Multi-Output Current Stimulator Circuit for Fast Low-Current Turn-On
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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
The system employs pairs of MOSFET transistors in a current mirror configuration with a cascode arrangement and an operational amplifier to maintain voltage equality, using resistor ratios to achieve current gain independently of transistor dimensions, thereby reducing turn-on time and eliminating the need for continuous calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If transistor W and L are increased to compensate for mismatch errors, then manufacturing precision is improved, but turn-on time increases due to increased transistor capacitances
Solution Approach 1:
The patent changes the parameter used for current gain from transistor W/L ratios to resistor ratios. By using resistors with precise ratios (e.g., 10:1) instead of relying on transistor dimension ratios, the system achieves accurate current control without the need for large transistor dimensions, thereby maintaining manufacturing precision while reducing turn-on time.
Solution Approach 2:
The patent introduces resistors as intermediary elements between the current source and the load. These resistors serve as mediators that establish precise current gain through their resistance ratios, eliminating the need to rely on transistor matching and allowing smaller transistor dimensions with faster response times.
2Reliability
If cascode transistors are used to increase output impedance, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent extracts the output impedance control function from the transistor cascode configuration and relocates it to the resistor-based current gain network. By separating the current gain function (handled by resistors) from the output impedance function, the system achieves stable output impedance without requiring complex cascode transistor arrangements.
3Adaptability or versatility
If multiple cascode transistors are used for multiple outputs, then adaptability is improved, but transistor die area increases
Solution Approach 1:
The patent segments the current gain control into separate resistor-based control paths for each output. Instead of using multiple large cascode transistors, each output channel uses a simple resistor to establish its current gain, significantly reducing the total die area while maintaining the ability to independently control multiple outputs.
4Adaptability or versatility
If high voltage transistors are used to accommodate high output voltages, then adaptability is improved, but turn-on time increases due to high threshold voltages
Solution Approach 1:
The patent changes the control mechanism from direct transistor gate voltage control to resistor-ratio-based current control. This allows the use of lower threshold voltage transistors that can turn on faster, while still achieving the required output voltage range through the resistor-defined current gains and standard supply voltages.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces turn-on time across various output current levels, including small currents, and minimizes transistor die area usage by relying on resistor values for current gain, enhancing accuracy and efficiency in multiple electrode applications.
Implementation Method 1
an operational or differential amplifier having one input coupled to a reference voltage and another input coupled to the output side transistors and an output which drives the supply transistor. In steady state, the operational amplifier maintains the voltage at the output side transistor equal to the reference voltage
Implementation Method 2
the current output from the DAC is normally amplified to the desired output current amplitude at the current stimulator. The current amplification is typically accomplished using a current mirror circuit and in particular for MOSFET devices, the current gain is achieved by controlling the device semiconductor die width (W) to length (L) ratio (W/L)
Implementation Method 3
One technique to compensate for the effects of the output impedance of the stimulator is to configure the circuit of FIG. 1 to include transistors MC1 and MC2 in a cascode arrangement in order to increase the output impedance of the stimulator
Implementation Method 4
using resistor ratios to achieve current gain independently of transistor dimensions
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.