Constant Current Circuit Using Differential Amplifier Feedback
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Solution Overview
Problem
Conventional constant current circuits face issues with nonlinear control responses and oscillation due to high-gain two-stage amplification, requiring high operational voltages and phase compensation, which limits their operability with lower power supply voltages and affects the accuracy of output current.
Innovation Solution
A constant current circuit design incorporating a differential amplifying unit, a current mirror circuit, and a constant current loading unit, which reduces the gain in the feedback path and operates at lower voltages by using diode-connected transistors and current mirror configurations to maintain a linear response and prevent oscillation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a high-gain two-stage amplification circuit is used in the feedback path, then the operational range of the input voltage can be widened, but nonlinear control responses and oscillation occur
Solution Approach 1:
The patent implements a feedback mechanism where the output current is converted to a feedback voltage and fed back to the inverting input terminal of the operational amplifier. This feedback loop enables the operational amplifier to automatically adjust the gate voltage of the N-MOS transistor to maintain a linear control response and prevent oscillation, while still allowing a wide input voltage range to be processed.
2Adaptability or versatility
If high-gain two-stage amplification is used, then the operational range is extended, but oscillation and nonlinear responses occur requiring phase compensation
Solution Approach 1:
The patent extracts the high-gain two-stage amplification circuit (N-MOS transistor and P-MOS transistor) from the feedback path of the operational amplifier. By removing this amplification stage, the feedback loop becomes simpler and more stable, eliminating the need for complex phase compensation while maintaining a wide operational range through the operational amplifier's inherent high gain.
3Adaptability or versatility
If conventional constant current circuit configuration is used, then wide input voltage range is achieved, but output current accuracy deteriorates
Solution Approach 1:
The patent replaces the conventional two-stage amplification mechanism with an operational amplifier-based feedback system. The operational amplifier, with its high open-loop gain and virtual short characteristic between input terminals, provides precise control of the N-MOS transistor gate voltage, ensuring accurate output current while accommodating a wide input voltage range through feedback adjustment.
Data Source
AI summary
A constant current circuit that generates a constant output current corresponding to an input voltage, comprises a differential amplifying unit to which the input voltage and a feedback voltage to be compared therewith are applied, the differential amplifying unit outputting a differential voltage, a first transistor with a first control electrode to which the differential voltage is applied, a first diode element that is connected to a power-supply side electrode of the first transistor, one or a plurality of second transistors that generates the output current, a feedback voltage conversion block that converts the duplicated current of the diode current flowing through the second transistor into the feedback voltage, and a constant current loading unit that is connected to a ground side electrode of the first transistor, the constant current loading unit making a voltage change in the ground side electrode follow a voltage change in the first control electrode.