Self-Configuring Precision Amplifier Output Stages for Ground Shift
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Solution Overview
Problem
Precision amplifiers face challenges in maintaining high gain accuracy and low offset, especially in bi-directional and unidirectional output applications, where ground shifts introduce errors and achieving zero output voltage is difficult due to parasitic diodes and leakage currents, and multiplexed systems require consistent supply voltage to prevent short circuits.
Innovation Solution
The amplifier output stage employs a high impedance cascoded-current mirror structure with Class AB operation, using Miller capacitors and additional amplifiers to control gate voltages and prevent current flow through parasitic diodes, allowing for bi-directional and unidirectional operations, and disabling output drivers to maintain high impedance even when powered down.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the amplifier output stage uses a conventional structure with parasitic diodes, then the amplifier can be manufactured with standard processes, but ground shifts between amplifier and sensing device introduce gain errors and offset errors
Solution Approach 1:
The patent introduces an external load resistor as an intermediary element between the amplifier output and ground. By referencing the output to an external voltage (Vext) rather than local ground, the amplifier becomes insensitive to ground shifts between the amplifier IC and the sensing device. The external load resistor serves as a common reference point that both the amplifier and sensing device can share, eliminating the harmful effect of ground potential differences.
2Adaptability or versatility
If the amplifier is designed for bi-directional output operation, then the output can swing above and below the reference voltage, but the output structure becomes more complex and cannot be implemented in a single monolithic IC
Solution Approach 1:
The patent employs dynamic control of the output stage transistors to achieve bi-directional operation. By using Class AB operation with dynamically adjusted gate voltages for the pull-up and pull-down transistors, the output can swing both above and below the external reference voltage. The dynamic biasing ensures that the appropriate transistor conducts during each half-cycle, enabling versatile bi-directional output without requiring a complex push-pull structure that would be difficult to implement monolithically.
3Measurement precision
If the amplifier uses an external load resistor for precision operation, then ground shift errors are eliminated, but the amplifier cannot achieve absolute zero output voltage due to parasitic diode conduction
Solution Approach 1:
The patent applies preliminary anti-action by preemptively preventing parasitic diode conduction through proper biasing of the output stage. By ensuring that the gate-to-source voltage of the output transistors remains below the threshold for parasitic diode turn-on, the design eliminates the harmful effect of leakage current before it can occur. This is achieved through careful control of the transistor operating points and gate voltages, particularly when the output approaches zero voltage.
4Ease of manufacture
If multiple amplifiers are multiplexed with a single load resistor, then component count is reduced and cost is lowered, but powered-down amplifiers short the output through body diodes
Solution Approach 1:
The patent implements preliminary action by disabling the output stage transistors before the amplifier is fully powered down or when not selected in a multiplexed configuration. By turning off the pull-up and pull-down transistors in advance, the output is placed in a high-impedance state that prevents short circuits through parasitic body diodes. This preliminary disabling action ensures that only the active amplifier can drive the external load resistor, eliminating the reliability issue in multiplexed systems.
Data Source
AI summary
Self configuring output stages of precision amplifiers that remain linear when operating into a load that may have a ground reference below the amplifier ground reference, that maintain full amplifier gain while approaching zero output, and that can provide a zero output even when operating into a load that may have a ground reference below the amplifier ground reference, that has a self configuring output stage operable with either a mid-rail or ground reference below amplifier ground, and which maintain a high output impedance when not selected even when the output is above the amplifier supply voltage, or when not powered, thereby allowing amplifier outputs from un-powered amplifiers or amplifiers operating at lower supply voltages to be connected in common for multiplexing to a common load.