Floating Input Op-Amp Topology for Low Offset at High Voltage
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Solution Overview
Problem
Operational amplifiers face challenges in achieving high precision with low offset voltage while maintaining a compact size, especially when operating at high voltages, as larger transistors are required to minimize mismatches but may not be practical due to size constraints and the need for high-voltage handling.
Innovation Solution
The design incorporates a floating input stage with a first and second low-voltage differential pair, where the first voltage offset is less than the second, and the second gain is greater, powered by a floating supply that spans from an upper rail voltage to a lower rail voltage, allowing the operational amplifier to operate effectively across a wide range of common mode voltages and supply voltages without the need for complex chopping or trimming techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the physical device size of transistors is increased to minimize mismatches and reduce offset voltage, then precision is improved, but device area increases
Solution Approach 1:
The input stage is divided into two separate gain blocks (G1 and G2), each with its own differential pair. This segmentation allows the use of smaller transistors in each block while achieving the required overall precision through the combination of both blocks, resolving the contradiction between transistor size and offset voltage precision.
Solution Approach 2:
Two gain blocks with different characteristics (G1 with lower offset voltage and G2 with higher gain) are merged together in the input stage. The combination allows the system to achieve both low offset voltage and high precision without requiring individual transistors to be large, thus reducing the overall die area while maintaining precision.
2Strength
If larger transistor sizes are used to handle high-voltages, then voltage handling capability is improved, but device area increases
Solution Approach 1:
The voltage handling function is segmented between the floating supply circuit and the differential pairs. The floating supply handles the high-voltage range (VDD to VSS) while the differential pairs operate at lower voltages (FP to FN), allowing smaller transistors to be used in the differential pairs while still achieving high-voltage operation capability through the floating supply architecture.
3Measurement precision
If chopping or trimming techniques are used to reduce offset voltage, then precision is improved, but device complexity and cost increase
Solution Approach 1:
The floating supply circuit preliminarily establishes a stable voltage reference and isolation mechanism before the differential pairs operate. This preliminary action of creating a floating voltage domain reduces offset voltage inherently through the circuit architecture itself, eliminating the need for additional chopping or trimming techniques and thereby reducing device complexity.
Data Source
AI summary
The operational amplifier disclosed includes an input stage configured to receive power from a floating supply circuit in in a low voltage range that can float according to the common mode voltage at the input. The low voltage supply facilitates the use of low voltage components that can improve the precision of the operational amplifier by lowering the offset voltage. The input stage utilizes a first gain block and a second gain block. The first gain block is configured to have a low offset voltage while the second gain h block is configured to have a high gain. Dividing these aspects over separate gain blocks improves the precision and noise performance of the operational amplifier. The operational amplifier has high gain at low frequencies and at high frequencies due to a topology that combines a low gain, high bandwidth path with a high gain, low bandwidth path at the output.