Constant Current Circuit Reducing Output Voltage Dependence
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Solution Overview
Problem
Conventional constant current circuits face issues with high output voltage dependence and complex circuit configurations, particularly when reducing output voltage dependence and maintaining stable current at low voltages, due to channel length modulation effects in MOSFETs.
Innovation Solution
A constant current circuit design incorporating a source-grounded transistor, an increase current generating circuit to counteract channel length modulation effects, and a current mirror circuit to supply equal current to the transistor, reducing output voltage dependence and simplifying the circuit configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a source-grounded MOSFET operates in saturation region with fixed gate-to-source voltage, then the circuit is simple, but output voltage dependence of output current is large (output resistance is small)
Solution Approach 1:
The patent introduces a feedback mechanism where a sense transistor senses the output current and feeds back a signal to adjust the gate voltage of the main MOSFET. This feedback loop compensates for channel length modulation effects, maintaining stable output current despite output voltage variations while keeping the circuit configuration relatively simple.
Solution Approach 2:
The patent dynamically adjusts the gate-to-source voltage parameter of the MOSFET based on the output voltage level. By changing this parameter in response to output conditions, the circuit compensates for channel length modulation and maintains constant current output without requiring complex additional circuitry.
2Reliability
If two MOSFETs are stacked in two stages to reduce output voltage dependence, then output resistance is enlarged, but a high output voltage is necessary
Solution Approach 1:
The patent introduces a sense transistor as an intermediary element that indirectly senses the output current and provides feedback control. This intermediary approach achieves high output resistance and stable current without requiring the high output voltage needed for two-stage cascode structures, thereby extending the operable output voltage range to lower voltages.
3Reliability
If a variable resistor operates in conjunction with output voltage to stabilize current, then constant current is achieved at low voltage, but the circuit becomes complicated
Solution Approach 1:
The patent implements a self-service mechanism where the sense transistor automatically adjusts its resistance based on the output current level. The transistor's inherent nonlinear characteristics provide the necessary compensation for channel length modulation without requiring external variable resistors or complex control circuits, achieving constant current with a relatively simple configuration.
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
The proposed circuit effectively minimizes output voltage dependence of the output current, reduces the lower limit of the operable output voltage range, and simplifies the circuit configuration, enabling stable constant current operation at lower voltages without the need for additional constant current sources or complex control circuits.
Implementation Method 1
an increase current generating circuit to generate an increase current equivalent to an increase of a current due to a channel length modulation effect of the transistor
Implementation Method 2
a current mirror circuit to generate a current having the same value as that of the increase current generated by the increase current generating circuit and supply the generated current to the drain terminal of the transistor
Data Source
AI summary
In a constant current circuit, a drain terminal is connected to an output terminal of a current, and a gate voltage operable in a saturation region is applied to a source-grounded transistor. An increase current generating circuit generates an increase current equivalent to an increase of a current due to a channel length modulation effect of the transistor. A current mirror circuit generates a current having the same value as that of the increase current generated by the increase current generating circuit and supplies the generated current to the drain terminal of the transistor.