of reference voltage as made prior to the trimming step, so as to form a voltage stabilizer together, said voltage stabilizer providing a stabilized output voltage U.sub.A at the output of a second voltage divider (R.sub.3, R.sub.4), and in which the desired stabilized voltage U.sub.A is obtained by variation of the first voltage divider (R.sub.1, R.sub.2) in the completion of the manufacture of the source of reference voltage as aforesaid, and in which, further, the second voltage divider (R.sub.3, R.sub.4) has a resistance ratio defined as ##EQU34## and the reference voltage U.sub.1o at the reference temperature T.sub.o is defined as satisfying the condition ##EQU35## where A is the constant portion of the temperature coefficient of the Zener diodes in .degree.K.sup.-.sup.1,

A.sub.d is the temperature coefficient of the conductively poled diodes in their conductive state, in .degree.K.sup.-.sup.1,

E is said desired temperature coefficient of said reference voltage U.sub.1, in .degree.K.sup.-.sup.1, and

U.sub.do is the voltage drop per diode in volts across the conductively poled diodes during conduction at the reference temperature T.sub.o,

and the temperature coefficient E of the reference voltage U.sub.1 is set equal to the desired temperature coefficient of the stabilized output voltage U.sub.A.

10. A method as defined in claim 1 in which said voltage divider is trimmed progressively by removal of portions of the resistors of said voltage divider at least in part by laser beam cutting.

11. A method as defined in claim 1 in which said voltage divider is trimmed progressively by modification of portions of the resistors of said voltage divider at least in part by electrochemical action.

12. A method of making a source of reference voltage having a desired temperature coefficient of said reference voltage and containing at least one Zener diode, the number of Zener diodes being designated k, and a number of conductively poled diodes, designated l, connected in series with said Zener diode(s) and comprising also a shunt circuit branch connected in parallel with said Zener diode(s) and the aforesaid conductively poled diodes, composed of a resistive voltage divider having an intermediate tap providing an output terminal for a reference voltage (U.sub.1) and, in series with said voltage divider, a number of conductively poled diodes, possibly zero and designated m, connected between a first pole of a current supply source and said voltage divider and also at least one conductively poled diode, the number thereof being designated n, connected between said voltage divider and a second pole of said current supply means, at least one of said last mentioned diodes being a dual-function diode by virtue of being also connected in cascade with said Zener diode(s) and counting also as one of said first mentioned group of conductively poled diodes, the number of which is designated l, which method comprises the steps of:

making a source of reference voltage having components as above set forth in such a way as to approximate a design criterion of ##EQU36## without regard to the scatter of Zener breakdown voltages of Zener diodes used, said scatter being within a reasonable manufacturing tolerance, and

progressively modifying the resistance values of the resistances (R.sub.1, R.sub.2) of said voltage divider and thereby trimming said voltage divider at a predetermined reference temperature T.sub.o until the value U.sub.1o that the output reference voltage U.sub.1 has at the aforesaid reference temperature T.sub.o reaches the predetermined value ##EQU37## where A is the constant portion of the temperature coefficient of the Zener diodes in .degree.K.sup.-.sup.1,

A.sub.d is the temperature coefficient of the conductively poled diodes in their conductive state, in .degree.K.sup.-.sup.1,

B is the voltage-dependent portion of the temperature coefficient of the Zener diode(s) in volts per .degree.K,

U.sub.do is the voltage drop per diode in volts across the conductively poled diodes during conduction at the reference temperature T.sub.o, and

E is said desired temperature coefficient of said reference voltage U.sub.1, in .degree.K.sup.-.sup.1.

13. A method as defined in claim 12 in which the aforesaid step preceding the step of modifying voltage divider resistance values is carried out by constituting a circuit in which at least one aforesaid dual-function diode is connected between the pole of the current source from which the output voltage at the voltage divider tap is measured and a function at which the current through said Zener diode(s) joins the current through said shunt circuit, said dual-function diode being counted both as one of the diodes determining the said number l and as one of the diodes determining the said number n.

14. A method as defined in claim 13 in which the aforesaid step preceding the step of modifying voltage divider resistance values is carried out by providing for the occurrence of a voltage drop U.sub.Do across the conductively poled diodes and for a temperature coefficient A.sub.D of the conductively poled diodes so chosen by reference to the current density that the condition ##EQU38## is exactly satisfied for the particular numbers l, k, m and n of diodes contained in said two parallel circuits.

15. A method as defined in claim 13 in which the aforesaid step preceding the step of modifying voltage divider resistance values is carried out by constituting a circuit in which the number of diodes in the two parallel circuits designated by l, k, m and n satisfies the condition ##EQU39## at least approximately equals 2.

16. A method as defined in claim 13 in which said voltage divider is trimmed progressively by removal of portions of the resistors of said voltage divider at least in part by sandblasting.

17. A method as defined in claim 13 in which said voltage divider is trimmed progressively by removal of portions of the resistors of said voltage divider at least in part by laser beam cutting.

18. A method as defined in claim 13 in which said voltage divider is trimmed progressively by modification of portions of the resistors of said voltage divider at least in part by electrochemical action.

19. A method as defined in claim 13 in which a reference voltage source is produced in monolithic integrated circuit form and in which the trimming of said voltage divider (R.sub.1, R.sub.2) is carried out on a monolithic integrated circuit chip.

20. A method as defined in claim 19 in which said voltage divider (R.sub.1, R.sub.2) is trimmed during the premeasurement of the chip on a wafer test equipment.

21. A method as defined in claim 13 in which at least one of the resistors (R.sub.1, R.sub.2) constituting said voltage divider is subdivided into at least two resistor portions for purposes of trimming and that the trimming step is carried out by the interruption of at least one conduction path by which said resistor portions were originally interconnected.

22. A method of manufacture as defined in claim 21 in which the resistance values of said resistor portions utilized in the trimming step are related to each other in the progressive ratio 1 : 2 : 4 : 8 : 16 : 32 : ... .

23. A method as defined in claim 13 in which the step of trimming a voltage divider is performed with a voltage regulator connected with said source of reference voltage as made prior to the trimming step, so as to form a voltage stabilizer together, said voltage stabilizer providing a stabilized output voltage U.sub.A at the output of a second voltage divider (R.sub.3, R.sub.4), and in which the desired stabilized voltage U.sub.A is obtained by variation of the first voltage divider (R.sub.1, R.sub.2) in the completion of the manufacture of the source of reference voltage as aforesaid, and in which, further, the second voltage divider (R.sub.3, R.sub.4) has a resistance ratio defined as ##EQU40## and the reference voltage U.sub.1o at the reference temperature T.sub.o is defined as satisfying the condition ##EQU41## where A is the constant portion of the temperature coefficient of the Zener diodes in .degree.K.sup.-.sup.1,

A.sub.d is the temperature coefficient of the conductively poled diodes in their conductive state, in .degree.K.sup.-.sup.1,

E is said desired temperature coefficient of said reference voltage U.sub.1, in .degree.K.sup.-.sup.1, and

U.sub.do is the voltage drop per diode in volts across the conductively poled diodes during conduction at the reference temperature T.sub.o,

and the temperature coefficient E of the reference voltage U.sub.1 is set equal to the desired temperature coefficient of the stabilized output voltage U.sub.A.