Bandgap Reference Circuit With Replica BJTs for β-Drift Compensation
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Solution Overview
Problem
Bandgap references in CMOS technology experience significant temperature and component drift over their lifetime, leading to variations in reference voltage that affect accuracy.
Innovation Solution
A bandgap reference circuit is designed with a core circuit comprising bipolar transistors and replica circuits that feed replica base currents into the emitters to cancel out β-drift effects, stabilizing collector currents and reducing aging effects by a factor of 3 to 10.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a proportional-to-absolute-temperature (PTAT) voltage is added to the base-emitter voltage of a bipolar transistor to achieve low temperature drift, then temperature stability is improved, but process variations cause significant drift in the reference voltage over the lifetime of the bandgap reference
Solution Approach 1:
The patent employs replica bipolar transistors that replicate the electrical characteristics and aging behavior of the main bipolar transistors. By copying the device properties and subjecting them to the same process variations and temporal drift, the differential measurement can cancel out common-mode drift effects, thereby improving long-term reference voltage stability while maintaining temperature compensation
Solution Approach 2:
The patent implements a feedback mechanism where the reference voltage is continuously monitored and compared against a stable reference. The error signal generated by this comparison is fed back to adjust the operating point of the bipolar transistors, compensating for drift caused by process variations and aging, thus maintaining reliable reference voltage output over the device lifetime
2Temperature
If bipolar transistors are used in the bandgap reference core circuit to achieve temperature compensation, then temperature drift is reduced, but β-drift effects cause collector currents to drift over time
Solution Approach 1:
The patent uses replica bipolar transistors with identical geometry and electrical characteristics to the main transistors. These replicas experience the same β-drift effects due to process variations and aging. By measuring the collector current in the replica transistors and using this information to compensate for drift in the main transistors, the system maintains stable collector currents over time while preserving temperature compensation capabilities
3Ease of manufacture
If standard CMOS process is used for manufacturing the bandgap reference, then manufacturing simplicity is maintained, but the reference voltage varies by several percent due to process variations
Solution Approach 1:
The patent implements replica bipolar transistors that are manufactured using the same CMOS process as the main transistors. This ensures identical process variation exposure. By measuring parameters in the replica devices and using differential measurement techniques, the system cancels out process-induced variations, achieving high reference voltage precision while maintaining standard CMOS manufacturing simplicity
Solution Approach 2:
The patent replaces precision mechanical trimming or laser adjustment methods with electrical compensation techniques. By using electronically controlled replica transistors and feedback circuits, the system achieves high manufacturing precision through electrical measurement and adjustment rather than mechanical means, maintaining ease of CMOS manufacturing
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution significantly reduces the aging effect on the bandgap voltage and proportional-to-absolute-temperature current, enhancing the stability and accuracy of the bandgap reference circuit.
Implementation Method 1
feed replica base currents into the emitters of the bipolar transistors of the bandgap reference core circuit, in order to cancel out a β-drift effect (β=IC/IB denotes a current gain of a bipolar transistor) that is caused by different drifts of the collector currents
Implementation Method 2
feed replica base currents into the emitters of the bipolar transistors of the bandgap reference core circuit, in order to cancel out a β-drift effect (β=IC/IB denotes a current gain of a bipolar transistor) that is caused by different drifts of the collector currents
Implementation Method 3
a differential amplifier circuit that controls first and second emitter currents through the first and second bipolar transistor, such that a sum of the second base-emitter voltage and a voltage drop across the resistor approximates or ideally is equal to the first base-emitter voltage
Data Source
AI summary
A bandgap reference circuit includes a bandgap reference core circuit that includes a first bipolar transistor having a first emitter current density and a first base-emitter voltage, a second bipolar transistor having a second emitter current density that is smaller than the first emitter current density and having a second base-emitter voltage, a resistor that is connected to the emitter of the second bipolar transistor, and a differential amplifier circuit that is configured to control first and second emitter currents through the first and second bipolar transistors, respectively, such that a sum of the second base-emitter voltage and a voltage drop across the resistor approximates the first base-emitter voltage. The bandgap reference circuit further includes a first replica bipolar transistor that emulates an operating point of the first bipolar transistor and a second replica bipolar transistor that emulates an operating point of the second bipolar transistor.