CFIA Back-Bias Trimming for Low Gain Error and Noise
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Solution Overview
Problem
Current current-feedback instrumentation amplifiers face challenges in minimizing gain error due to mismatches between transistors and tail currents, which are not adequately addressed by existing architectures that rely on resistor matching, leading to noise and complex frequency compensation issues.
Innovation Solution
The proposed solution involves a circuit architecture with trimming circuits that adjust the back-bias voltage of differential transistor pairs and a modulator circuit to reduce gain error by minimizing mismatches between transconductances, using series-connected resistors and switches to vary the back-bias voltage and alternating phase states to swap tail current sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If preamplifiers are added to each transconductor to create a transfer function proportional to the ratio between degeneration resistors, then gain error is reduced and depends almost exclusively on resistor matching, but noise increases and frequency compensation becomes complex
Solution Approach 1:
The patent changes the operating parameters of the transconductor by adjusting the back-bias voltage of the differential pair transistors. By varying this voltage, the transconductance can be tuned to match between input and feedback transconductors, reducing gain error without adding noisy preamplifier stages.
2Measurement precision
If preamplifiers are added to each transconductor, then gain error is reduced and depends almost exclusively on resistor matching, but device complexity increases due to feedback loops
Solution Approach 1:
The patent extracts the gain error correction function from the signal path by using a separate trimming circuit that adjusts the back-bias voltage. This removes the need for noisy preamplifier feedback loops while maintaining the ability to correct gain error through transistor parameter adjustment.
3Object-generated harmful factors
If differential pairs with degeneration are used without preamplifiers, then noise is significantly reduced and feedback loops are eliminated, but gain error increases due to transistor mismatch
Solution Approach 1:
The patent implements self-service by enabling the differential pair transistors to self-adjust their transconductance through back-bias voltage trimming. The trimming circuit allows each transconductor to self-correct its gain to match the other, eliminating the need for noisy preamplifiers while maintaining low gain error.
4Device complexity
If transistor matching is relied upon for gain accuracy, then device complexity is reduced, but manufacturing precision becomes difficult due to poor transistor matching compared to resistors
Solution Approach 1:
The patent introduces dynamic adjustability through the back-bias voltage trimming circuit. Instead of relying on static manufacturing precision of transistor matching, the system dynamically adjusts the transistor operating point to achieve precise gain matching, compensating for manufacturing variations.
Data Source
AI summary
A current feed-back instrumentation amplifier (CFIA) comprises a differential pair with degeneration for amplifying small differential voltages in the presence of large common-mode voltages. The CFIA includes input and feedback transconductors and a trimming circuit that trims the back-bias voltages of the transistors in each transconductor. The trimming circuit includes a plurality of selectable resistors disposed in the signal path of the tail current in each transconductor. Each of the plurality of selectable resistors has a switch coupled to it. When a switch is closed, only the resistors up to the respective switch are in the signal path of the bulk-to-source voltage of the differentially paired transistors. The resistor trimming circuit reduces the mismatch between transconductances of the respective differential pair transistors, in turn reducing mismatch of the overall transconductances of the transconductors, and thereby reducing the CFIA's gain error.