Circuit to correct non-linear terms in bandgap voltage references

#Reference voltages have a broad appliability to several aspects of solid state electronics. Voltage regulators, analog-to-digital converters and digital-to-analog converters are some examples which require high precision reference voltages for optimum operation.

#One problem has been observed in that the output of a reference will tend to vary with changing temperature. This has been attributed to the physical properties of the components used. This temperature variation has a linear component as well as a non-linear component. For example, see the Robert J. Widlar article entitled "Low Voltage Techniques" presented Feb. 17, 1978 at the I.E.E.E. International Solid State Circuits Conference.

#Accordingly, it is an object of the present invention to provide a circuit which compensates for the non-linear temperature caused variation in a reference voltage.

#The novel featurs believed to be characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof may best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings in which:

#FIG. 1a represents a circuit diagram of the present invention isolated from associated circuitry.

#FIG. 1b represents a typical linear variation compensation circit without circuit means for non-linear compensation.

#Referring now to FIG. 1a, transistor Q.sub.1 is a set of four transistors operated in parallel. Four transistors were found experimentally to have the proper amount of temperature responsivity when the base was connected to the voltage divider circuit shown by R.sub.1 and R.sub.2. The number of transistors, and the values for R.sub.1 and R.sub.2 are governed by formulas disclosed herein. In the preferred embodiment, R.sub.1 is 4.96 K.OMEGA. and R.sub.2 is 178.OMEGA.. The junction ratio between Q.sub.1 and Q.sub.2 is 4 to 1 which results in smaller resistance values for R.sub.1 and R.sub.2 and thus less area on the chip.

#The background of the mathematical derivation of the required formulas is contained in the article by Yannis P. Tsividis entitled "Accurate Analysis of Temperature Effects in I.sub.c -V.sub.BE Characteristics With Application to Bandgap Reference Sources," published in the I.E.E.E. Journal of Solid State Circuits, Vol. SC-15, No. 6, December, 1980, which is incorporated herein by reference.

#The approach in the preferred embodiment requires a derivation for the temperature variated current at point 11, in FIG. 1a. By appropriate control of that current with Q.sub.2, a non-linear current component is subtracted which corresponds very closely with the observed non-linear variation caused by temperature fluctuation in a circuit such as shown in FIG. 1b. By deriving the temperature equation for the current at point 11, the formula