A precision CMOS bandgap reference

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Abstract
The simple circuit uses naturally occurring vertical n-p-n bipolar transistors as reference diodes. Use is made of p-tub diffusions as temperature-dependent resistors to provide current bias, and an op-amp is used for voltage gain. Only two reference diodes, three p-tube resistors, and one op-amp are necessary to produce a reference with fixed voltage of -1.3 V. An additional op-amp with two p-tub resistors will adjust the output to any desired value. The criteria for temperature compensation are presented and it is shown that the properly compensated circuit can in principle produce thermal drift which is less than 10 p.p.m.//spl deg/C. Process sensitivity analysis shows that in practical applications it is possible to control the output to better than 2%, while keeping thermal drift below 40 p.p.m.//spl deg/C. Test circuits have been designed and fabricated. The output voltage produced was -1.3/spl plusmn/0.025 V with thermal drift less than 7 mV from 0/spl deg/C to 125/spl deg/C. Significant improvements in performance, at modest cost in circuit complexity, can be achieved if the op-amp offset contribution to the output voltage is reduced or eliminated.

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