Temperature Compensated Voltage Reference Circuit

The present invention relates to solid state integrated band-gap type voltage reference circuits. More particularly, this invention relates to band-gap reference circuits wherein the output voltage can be made any multiple of the band-gap voltage in which the output voltage remains substantially constant with temperature variation.

Solid state band-gap references are well known to those skilled in the art which rely on certain temperature dependent characteristics of the base-emitter voltage (VBE) of a bipolar transistor. For example, U.S. Pat. No. 3,617,859 describes such a band-gap reference wherein the negative temperature coefficient of the base-to-emitter voltage of a first transistor in conjunction with the positive temperature coefficient of the base-to-emitter voltage differential between two additional transistors operating at different current densities is used to achieve a zero temperature coefficient reference potential.

Another voltage reference circuit of the type referred to incorporates four transistors which are interconnected, with respective pairs of the transistors having ratioed emitter areas to establish a difference voltage across a reference resistor having a positive temperature coefficient. This positive temperature coefficient voltage across the reference resistor can be used to negate the negative temperature coefficient of the base-to-emitter voltage of another transistor. This particular reference circuit is shown and described in U.S. Pat. No. 3,908,162.

Although prior art voltage reference circuits based on the V.sub.BE characteristics of transistors and discussed above have advantages associated therewith, these types of circuits suffer from some limitations. For instance, these circuits may suffer on accuracy and TC compensation as well as having beta dependent characteristics which are not desired. Therefore, there is a need for an improved temperature compensated voltage reference circuit which overcomes the aforementioned limitations as well as having superior load rejection characteristics. In addition such improved circuit would desirably have load driving capability.

Accordingly, it is an object of the present invention to provide an improved voltage reference circuit.

Another object of the present invention is to provide an improved solid state voltage reference circuit.

Still another object of the invention is to provide an improved solid state band-gap voltage reference wherein the reference voltage has a value that can be made any integral multiple of the band-gap voltage below the positive power supply conductor rail.

A further object is to provide a solid state band-gap voltage reference having both a low temperature coefficient associated therewith and load rejection capability.

In accordance with the above and other objects there is provided a temperature compensated voltage reference comprising a thermal source circuit responsive to a first or initial current for producing a second current at an output having a predetermined temperature coefficient and further including an output circuit responsive to the second current which produces a temperature compensated voltage at an output thereof.

FIG. 1 is a schematic diagram illustrating the temperature compensated voltage reference of a first embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating a temperature compensated voltage reference of a second embodiment of the invention; and

FIG. 3 is a schematic diagram illustrating a temperature compensated voltage reference of an additional embodiment of the invention.

Turning to FIG. 1 there is shown temperature compensated voltage reference 10 of the present invention. Reference 10 is suited to be manufactured in monolithic integrated circuit form. Reference 10 includes a thermal source circuit comprising cross-connected NPN transistors 12 and 14 which are interconnected with NPN transistors 16 and 18 as well as resistor 20. A first current I1 is produced through resistor 22, which is connected between power supply conductor 24 to an input of the thermal source circuit. Assuming the transistors have a high beta (current amplification factor), to the first order, base current errors can be neglected such that ideally the current I1 flows through the collector-emitter conduction path of diode connected transistor 16 and the collector-emitter conduction path of transistor 12 to power supply conductor 26.

Current I1 flowing through diode 16 and, thus, transistor 12 establishes a current I2 to flow through the collector-emitter conduction path of transistor 18 as the base of transistor 18 is connected to diode 16. This current I2 flows from the output of the thermal source circuit (at the collector of transistor 18) through the collector-emitter of transistor 14 and through resistor 20. The base of transistor 12 is cross coupled to the collector of transistor 14 and vice-versa such that a .DELTA.V.sub.BE voltage is developed across the resistor. The magnitude of .DELTA.V.sub.BE is equal to (I2.multidot.R.sub.20). As illustrated, transistors 14 and 16 are emitter area ratioed with respect to transistors 12 and 18 such that a different current density is established through the respective transistors. This causes a positive temperature coefficient voltage to be developed across resistor 20 as is well understood. The magnitude and temperature dependent characteristics of current I2 can be found by writing the voltage loop equation around the transistor circuit loop formed by the thermal source circuit. Thus, it can be shown that: ##EQU1## substituting the well known diode-current expression for each V.sub.BE term of equation 1 and rearranging yields the following: ##EQU2## Equation 2 shows that the current I.sub.2 flowing at the output of the thermal source circuit has a predetermined positive temperature coefficient and has a magnitude that is a factor of the emitter area ratios A and B.

Current I.sub.2 is sourced through an output circuit comprising diode 28 and resistor 30 which are series connected between power supply conductor 24 and node 32. The positive temperature coefficient of the resulting voltage developed across resistor 30 cancels the negative temperature coefficient of the voltage of diode 28 to produce an output voltage, V.sub.OUT, at output terminal 34 of voltage reference 10 that has a substantially zero temperature coefficient.

In general, V.sub.OUT can be made any multiple of the band-gap voltage, 1.2 volts below +V by changing the value of resistor 30 simultaneously with adding or decreasing the number of diode series connected therewith. For instance, if two diodes are series connected to node 32, resistor 30 would be doubled in value.

Referring now to FIGS. 2 and 3 there are illustrated voltage references comprising the thermal source circuit described above. Therefore, circuit components illustrated in FIGS. 2 and 3 corresponding to like components in FIG. 1 are indicated with the same reference numerals.

Voltage reference 40 of FIG. 2 enjoys improved thermal rejection over voltage reference 10. A preregulator circuit comprising resistor 36 series connected with diodes 38, 42 and 48 between power supply conductors 24 and 26 provides a voltage level at the interconnection between resistor 36 and the anode of diode 38 which is substantially proportional to absolute temperature by the same equations as shown above. The positive temperature coefficient of the voltage developed across resistor 22 due to the present circuit configuration, including the preregulator, helps reduce or inhibit variations in the output that might otherwise occur due to higher order base current error effects. Hence, the overall effect of voltage reference 40 is to provide a temperature regulated output voltage having better temperature compensation and regulation over that of the voltage reference 10.

Voltage reference 50 illustrated in FIG. 3 not only enjoys the better performance described above in reference to the voltage reference circuit of FIG. 2 but also provides improved output impedance and load rejection characteristics with respect to either reference 10 or reference 40. Moreover, voltage reference 50 has the additional advantage of being able to supply large drive currents at output 34.

As illustrated, resistor 30 is connected between power supply conductor 24 and node 32 with output 34 being taken at the emitter of the emitter follower transistor 48, the base of which is connected to node 32. Transistor 52 has its collector-emitter path connected between the emitter of transistor 48 and power supply conductor 26 and its base connected to the base of transistor 14 wherein the collector current of this transistor is mirrored with respect to output current I.sub.2 of the thermal source circuit.

The voltage drop across resistor 30 is amplified up from the voltage drop developed across resistor 20 and has a positive temperature coefficient associated therewith as aforedescribed. However, when the voltage drop across resistor 30 is added with the negative temperature coefficient base-emitter voltage of transistor 48, a substantially zero temperature coefficient output voltage is developed at output 34 wherein the magnitude is approximately one band-gap voltage drop below the voltage supplied at power supply conductor 24.

What has been described above is an all NPN temperature regulated band-gap voltage reference. The voltage reference is comprised of a thermal source circuit including cross-coupled and interconnected emitter area ratioed transistor pairs for producing an output current having a positive temperature coefficient which is utilized by an output circuit in conjunction with the negative temperature coefficient of a PN junction to establish a temperature compensated output voltage.