Cascode MOSFET Current Mirror for Fast Multi-Electrode Stimulation
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Solution Overview
Problem
Current FES systems face challenges in delivering accurate and fast output currents to multiple electrodes due to transistor mismatch errors, high output impedance, and long turn-on times, especially for small current levels, which complicates the calibration and increases transistor die area requirements.
Innovation Solution
A current mirror circuit design using pairs of MOSFET transistors in cascode arrangement with operational amplifiers and resistors to establish current gain, independent of transistor width and length, ensuring accurate and rapid current delivery across multiple electrodes by maintaining voltage equality through differential amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If transistor W and L are increased to compensate for mismatch errors, then current delivery accuracy is improved, but response time deteriorates due to increased transistor capacitances
Solution Approach 1:
The current mirror is divided into multiple independent cascode current mirror pairs, each serving a specific output channel. This segmentation allows each transistor pair to be optimized independently, using smaller W/L ratios that provide faster response times while maintaining accuracy through the regulated cascode architecture and operational amplifier control.
Solution Approach 2:
Operational amplifiers are introduced to provide feedback control for each cascode current mirror pair. The op amps regulate the gate voltages of the cascode transistors to maintain precise current mirroring ratios, eliminating the need for oversized transistors and enabling fast response times with accurate current delivery.
2Measurement precision
If cascode transistors are used to increase output impedance, then current amplification accuracy is improved, but turn-on time deteriorates due to high voltage transistor threshold charging requirements
Solution Approach 1:
The operational amplifiers are configured to provide preliminary voltage preparation at the gates of the cascode transistors. By pre-charging the gate capacitances through the feedback mechanism, the transistors can transition more rapidly from off to on state, reducing turn-on time while maintaining the high output impedance benefits of the cascode configuration.
Solution Approach 2:
The invention dynamically adjusts the gate-source voltages of the cascode transistors through operational amplifier control, allowing the transistors to operate with optimized voltage parameters during switching transitions. This parameter control enables fast turn-on times by reducing the effective threshold voltage charging requirement while maintaining accurate current amplification.
3Adaptability or versatility
If multiple cascode transistors are used for multiple outputs, then current delivery to multiple electrodes is enabled, but variations in output currents increase due to different electrode/tissue impedances
Solution Approach 1:
Each output channel is equipped with its own operational amplifier that continuously monitors the output current and adjusts the cascode transistor gate voltages to maintain the desired current level. This per-channel feedback compensation eliminates the effect of varying electrode and tissue impedances, ensuring accurate current delivery to each electrode independently.
Solution Approach 2:
The invention implements independent regulation for each output channel through dedicated operational amplifiers and cascode pairs. This local quality approach allows each channel to be optimized and regulated separately, compensating for local variations in electrode impedance and tissue characteristics without affecting other channels.
4Strength
If high voltage transistors are used for cascode to accommodate high output voltages, then voltage handling capability is improved, but turn-on time deteriorates due to high threshold voltages requiring long charging times
Solution Approach 1:
Operational amplifiers are introduced as intermediary devices that control the gate voltages of the high-voltage cascode transistors. The op amps provide precise voltage control and can rapidly charge the gate capacitances through their high-output-current capability, enabling fast turn-on times for high-voltage transistors that would otherwise have slow switching responses.
Data Source
AI summary
A multiple output current stimulator circuit with fast turn on time is described. At least one pair of input side and output side transistors is arranged in a current mirror connected to a supply transistor by cascode coupling. The output side transistor supplies stimulation current to an electrode in contact with tissue. An operational amplifier connected to a reference voltage and to the output side transistor drives the supply transistor to maintain the voltage at the output side transistor equal to the reference voltage. The at least one pair of transistors includes multiple pairs of transistors whose output side transistors drive respective electrodes with stimulation currents. The stimulator determines the initiation and duration of stimulation current pulses supplied to each electrode. At circuit activation, large currents are generated which discharge capacitances in the output side transistors causing rapid output side transistor turn on.