Parallel Transconductance Current Source for Accuracy and Dynamic Range
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Solution Overview
Problem
Existing transconductance current source circuits face challenges in achieving a balance between dynamic range, accuracy, and mismatch tolerance, particularly when two sources are driven by the same voltage, with high transconductance sources providing wide current ranges but poor accuracy and low transconductance sources offering better accuracy but limited range.
Innovation Solution
A transconductance circuit comprising multiple current sources with different transconductances connected in parallel, where a low transconductance source provides accurate low output currents and a high transconductance source offers wide dynamic range, with a source follower and voltage drop circuit ensuring the high transconductance source is only activated at increased input voltage, thereby improving dynamic range and accuracy while maintaining tolerance to mismatch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a high transconductance current source is used, then the dynamic range is improved, but the accuracy deteriorates
Solution Approach 1:
The current source is segmented into multiple parallel current sources with different transconductance values. Each current source is responsible for a specific portion of the output current range, with low transconductance sources handling small currents for high accuracy and high transconductance sources handling large currents for wide dynamic range. This segmentation resolves the contradiction by allowing each segment to optimize for its specific operating range.
2Measurement precision
If a low transconductance current source is used, then the accuracy is improved, but the dynamic range deteriorates
Solution Approach 1:
Multiple current sources with different transconductance characteristics are merged in parallel to form a composite current source. The low transconductance source provides accurate small current output, while the high transconductance source extends the dynamic range for larger currents. The merging of these complementary sources resolves the contradiction between accuracy and dynamic range.
3Measurement precision
If large area devices are used to improve accuracy, then the mismatch tolerance is improved, but the speed deteriorates
Solution Approach 1:
The device area is segmented into multiple smaller parallel current sources instead of using a single large area device. This segmentation maintains accuracy through parallel redundancy while reducing the capacitance associated with large single devices, thereby improving speed. Each small current source contributes to the overall accuracy without suffering from the speed penalties of large area devices.
Data Source
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AI summary
A transconductance circuit has an input terminal (VIN) and an output terminal (Out), a first current source (4) having a gate connected to said input terminal (VIN); and a second current source (5), in parallel with said first current source, and having a higher transconductance and a wider dynamic range than the first current source. The current sources are configured so that at a low input voltage only the first current source (4) is on. A voltage drop circuit (2) provides a lower bias voltage for the second current source than for the first current source.