(5
`United States Patent
`[11]
`Widlar
`[45]
`
`4,249,122
`Feb. 3, 1981
`
`(54] FEMPERATURE COMPENSATED
`BANDGAPIC VOLTAGE REFERENCES
`
`[75]
`
`Inventor:
`
`Robert J. Widlar, Puerto Vallarta,
`Mexico
`
`4,103,219
`
`7/1978 Wheatley vo..ccsscsccssceseessenee 323/8
`
`Primary Examiner—A. D. Pellinen
`Attorney, Agent, or Firm—Gail W. Woodward; James
`A. Sheridan
`
`(73] Assignee: National Semiconductor Corporation,
`Santa Clara, Calif.
`
`[21] Appl. No.: 928,631
`
`[22] Filed:
`
`Jul. 27, 1978
`
`ABSTRACT
`[57]
`Bandgap voltage reference circuits have been devel-
`oped for integrated circuit applications. Typically, a
`negative temperature coefficient first voltage is devel-
`oped related to the base to emitter potentialof a transis-
`tor. A positive temperature coefficient second voltage
`related to the difference in base to emitter potential
`between twotransistors operating at different current
`densities is developed and combined with thefirst volt-
`age so as to produce a temperature compensated refer-
`ence voltage. Such first order compensation leaves
`second order effects uncompensated. In the invention, a
`third voltage having a suitable temperature coefficient
`3,617,859
`11/1971
`Dobkin etal... 323/19 X
`is combined with the first and second voltages so that
`3,893,018=7/1975 Marley .....cceeccecessenreerenteee 323/19
`the resultant reference voltage is compensated to a
`3,908,162
`9/1975 Marley etal.
`.. 323/19
`second order.
`
`4,088,941
`5/1978 Wheatley.........
`ws 32348
`5/1978 Hanna oo...ee cereeneseeee 323/19
`4,091,321
`4,100,436
`7/1978
`van de Plassche .............. 307/296 R
`
`[SE] Ent. CDS cece reeeeenereeeeennes GOSF 3/20
`[$2] US. CI. oe
`eeeeenseecees 323/313; 330/297
`
`(58} Field of Search ...........000000 307/296 R, 297;
`330/296, 297; 323/1, 4, 8, 19, 22 T, 68
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`8 Claims, 6 Drawing Figures
`
`CURRENT
`
`
`
`
`SOURCE
`
`EXHIBIT 1011
`
`1
`
`EXHIBIT 1011
`
`

`

`Feb. 3, 1981
`
`Sheet 1 of 3
`
`4,249,122
`
`10
`
`CURRENT
`
`U.S. Patent
`
`SOURCE
`
`2
`
`

`

`U.S. Patent
`
`Feb. 3, 1981
`
`Sheet 2 of 3
`
`4,249,122
`
`
`
`L 10
`
`SOURCE
`
` CURRENT
`
`3
`
`

`

`U.S. Patent
`
`Sheet 3 of 3
`
`Feb. 3, 1981
`
`4,249,122
`
`4
`
`

`

`1
`
`4,249,122
`
`TEMPERATURE COMPENSATED BANDGAPIC
`VOLTAGE REFERENCES
`
`BACKGROUNDOF THE INVENTION
`
`The invention relates to an improvement in tempera-
`ture compensated voltage reference circuits. U.S. Pat.
`No.3,617,859 issued to Robert C. Dobkin and RobertJ.
`Widlar on a basic voltage reference circuit and is incor-
`porated herein by reference.
`An improved form of temperature compensated volt-
`age reference circuit is disclosed in copending applica-
`tion Ser. No. 888,721 filed Mar. 21, 1978, by Robert C.
`Dobkin and titled AN IMPROVED BANDGAP
`VOLTAGE REFERENCE.
`In the design of electronic circuits constant voltage
`references are often useful. The object is to develop a
`potential that has an absolute known magnitudethatis
`substantially independent of current supply and load
`conditions. The avalanche or zener diode is characteris-
`tic of such a device but it has a temperature responsive
`voltage characteristic that
`is established by physical
`parameters. Furthermore, such devices have a knee, or
`transition region from variable to constant voltage, that
`produces noise. The so-called bandgap voltage refer-
`ence devices have been developed in integrated circuit
`(IC) form in which the fundamental electronic proper-
`ties of the semiconductor material are employed to
`develop a reference potential.
`DESCRIPTION OF THE PRIOR ART
`
`The prior art circuits are arranged to develop an
`output potential
`that
`is obtained by combining two
`potentials, one having a positive temperature coefficient
`and one having a negative temperature coefficient, in
`such a way that a temperature compensated output
`potential is produced.
`The base to emitter voltage (Vaz) ofa transistor is
`typically the source of potential with a negative temper-
`ature coefficient. The differential
`in base to emitter
`voltage (AV£) of two transistors operating at different
`current densities is typically the source of potential with
`a positive temperature coefficient. When those poten-
`tials are combined to produce a potential equal to the
`semiconductor bandgap extrapolated to 0° K., the tem-
`perature dependent terms cancel for zero coefficient.
`Hence, the devices are often called bandgap references.
`Using silicon devices Vag at 300° K.is typically about
`600 mV. With a current density ratio of abut ten, AVae
`is typically about 60 mV at 300° K.Since the extrapo-
`lated bandgapis about 1.205 volts, AVgz is multiplied
`by ten and combined with Var to produce 1.2 volts. It
`has been determined that if the reference is actually
`adjusted to 1.237 volts, the drift over the range of 220°
`to 400° K.is minimized, provided that the currentin the
`Veetransistor varies directly with temperature. Thus,
`in the vicinity of 300° K. (close to normal room temper-
`ature) the reference voltage will not vary significantly
`with temperature.
`In effect, as Var falls.at about 2mV for each degree
`K. rise in temperature, AVg¢ will rise about 0.2 mV for
`each degree K. temperature rise. When AVgeis multi-
`plied by ten the rise compensates thefall.
`The AVge potentialis linearly related to temperature,
`as shown in patent 3,617,859. However, Vaz, while
`linear with respect to temperature to a first order, in-
`cludes second order dependencies that make the tem-
`
`5
`
`10
`
`20
`
`30
`
`40
`
`45
`
`50
`
`55
`
`65
`
`2
`perature compensation imperfect, particularly over
`large temperature ranges.
`In practice if a curve of potential versus temperature
`is plotted, it is quite flat in the vicinity of 300° K. but
`shows curvature at temperatures remote from 300° K.
`For example, even a good reference will display a
`changein excess of 0.5% over a +80° K.range.
`SUMMARYOF THE INVENTION
`
`It is an object of the invention to improve the temper-
`ature compensation of bandgap voltage reference cir-
`cuits.
`.
`It is a further object of the invention to reduce the
`curvature of the temperature-voltage characteristic in a
`bandgap voltage reference.
`It is a still further object of the invention to produce
`a bandgap voltage reference in which second order
`temperature dependence is compensated.
`These and other objects are achieved as follows. A
`bandgap voltage reference circuit is employed in the
`conventional manner. A Vz¢ potential is generated and
`combined with a AVaz related potential to produce a
`first order temperature-compensated reference poten-
`tial. A third potential is developed, having a character-
`istic that matches the second order Vee temperature
`dependence, and combined with the first order terms to
`provide a reference potential, that is, compensates for
`the second order temperature dependence. In one em-
`bodiment the third potential
`is caused to vary with
`temperature by changing the current in a Vaz transistor
`as a function of temperature raised to some power. The
`exponentis selected to be in the range of about1.5 to 4,
`with 3 being preferred. In another embodiment
`the
`AVze potential is caused to vary by changing the ratio
`of current densities as a function of temperature.
`
`BRIEF DESCRIPTION OF THE DRAWING
`FIG.1 is a schematic diagram showing a temperature
`compensated reference circuit with provision for com-
`pensating. second order temperature dependency ef-
`fects;
`FIG.2 is a schematic diagram of a practical imple-
`mentation of the circuit of FIG. 1,
`FIG.3 is a schematic diagram of a basic reference
`circuit with second order temperature compensation;
`FIG. 4 is a schematic diagram of a very low voltage
`reference having second order temperature compensa-
`tion.
`FIG. § is a schematic diagram of the reference of
`FIG. 1 with discontinuous second order temperature
`compensation and;
`FIG. 6 is a schematic diagram of a basic reference
`circuit with discontinuous temperature compensation.
`DESCRIPTION OF THE INVENTION
`
`In the following discussions transistor base current
`will be largely ignored. Since IC transistors can consis-
`tently be manufactured to have beta values of 200, the
`base current typically represents only about 0.5% of the
`collector current. Accordingly, the simplification will
`not introduce serious error. In those instances where
`base current cannot be ignored without introducing a
`serious error, it will be accounted for.
`FIG,1 shows a bandgapreferencecircuit of the kind
`disclosed in the above-referenced Dobkin application
`Ser. No. 888,721. A pair of terminals 11 and 12 define
`the circuit which is energized by current source 10
`supplying Isource. Transistors 13 and 14are differentially
`
`5
`
`

`

`3
`connected and current source 15 supplies their com-
`bined current. Transistors 13 and 14 are operated at
`different current densities to generate AVag. Since tran-
`sistor 14 is at the lower current density, its base will be
`of a lower potential than the base oftransistor 13. Ratio-
`ing can be achieved by designing transistor 14 to have
`about ten times the area of transistor 13. In this case,
`load resistors 16 and 17 are matchedso that equal cur-
`rents will flow in the transistors. However, the current
`density ratio can be achieved by ratioing the load resis-
`tors 16 and 17 and using equalarea transistors. Further-
`more, the resistors can be ratioed as well as the transis-
`tor areas to achieve the desired current density ratio.
`A voltage divider consisting of resistors 18-20 and
`diodeconnected transistor 21 in series is connected
`across terminals 11 and 12. Resistor 19 is coupled be-
`tween the bases of transistors 13 and 14 to develop the
`AVee component. As shownin the drawingresistor 19
`is R, resistor 18 is xR, and resistor 20 is yR. Thus if
`AVze appears across register 19, the combined resistor
`voltage drop will be (x+ y+ DAVa<. If a current den-
`sity ratio of ten is used, AVger will be about 60 mV at
`300° K.If resistors 18 and 20 have a combined value of
`nine times the value of resistor 19, the three resistors
`will develop a potential of about 600 mV at 300° K.
`Since transistor 21 will develop a Vag of about 600 mV
`at 300° K., the total potential across the series combina-
`tion is about 1.2 volts at 300° K. As pointed out above,
`the temperature coefficients of the two potentials will
`be substantially equal and opposite thus compensating
`the circuit for temperature to a first order.
`Thecircuit is stabilized by amplifier 22 which senses
`the differential voltage at the collectors oftransistors 13
`and 14 and, by shunting a portion of Isource, forces the
`potential across terminals 11 and 12 to produce zero
`differential collector voltage.
`In accordance with the invention, the temperature
`compensation of the circuit can be improved by ac-
`counting for second order effects. This can be done by
`inserting a temperature dependent imbalance into the
`circuit as shown by the current source at 25 or the
`current source at 26. By making Is in source 25 and/or
`Ine in source 26 temperature dependent, as will be
`shown hereinafter, the circuit can be compensated for
`second order temperature effects as well as the first
`order compensated of the prior art. A key point is that
`Ty5 and I¢ vary as a function of temperature in a differ-
`ent way than Ij.
`The formula for AVg<is:
`
`AVRE=AT/qinJ1/22)
`
`q)
`
`Where:
`q is the electron charge
`k is the Boltzmann’s constant
`T is absolute temperature
`31/J2 is the transistor current density ratio.
`The formula for Vgzis:
`
`Vai = Vel — T/T) + VEEAT/To) + 0kT/q In
`(To/T) + KTql ic/ICo)
`
`60
`
`Q)
`
`Where:
`Vo is the semiconductor bandgap extrapolated to
`absolute zero.
`VBEis the base to emitter voltage at T, and Ic,
`Icis collector current
`n is a transistor structure factor and is about 3
`
`65
`
`4,249,122
`
`4
`for NPN double-diffused IC transistors.
`For the best compensation using silicon devices over
`a 220° K. to 400° K. temperature range:
`
`1.237V=Vag+adVgg
`
`Q)
`
`Where:
`ais a multiplying factor.
`Formula (1) shows that the AVgg term is a linear
`‘function of temperature. However, Vge is not. The
`third term in Formula (2) is the one that causes the basic
`circuit of FIGS. 1 and 3 to depart from compensation
`and constitutes a significant second order effect. For
`small
`temperature changes T,/T=1 and In T,/T is
`: small and insignificant. However, over the temperature
`range demanded of operating devices, the logarithmic
`temperature ratio term becomessignificant.
`The current sources 25 and 26 of FIG. 1 will act to
`introduce an effective offset potential into the circuit
`and shift the current ratio in transistors 13 and 14 as a
`function of temperature. The feedback loop around
`amplifier 22 will still force the differential collector
`voltage to zero. This offset will then cause AVagg to
`vary with temperature differently.
`Thecircuit of FIG, 2 is a practical realization of the
`circuit of FIG. 1. In addition, it discloses a three-termi-
`nal circuit representation. It is to be understood thatall
`of the circuits to be discussed herein can be imple-
`mented with a similar three-terminal equivalent.
`A source of potential is applied between terminals
`101 (+ V) and 112 (~V). This would be the conven-
`tional voltage supplied to the IC. The reference poten-
`tial shownat terminal 111 (Vzep)is in relation to termi-
`nal 112. A positive potential (+ V)is applied to differen-
`tial operational amplifier 122 as a power supply so that
`the output terminal, when coupled to terminal 111, will
`supply current thereto. Thus, the current source 10 of
`FIG.1 is inherent in the circuit.
`Transistors 113 and 114 are operated at raticed cur-
`rent densities and AVge appears across resistor 119.
`Amplifier 122 drives the potential between terminals
`111 and 112 to force the input differential to zero. Basi-
`cally the circuit functions as was described for FIG. 1.
`However, it can be seen that the voltage divider that
`includes resistors 118, 119, and 120 also includes two
`diode connectedtransistors, 102 and 121. Since resistor
`119 develops about 60 mV at 300° K., resistors 118 and
`120 should develop a total of about 1.24 volts to provide
`a Vrer of about 2.5 volts, for basic compensation.
`Transistor 104 is connected to diode 121 toprovide a
`current inverter. Thus the current flowing in resistor
`103 mirrors the current flowing in resistor 119 whichis
`proportional to AVge. Resistor 103 has a relatively
`small value so that it develops a few tens ofmillivolts at
`300° K. and this voltage has a positive temperature
`coefficient. This voltage appears in series with resistor
`117 and constitutes an offset potential at the input to
`amplifier 122. The amplifier will still act on the voltage
`at terminal 111 to force its differential input to zero.
`Transistor 115 acts as a current sourceto transistors
`113 and 114. Since the base of transistor 115 is biased up
`two Vpevalues, the voltage across resistor 105 will be
`equal to one Vge. Thusresistor 105 sets the combined
`current flowing in transistors 113 and 114 and this cur-
`rent has a negative temperature coefficient because it is
`directly proportional to V ge.
`As temperature rises, the total current in transistors
`113.and 114 will fall and the potential across resistor 103
`
`20
`
`23
`
`45
`
`35
`
`6
`
`

`

`4,249,122
`
`5
`will rise. These values can be proportioned so that the
`curvature of the temperature voltage curve of the un-
`compensatedcircuit is largely cancelled and the circuit
`is temperature compensated to a second order.
`FIG. 3 shows a bandgapreference designed to work
`at twice the semiconductor bandgap voltage when ener-
`gized by current source 10. The basic operation is simi-
`jar to the circuit disclosed in U.S. Pat. No. 3,617,859.
`The AVagg term is generated by transistors 32 thru 35
`and appears across resistor 39. The actual value of 19
`AVge will be:
`
`ASVee= Vee 32+ Vee x3— VBE 4 VBE 3s
`
`4)
`
`where the numbersubscripts denote the transistor. The
`current through transistor 32 is established by resistor
`36, the current through transistor 33 by resistors 37 and
`44, the current through transistor 34 byresistor 38, and
`the current through transistor 35 by resistor 39. Thus,
`each transistor can have its current independently set.
`The AVage of formula (4) will appear across resistor 39.
`If resistor 40 is ratioed with respect to resistor 39,it will
`develop a multiple of AVg£ equal to the ratio. In opera-
`tion, the Vgg values of transistors 41 and 42 will com-
`bine with the AVg¢ multiple across resistor 40 to pro-
`vide a bandgap reference of about 2.5 volts across ter-
`minals 30-31.
`Transistors 41 and 42 are connected into a Darlington
`configuration along with resistor 43 Node 43 will be
`VBE 41+ VBE 42 above terminal 31 and at 300° K.will
`develop about 1.25 volts. This combined with the
`AVzerelated drop across resistor 40 will provide the
`temperature compensated 2.5 volts between terminals
`30 and 31.
`As explained above, the compensation is to a first
`order and the temperature versus voltage characteristic
`is curved. Transistor 43 and resistor 44 are added to the
`circuit to provide the desired second order compensa-
`tion. As temperature rises, the Vag acrosstransistors 43
`and 32 falls with the Vag of 43 falling more rapidly
`since it operates at lower current density. This action
`increases the relative current in transistor 33. Thus,
`while AVgg varies normally with temperature, an addi-
`tional or compensating variation is introduced to pro-
`vide a second order temperature compensation.
`FIG. 4 shows a very low voltage reference circuit
`that is compensated for second order temperature ef-
`fects. In the circuit of FIG. 4 operation is from current
`source 10 supplying I). A portion of I), labeled I2, will
`flow through the voltage divider consisting ofresistors
`50-52. Another portion, I3, flows through transistor 53
`and the remainder, 14 flows through transistor 54 and
`back to node 55 by wayofresistor 56.
`Transistor 54 is manufactured to have an emitter area
`large with respect to the emitter area of transistor 53
`and the current in transistor 54 is made small with re-
`spect to the current in transistor 53. Thus, the current
`density in transistor 54 is much smaller than the current
`density in transistor 53.
`Thecircuit functions to develop a reference potential
`(Vref) at terminal 60 and is arranged to maintain this
`potential constant as a function of temperature.
`The Vg¢ potential of transistor 53.appears at node 37.
`The voltage divider action of resistors 50-52 results in a
`fraction of this Vgeto appear acrossresistor 50. Thus,at
`node 61 a potential of Vaz plus a fraction thereof ap-
`pears. Assuming resistor 59 to be zero for the moment,
`it can be seen that, with respect to terminal 60, the Var
`of transistor 54 will subtract from the potential at node
`
`25
`
`45
`
`35
`
`@
`
`65
`
`6
`61 so that Vref will contain a AVge term. This term
`will be:
`
`. AVag=kT/glallsa/¥s4)
`
`(5)
`
`Where:
`k is Boltzman’s constant
`T is absolute temperature
`q is electron charge
`Js53 is current density in transistor 53
`Jsq is current density in transistor 54
`If the current density ratio is set, for example, at 50,
`AVze at 300° kelvin will be about 100 mV. If the frac-
`tion of Vaz appearing across resistor 50 is made about
`100 mV at 300 ° kelvin, Vrer will be about 200 mV.
`Accordingly, Vreris:
`
`Vrer=AVpe+ (Vg 53/6
`
`(6)
`
`The first term has a positive temperature coefficient
`and the second term has an equal negative temperature
`coefficient so that, to a first order, temperature compen-
`sation is achieved.
`Resistor 59 is present in the circuit to permit correc-
`tion for current source variations. A portion of I; will
`flow into the base of transistor 53 which will act as an
`inverting amplifier to node 58. Thus, if resistor 59 is
`made equal to the reciprocalof the transconductance of
`transistor 53, node 58 will be compensated for varia-
`tions in I|.
`As shown above,the circuit is compensated forfirst
`order temperature effects. By returning resistor 56 to a
`tap, node 5§, on the resistance associated with the Vac
`of transistor 53, a second order temperature compensa-
`tion is achieved.
`Resistor 56 will determine the current flowing in
`transistor 54 and henceits current density, J54 of equa-
`tion (5). Since the potential at node 55 will fall within
`rising temperature, due to the Vazof transistor 53, the
`current flowing in transistor 54 and hence its current
`density will increase with a rising temperature but less
`rapidly than the current in 53. Thus, the AVge term is
`varied nonlinearly as a function of temperature in such
`a direction as to compensate for the curvature in Vaz
`{and that introduced by the temperature drift of diffused
`resistors). The degree of compensation can be adjusted
`by the ratio of resistors 51 and 52, to compensate the
`curvature of the first order compensation described
`above.
`FIG. § represents an alternative compensation
`method for the circuit of FIG. 1. However, the com-
`pensation in FIG. 5 is discontinuous. All of the part
`designations are as used in FIG. 1 and the first order
`compensation is as was described for FIG. 1.
`The second order compensation is achieved by the
`action of transistor 65.and resistor 66. At the design
`temperature, for example, 300° K. where AVg¢ would
`be set to 60 mV which appears across resistor 19, tran-
`sistor 65 is inoperative. Thatis, the potential developed
`across resistors 18’, 19, and 20 is less than one VE so
`that negligible current will flow in resistor 66. As tem-
`perature rises and AVgzincreases, and Vge decreases, a
`point will be reached where transistor 65 will be turned
`on. As temperature further increases the current
`in
`transistor 65 will
`increase. Resistor 66 will determine
`how much the current in transistor 65 will rise and the
`tap on resistor 18 which sets the relative values ofresis-
`
`7
`
`

`

`7
`tors 18' and 18” will determine the temperature at
`which transistor 65 will turn on. This is selected to be
`the temperature at which curvature exceeds a certain
`valuein thebasic circuit. The increasing current flow in
`transistor 21 will cause its Vge value to increase. This
`will offset the normal tendency of Vg¢ to decline exces-
`sively with temperature. The degree of compensation at
`the higher temperatures will be established by the value
`of resistor 66.
`FIG. 6 represents a discontinuously compensated
`bandgapreferenceof the kind disclosed in U.S. Pat. No.
`3,617,859. Source 10 supplies Isource to terminals 11 and
`12. Transistors 70 and 71 generate AVg¢ which appears
`across resistor 72. Assuming a ten to one current density
`ratio, AVgz will be about 60 mV at 300° K.If resistor 72
`is made 600 ohms, 100 microamperes will flow in tran-
`sistor 71 at 300° K. If resistor 73 is made ten times the
`value of resistor 72, it will develop a drop of about 0.6
`volt, proportional to Vg. Since this drop is combined
`with the Vazof transistor 74, a compensated 1.2 volts
`appears across terminals 11 and 12. Clearly the required
`current density ratio can be established by current ratio-
`ing, area ratioing, or the combination of current and
`area ratioing.
`The circuit described thus far is temperature compen-
`sated to a first order. Transistor 77 and resistor 78 pro-
`vide the second order compensation. Since the base is
`tapped into the divider consisting of resistors 75 and 76,
`less than a Vgf at 300° K.will be applied to the emitter-
`base circuit of transistor 77. It will therefore be non-
`conductive. As temperature rises the Vaz in transistor
`70 will drop thereby increasing the potential across
`resistor 75. At some temperature, as determined by the
`values of resistors 75 and 76, transistor 77 will turn on
`and act to shunt resistor 75 thereby tending to increase
`the Vae of transistor 70 and offset its tendency to fall
`excessively with rising temperature. The amount of
`compensation is established by the value of resistor 78.
`This provides a discontinuous compensation of the sec-
`ond order temperature effect
`EXAMPLEI
`
`The circuit of FIG. 4 was constructed using standard
`bipolar IC techniques. The transistors had a Beta of
`about 200. The following resistor values were estab-
`lished using ion implanted resistors;
`
`Resistor
`Value/ohms
`50
`14.8K
`51
`82.4K
`52
`2.5K
`56
`135K
`
`59 2.8K
`
`The circuit was operated at about 20 microamperes.
`The reference voltage drift was less than 0.1% over the
`range of 220° K. to 400° K.
`EXAMPLEII
`
`Thecircuit of FIG. 2 was constructed as described in
`EXAMPLE|. All transistors were designed to have
`the same emitter area. The following resistor values
`were used:
`
`Resistor
`Value/ohms
`103
`400
`
`4,249,122
`
`-continued
`Resistor
`Value/ohms
`
`105
`6K
`116
`3K
`117
`30K
`118
`6.2K
`119
`600
`
`120 6.2K
`
`Amplifier 122 was a conventional high gain differen-
`tial operational amplifier trimmed to have substantially
`zero Offset voltage.
`VreFwas2.44 volts and varied less than 0.5 mv. over
`a temperature range of —55° to + 100° C.
`Theinvention has been described and examplesofits
`implementation set forth. A person skilled in the art
`when reading the foregoing disclosure will appreciate
`that there are other obviousalternatives and equivalents
`that come within the intent of the invention. Accord-
`ingly, it is intended that the scope of the invention be
`limited ‘only by the following claims.
`Weclaim:
`1. In a voltage reference circuit comprising:
`means for supplying operating current to said circuit;
`means for developing a first potential based upon the
`base to emitter potential of a transistor, said first po-
`tential having a negative temperature coefficient;
`means for developing a second potential based upon the
`difference in the base to emitter potentials offirst and
`secondtransistors operating at different current den-
`sities to produce a current density ratio, said second
`potential having a positive temperature coefficient;
`and
`means for combiningsaid first and second potentials to
`obtain a reference potential, said means for combin-
`ing operating to produce a reference potential that is
`temperature compensated to a first order;
`the im-
`provement comprising:
`means for varying said current density ratio as a func-
`tion of temperature to temperature compensate said
`reference potential to a second order.
`2. The circuit of claim 1 wherein said means for vary-
`ing said current density ratio comprise means respon-
`sive to said first potential and coupledto at least one of
`said first and second transistors thereby to vary the
`current in said one transistor relative tothe current in
`the other transistor.
`3. The circuit of claim 2 wherein said current density
`ratio is increased with increasing temperature.
`4. The circuit of claim 3 further comprising means
`responsive to a potential having a negative temperature
`coefficient and operative to vary the currents in both of
`said first and second transistors.
`5. The circuit of claim 3 wherein said means for vary-
`ing said current density is only operative above a prede-
`termined temperature.
`_ 6. A voltage reference circuit comprising:
`a pair of terminals across which a reference potential
`can be developed in response to the passage of an
`operating current;
`means coupled to said terminals for developing a first
`potential having a positive temperature coefficient,
`said first potential being developed in proportion to
`the difference in base to emitter potentials produced
`in a pair of transistors operating at different current
`densities to establish a current density. ratio;
`
`0
`
`15
`
`50
`
`55
`
`65
`
`8
`
`

`

`4,249,122
`
`10
`9
`'
`means for varying said current density ratio as a func-
`meanscoupled to said terminals for developing a second
`tion of temperature to provide second order tempera-
`potential having a negative temperature coefficient,
`ture compensation of said reference potential.
`4, The circuit of claim 6 wherein said means for vary-
`said second potential being developed in proportion
`ing is operativeabovea critical temperature and consti-
`to the base emitter potential of a current conducting
`tutes a discontinuous second order temperature com-
`transistor;
`pensation.
`8. The circuit of claim 7 wherein said meansfor vary-
`means for combining said first and second potentials so
`ing includes a transistor biased in response to a fraction
`that said positive and negative temperature coeffici-
`of said first potential, said fraction being selected to
`ents cancel to produce a potential available at said
`render said transistor conductive at
`temperatures in
`terminals that is temperature compensated toafirst
`excess of a predetermined value.
`*
`*
`*
`*
`*
`order; and
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50:
`
`60
`
`65
`
`9
`
`