Miscellaneous active electrical nonlinear devices – circuits – and – External effect – Temperature
Reexamination Certificate
2003-02-27
2004-12-07
Riley, Shawn (Department: 2838)
Miscellaneous active electrical nonlinear devices, circuits, and
External effect
Temperature
C327S539000, C323S316000
Reexamination Certificate
active
06828847
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a bandgap voltage reference circuit for producing a stable TlnT temperature curvature corrected voltage reference, which preferably is suitable for fabrication in a CMOS process, and the invention also relates to a PTAT voltage generating circuit for generating a PTAT voltage with a temperature curvature complementary to an uncorrected TlnT temperature curvature CTAT voltage of the type developed across a base-emitter of a transistor, which preferably is suitable for fabrication in a CMOS process. The invention also relates to a method for producing such a voltage reference and a PTAT voltage.
BACKGROUND TO THE INVENTION
Most electronic circuits require a stable DC voltage reference, and in particular, a temperature stable DC voltage reference. Bandgap voltage reference circuits for producing a reasonably temperature stable DC voltage reference are known. Such bandgap voltage reference circuits rely on the property of a bipolar transistor to produce a substantially constant base-emitter voltage, and when fabricated in silicon, rely on the property of silicon which when a bipolar transistor is fabricated in silicon produces a base-emitter voltage in the range of 0.5 volts to 0.8 volts. However, the voltage produced by the base-emitter of a transistor has a negative temperature coefficient, in other words, the voltage is complementary to absolute temperature (CTAT). In known bandgap voltage reference circuits a pair of transistors are operated at different current densities and are arranged to develop a voltage which is proportional to the difference in the base-emitter voltages of the two transistors. This difference voltage has a positive temperature coefficient, in other words, the voltage is proportional to absolute temperature (PTAT). The PTAT voltage provided by the difference in the base-emitter voltages is properly scaled and summed with the CTAT voltage of one of the transistors to produce the voltage reference. However, as well as the linear relationship with temperature of the CTAT base-emitter voltage of a transistor, the CTAT base-emitter voltage also exhibits a non-linear temperature relationship which is referred to as temperature curvature. This non-linear relationship of the CTAT voltage to temperature is commonly represented by the term K.TlnT where K is a constant and T is absolute temperature in degrees Kelvin (° K). Thus, in order to produce a voltage reference which is entirely temperature stable over a reasonable temperature range, the TlnT temperature curvature of the CTAT base-emitter voltage must also be corrected for.
Various attempts have been made to correct for the TlnT non-linearity of the CTAT voltage of the base-emitter of a transistor. U.S. Pat. No. 5,352,973 of Audy discloses a bandgap voltage reference circuit where the TlnT temperature curvature is corrected for. The bandgap voltage reference circuit of Audy comprises a Brokaw bandgap voltage reference cell and a correction cell. The Brokaw cell comprises first and second bipolar transistors which are arranged to develop a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors. The PTAT voltage difference is developed across a first resistor. The first and second transistors are operated with PTAT collector currents, and the collectors of the two transistors are held at a common voltage by an operational amplifier.
The correcting cell corrects for the TlnT curvature term, and comprises a third bipolar transistor which co-operates with one of the second transistor of the bandgap cell for developing a voltage across a second resistor which is proportional to the difference in the base-emitter voltages of the third transistor and the second transistor of the Brokaw cell. An operational amplifier drives the emitter of the third transistor until its collector current is at a substantially constant temperature insensitive value. This, thus, causes the difference voltage developed across the second resistor to have a TlnT curvature which is complementary to the TlnT curvature of the base-emitter CTAT voltage. Currents which flow through the first resistor in the Brokaw cell and the second resistor in the correction cell are summed in a third resistor embedded in the Brokaw cell for developing a corresponding voltage with a TlnT curvature complementary to the CTAT base-emitter voltage. The voltage developed across the third resistor is summed with the CTAT base-emitter voltage of the second transistor of the bandgap cell to provide a temperature stable and TlnT curvature corrected voltage reference.
However, while the voltage reference developed by the bandgap circuit of Audy is TlnT curvature corrected, and is thus temperature stable within a relatively wide temperature range, unfortunately, the bandgap circuit of Audy does not lend itself to easy implementation in a CMOS process. Furthermore, Audy relies on the PTAT current through the first resistor and the current through the second resistor which has a TlnT curvature complementary to the CTAT base-emitter voltage for developing the PTAT voltage with TlnT curvature across the third resistor.
U.S. Pat. No. 5,424,628 of Nguyen discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of bipolar transistors arranged in similar fashion to that of Audy in U.S. Pat. No. 5,352,973 for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors, which is then summed with a CTAT base-emitter voltage of one of the transistors of the bandgap cell. The Nguyen bandgap voltage reference circuit includes additional circuitry for providing a correction current signal, which is generated by a current squaring circuit, and is injected into the collector of one of the two transistors of the bandgap cell such that the collectors of the two transistors have unequal current values. The correction current is injected into the transistor which is to provide the CTAT base-emitter voltage of the voltage reference, and it is alleged that the collector current difference between the two transistors enables the elimination of the TlnT curvature of the CTAT base-emitter voltage. However, the circuitry required for implementing the bandgap voltage reference circuit of Nguyen is relatively complex, and additionally, it does not lend itself to a CMOS process.
U.S. Pat. No. 6,157,245 of Rincon-Mora discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of transistors arranged to develop a PTAT voltage proportional to the difference of the base-emitter voltages of the transistors, and this voltage is used to generate a PTAT current which is applied to one resistor of a resistor divider circuit comprising two resistors, across which the voltage reference is developed. The bandgap voltage reference circuit of Rincon-Mora also comprises a compensating circuit which generates a logarithmic operating temperature dependent current which is applied to the second resistor of the voltage divider network for developing a logarithmic temperature dependent correcting voltage across the second resistor. The voltages across the first and second resistors are summed to provide a voltage reference, which is allegedly temperature stable and TlnT curvature corrected. The circuitry of the Rincon-Mora bandgap voltage reference circuit is relatively complex, and does not easily lend itself to implementation in a CMOS process.
U.S. Pat. No. 5,512,817 of Nagaraj discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of bipolar transistors arranged for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors. The PTAT difference voltage is developed across a first resistor, and the developed PTAT difference voltage on the first resistor is scaled onto a second resistor through a current mirror circuit. The scaled voltage on the second resistor is summed with the CTAT base-emitter voltage of one
Analog Devices Inc.
Riley Shawn
Wolf Greenfield & Sacks P.C.
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