Low-Offset Bandgap Circuit with Offset-Cancelling Mechanism
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Solution Overview
Problem
Bandgap circuits are affected by operational amplifier offset voltages, leading to unstable output voltages that are sensitive to temperature and power supply fluctuations.
Innovation Solution
A low-offset bandgap circuit is designed with an offset-cancelling circuit that generates a compensation current to counteract the offset voltage of the core operational amplifier, using multiple operational amplifiers and transistors to stabilize the output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional bandgap circuit is used, then the circuit structure is simple, but the output voltage is unstable due to operational amplifier offset voltage
Solution Approach 1:
The bandgap circuit is divided into a core bandgap circuit and a separate offset-cancelling circuit. The offset-cancelling circuit includes two operational amplifiers (first and second operational amplifiers) that are segmented to respectively cancel the offset voltages at the positive and negative input terminals of the core operational amplifier. This segmentation allows independent compensation of offset errors without redesigning the entire bandgap circuit.
Solution Approach 2:
The first and second operational amplifiers act as intermediary components that generate compensation currents to counteract the offset voltage effects. These intermediary operational amplifiers process the input voltages and generate corrective signals that are fed back to cancel the offset effects on the core operational amplifier, thereby stabilizing the output voltage.
2Reliability
If no offset compensation is applied, then the circuit complexity is low, but the output voltage is sensitive to temperature and power supply fluctuations
Solution Approach 1:
The offset-cancelling circuit implements feedback mechanisms where the first operational amplifier receives feedback from its output through a feedback transistor to its negative input terminal, and similarly the second operational amplifier receives feedback. This feedback ensures that the compensation currents dynamically adjust to maintain stable output voltage despite temperature and power supply variations.
Solution Approach 2:
The circuit utilizes parameter changes in the compensation currents generated by the first and second operational amplifiers to counteract the effects of temperature and power supply fluctuations. By dynamically adjusting the compensation currents based on the input voltages at the core operational amplifier terminals, the circuit maintains stable output characteristics across varying environmental conditions.
3Measurement precision
If an offset-cancelling circuit is added, then the output voltage becomes independent of offset voltage, but the number of operational amplifiers increases
Solution Approach 1:
The offset-cancelling circuit uses two additional operational amplifiers that are essentially copies of each other, with the first operational amplifier compensating for the positive input terminal offset and the second operational amplifier compensating for the negative input terminal offset. This copying approach allows systematic cancellation of offset errors while maintaining circuit symmetry and ease of implementation.
Data Source
AI summary
A low-offset bandgap circuit including a core bandgap circuit and an offset-cancelling circuit is provided. The low-offset bandgap circuit provides a reference voltage at an output node. The core bandgap circuit includes a core operational amplifier to generate a core current. The offset-cancelling circuit is coupled to two input terminals of the core operational amplifier. The offset-cancelling circuit is configured to generate a compensation current according to the voltages at the two input terminals of the core operational amplifier so as to compensate for an offset voltage of the core operational amplifier. The reference voltage is generated according to the core current and the compensation current.