Reference Voltage Source Circuit

The present invention relates to a reference voltage source circuit, and more particularly to an integrated reference voltage source circuit for generating a reference voltage stabilized with respect to variations in temperature.

Recently a temperature-independent, stable reference voltage has been needed for use in electronic devices, e.g., digital-analog converters. In such applications a reference voltage source is expected to provide an output voltage with a temperature coefficient, i.e., a variation with temperature, controlled to a value within .+-.50 PPM/.degree. C. A reference voltage source circuit comprising a Zener diode and a transistor has heretofore been known, in which the positive temperature coefficient of the Zener diode is compensated by a negative coefficient of the forward transistor voltage. This approach, however, is not practical since it is extremely difficult to control the temperature coefficient of a reference voltage within .+-.50 PPM/.degree. C because Zener diodes are not always consistent quality. In addition, Zener diodes exhibit an inferior noise characteristic.

Another prior art approach uses a silicon energy bandgap for a reference voltage source in the form of a monolithic integrated circuit. Again, this approach is impractical since values of resistors formed on a monolithic chip by the diffusion of impurities deviate due to the diffusion process, resulting in variations in the output voltage of the silicon bandgap voltage source circuit and in temperature coefficient. This has made it difficult to control the temperature coefficient to a value within .+-.50 PPM/.degree. C. One solution to this problem has been to use thin-film resistors. However, the resistance values have had to be precisely adjusted by LASER trimming or like techniques, thus increasing production costs considerably.

It is therefore an object of the present invention to provide a reference voltage source which can readily be fabricated into a monolithic integrated circuit.

It is another object of the invention to provide a reference voltage source circuit capable of compensating for deviation in resistance values of resistors formed in monolithic integrated circuit, and thus generating an output reference voltage with a minimum temperature coefficient.

It is another object of the invention to provide a reference voltage source circuit suited for digital-analog converters.

A silicon energy bandgap reference voltage source circuit to be improved by the present invention comprises a differential amplifier, a pair of transistors having their bases connected in common and collectors respectively connected to different input terminals of the differential amplifier, and load resistors connected to the collectors of the transistor pair, respectively, in which the transistors are supplied with collector currents through the load resistors and the output of the differential amplifier is coupled with the common base junction of the transistors. In accordance with a feature of the invention, collector currents of the transistors are adjusted by a variable resistor such that the sum of the transistors and .alpha.-times a difference voltage .DELTA. V.sub.BE between the voltages V.sub.BE of the pair of transistors (.alpha. being a constant, positive number) be equal to a silicon energy bandgap voltage. An output reference voltage of this circuit is equal to the silicon energy bandgap voltage, i.e., the sum of V.sub.BE + .alpha..multidot..DELTA. V.sub.BE, where the common base junction is directly connected to the output of the differential amplifier. Where the base common junction is connected to the output of the amplifier through a resistive voltage dividing circuit, the output of the reference voltage circuit is larger than the silicon energy bandgap voltage by a ratio determined by the voltage division circuit.

Further objects, features and advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a circuit diagram showing a conventional silicon energy bandgap reference voltage source circuit,

FIG. 2 is a circuit diagram illustrating one embodiment of the present invention,

FIG. 3 is a circuit diagram depicting a second embodiment of the present invention,

FIG. 4 is a circuit diagram showing a third embodiment of this invention, and

FIG. 5 is a circuit diagram showing still another embodiment of the invention.

With reference to FIG. 1, there is schematically shown a prior art silicon energy bandgap reference voltage source circuit described in "A Simple Three Terminal IC Bandgap Reference" by A. P. Brokaw, IEEE Journal Solid-State Circuits, Vol. SC-9, No. 6, December 1974. In FIG. 1, the bases of a pair of NPN transistors T.sub.r1 and T.sub.r2 are connected in common, and a voltage at an output terminal 3 of a differential amplifier 1 is fed back to this common base. The collectors of the transistors T.sub.r1 and T.sub.r2 are respectively connected to a noninverting input and an inverting input of a differential amplifier 1 and further connected to a positive terminal 2 of a power source through load resistors R.sub.3 and R.sub.4, respectively. The transistor T.sub.r1 has its emitter connected to a negative terminal 4 of the power source, for example to a ground potential, through resistors R.sub.1 and R.sub.2. The transistor T.sub.r2 has its emitter connected to the connection point of the resistors R.sub.1 and R.sub.2. The differential amplifier 1 is supplied with power through the power terminals 2 and 4.

The sum of the base-emitter voltage V.sub.BE of the transistor T.sub.r2 and the voltage being .alpha.-times (.alpha.: a positive constant) a voltage across the resistor R.sub.1, i.e., the difference .DELTA.V.sub.BE between the base-emitter voltages V.sub.BE of the transistors T.sub.r1 and T.sub.r2, is generated at the output terminal 3. This output voltage is made to be equal to the silicon energy bandgap voltage V.sub.GO and thus a temperature-independent reference voltage is provided.

The output voltage V.sub.OUT is given by Eq. (1) below, where the load resistors R.sub.3 and R.sub.4 are assumed to be the same in resistance value. ##EQU1## Here, .alpha. is equal to 2R.sub.2 /R.sub.2. In Eq. (1), the difference voltage V.sub.BE is expressed as

where "K" denotes the Boltzmann's constant, q a unit charge, "T" the absolute temperature, I.sub.s1 and I.sub.s2 saturation currents of transistors T.sub.r1 and T.sub.r2, and I.sub.1 and I.sub.2 collector currents of transistors T.sub.r1 and T.sub.r2.

By selecting the output voltage V.sub.OUT as defined in Eqs. (1) and (2) to be equal to the silicon bandgap voltage V.sub.GO (.div. 1.205 V), the temperature drift of the output voltage V.sub.OUT can be reduced. In practice, the output voltage V.sub.OUT varies due to deviations in resistance values of the resistors R.sub.1 to R.sub.4 formed on a monolithic chip. This has made it extremely difficult to control the temperature coefficient to a value within .+-.50 PPM/.degree. C.

FIG. 2 is a circuit diagram showing one embodiment of the present invention. Like constituent components are indicated by the identical reference numerals in FIGS. 1 and 2. Instead of the load resistors R.sub.3 and R.sub.4 in FIG. 1, collector load resistors R.sub.3 ' and R.sub.5, and R.sub.4 ' and R.sub.6 are used for transistors T.sub.r1 and T.sub.r2 respectively, as shown in FIG. 2. In other words, the transistor T.sub.r1 has its collector connected to a positive power terminal 2 through the resistors R.sub.3 ' and R.sub.5, and the transistor T.sub.r2 has its collector also connected to the positive power terminal 2 through the resistors R.sub.4 ' and R.sub.6. In addition, a variable resistor R is connected across a junction point 5 between the resistors R.sub.3 ' and R.sub.5 and a junction point 6 between the resistors R.sub.4 ' and R.sub.6. A variable tap 7 of the variable resistor R is connected to the power terminal 2. The resistors R.sub.3 ', R.sub.4 ', R.sub.5 and R.sub.6 are formed together with the transistors T.sub.r1 and T.sub.r2 on a monolithic semiconductor integrated circuit chip with terminals 5, 6 and 7 among others, while the variable resistor R is attached to this monolithic chip by being electrically connected to the terminals 5, 6 and 7 of the chip. In this circuit, the temperature coefficient of the output voltage can be controlled to a value smaller than a specific value in the following manner.

An output voltage V.sub.OUT at a terminal 3 in FIG. 2 is given as ##EQU2## where I.sub.1 and I.sub.2 denote collector currents of T.sub.r1 and T.sub.r2, I.sub.s1 and I.sub.s2 saturation currents of T.sub.r1 and T.sub.r2, and R.sub.L1 = R.sub.3 ' + R.sub.5 //R.sub.x and R.sub.L2 = R.sub.4 ' + R.sub.6 //R.sub.y (R.sub.x : resistance across terminals 5 and 7 of the variable resistor R, and R.sub.y : resistance across terminals 7 and 6 of the variable resistor R).

The forward junction voltage V.sub.BE at a temperature "T" is given as:

where V.sub.BE (T.sub.o) denotes the value of V.sub.BE at a temperature T.sub.o. Substituting Eq. (4) for Eq. (3) and substituting the condition dV.sub.OUT /dT = O at T = T.sub.o for Eq. (3), ##EQU3##

Substituting the condition .DELTA.T = T - T.sub.o for Eq. (5), ##EQU4## where .eta. denotes a device constant, "A" a factor coefficient, F(R.sub.x, R.sub.y) a function depending on R.sub.x and R.sub.y, and .theta. a difference in temperature coefficients difference between the fixed resistors (R.sub.3 ', R.sub.4 ', R.sub.5 and R.sub.6) and the variable resistor R. In Eq. (6), the third term shows a temperature drift which accounts for the variable resistor R. The temperature coefficient on the third term can be adjusted to a value within .+-.10 PPM/.degree. C by suitably selecting the values of A, R, R.sub.3 ', R.sub.4 ', R.sub.5 and R.sub.6. The temperature coefficient value of the output voltage V.sub.OUT depends on the second and third terms of Eq. (6) when the output voltage of the differential amplifier 1 which depends on Eqs. (2) and (3) is made equal to the first term {V.sub.GO + (.eta.--1).multidot.KT.sub.o /q} of Eq. (6) at the ordinary temperature (T = T.sub.o) by adjusting the variable resistor R. This temperature coefficient comes within .+-.10 PPM/.degree. C at a temperature in the range of T.sub.o .+-.30.degree. C. As previously mentioned, V.sub.GO is nearly equal to 1.205 V and (.eta. - 1).multidot.KT.sub.o /q is about 0.02 V. Hence, by making the output V.sub.OUT of the differential amplifier 1 approximately equal to the silicon energy bandgap voltage V.sub.GO at the ordinary temperature by means of the variable resistor R, it is possible to realize a reference voltage source circuit having a temperature coefficient controlled to a value about within .+-.20 PPM/.degree. C.

Referring to FIG. 3, there is shown a circuit diagram of another embodiment of the invention. Like constituent components are indicated by the identical reference numerals in FIGS. 2 and 3. This embodiment differs from the one shown in FIG. 2 in the voltage supply to the load resistors R.sub.5 and R.sub.6 and to the variable tap 7 of the variable resistor R, as well as in the base input supply to the transistors T.sub.r1 and T.sub.r2. The common terminal of the resistors R.sub.5 and R.sub.6, and the variable tap 7 are connected to the output terminal 3, which is grounded through resistors R.sub.7 and R.sub.8. The bases of the transistors T.sub.r1 and T.sub.r2 are commonly connected to the junction point 8 between the resistors R.sub.7 and R.sub.8. In this circuit, when the voltage at the junction point 8 is V.sub.GO, the output voltage V.sub.OUT at the output terminal 3 is V.sub.GO .multidot.(R.sub.7 + R.sub.8)/R.sub.8. This constant voltage V.sub.OUT is supplied to the collector load resistors of the transistors T.sub.r1 and T.sub.r2. As a result, the supply voltage rejection ratio (i.e., variations in output voltage at terminal 3 for variations in supply voltage) can be improved and the output voltage V.sub.OUT can be arbitrarily determined by suitably selecting R.sub.7 and R.sub.8. Thus, as in the embodiment shown in FIG. 2, a reference voltage source circuit minimally affected by temperature variations can be realized.

Referring to FIG. 4, there is shown a circuit diagram of another embodiment of the invention. Again, like constituent components are indicated by the identical reference numerals in FIGS. 2 and 4. This circuit differs from the one shown in FIG. 2 in the voltage supply to the collector load resistors R.sub.5 and R.sub.6 and to the tap 7 of the variable resistor R. The common terminal of the resistors R.sub.5 and R.sub.6 and the variable tap 7 are connected in common to the positive power terminal 2 through a current source 9 and also to the output terminal 3 through level shifting diodes D.sub.1 and D.sub.2. In this embodiment, the level shifting diodes are connected in series in two stages. Alternatively, the level shifting diodes may be installed in the desired stages according to the voltage supplied to the power terminal 2. In this embodiment also, the supply voltage rejection ratio is improved because a constant voltage provided from the constant reference voltage V.sub.GO clamped by the level shifting diodes is supplied to the collector load resistors of the transistors T.sub.r1 and T.sub.r2.

Referring to FIG. 5, there is shown a circuit diagram of another embodiment of the invention. Like constitutent components are indicated by the identical reference numbers in FIGS. 2 and 5. This FIG. 5 circuit differs from the one shown in FIGS. 2 in the collector load resistor part of the transistors T.sub.r1 and T.sub.r2. The fixed resistors R.sub.3 ', R.sub.4 ', R.sub.5 and R.sub.6 of FIG. 2 are omitted. The transistor T.sub.r1 has its collector connected to one end of the variable resistor R, and the transistor T.sub.r2 has its collector connected to the other end thereof. The tap 7 of the variable resistor R is connected to the positive power terminal 2. In this circuit, the resistance value of the variable resistor R between the collector of T.sub.r1 and the tap 7 corresponds to R.sub.L1 in Eqs. (3), (4), (5) and (6) described with reference to FIG. 2, and the resistance value of the variable resistor R between the collector of T.sub.r2 and the tap 7 corresponds to R.sub.L2 in the same equations. Thus this circuit can also generate a reference voltage with a small temperature drift as in the circuit shown in FIG. 2.

According to the invention, as has been described above, an output reference voltage with a minimum temperature coefficient can be obtained. The reference voltage source circuit of the invention is therefore highly suited for digital-analog converters and the like. Furthermore, the invention obviates the need for intricate adjustment of the resistance values of the transistor load resistors such as by LASER trimming, thus permitting the circuit of the invention to be fabricated into a monolithic IC except for the variable resistor. Although the disclosed embodiments employ NPN transistors, it is apparent that PNP transistors may be used instead of the NPN transistors. Also, instead of the variable resistor R, a fixed resistor whose resistance value has been adjusted for a specific one may be used.

While several preferred embodiments of the invention and particular modifications thereof have been described, it is to be understood that numerous variations may occur to those skilled in the art without departing from the true spirit of the invention.