Self-Oscillating Class D Transconductance Amplifier With Current Feedback
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Solution Overview
Problem
Class D amplifiers have not been used for current sources due to their output being a voltage, whereas the requirement is for a current output proportional to the input voltage, which is not met by conventional transconductance amplifiers.
Innovation Solution
A self-oscillating transconductance Class D amplifier circuit using current feedback to ensure the output current is proportional to the input voltage, maintaining linearity and constant output current regardless of load impedance, achieved through a configuration including comparators, transistors, inductors, and differential amplifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a Class D amplifier is used, then power conversion efficiency is improved, but the output is a voltage signal instead of a current signal
Solution Approach 1:
The patent applies feedback by sensing the output current through a resistor and feeding it back to the comparator input. This current feedback mechanism forces the Class D amplifier to operate as a transconductance amplifier, where the output current becomes proportional to the input voltage, thus resolving the contradiction between maintaining high efficiency and achieving current output.
Solution Approach 2:
The patent changes the operating parameters of the Class D amplifier by introducing a feedback loop that modifies the control voltage based on output current. This parameter change transforms the amplifier's behavior from voltage-output to current-output mode while preserving the high efficiency characteristics of Class D operation.
2Ease of operation
If conventional transconductance amplifiers are used to produce current output proportional to input voltage, then the desired current source function is achieved, but power efficiency is reduced
Solution Approach 1:
The patent employs periodic switching action characteristic of Class D amplifiers, where the output transistor switches between on and off states. This periodic action, combined with feedback, allows the amplifier to deliver current proportional to input voltage while maintaining high efficiency through pulse-width modulation rather than linear operation.
Solution Approach 2:
The patent substitutes the traditional linear transconductance amplifier mechanism with a switching-based Class D architecture. By replacing the continuous analog control with periodic switching and feedback, the system achieves the same current proportionality function with significantly improved power efficiency.
3Manufacturing precision
If current feedback is added to force oscillation and improve linearity, then output current proportionality is improved, but device complexity increases
Solution Approach 1:
The patent achieves multi-functionality by using the same feedback path to accomplish multiple objectives: forcing oscillation, improving linearity, and ensuring current proportionality. The feedback resistor and comparator configuration serve universal purposes, reducing the need for additional dedicated components and thereby limiting the increase in complexity.
Solution Approach 2:
The circuit employs self-oscillation where the feedback mechanism automatically generates the necessary oscillating control signal without requiring an external oscillator. The system serves itself by using the output current feedback to directly control the switching action, eliminating the need for separate oscillation-generating components and reducing overall complexity.
Data Source
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AI summary
A circuit that outputs a current which is proportional to an input voltage includes input and output terminals, a comparator, first and second transistors, an inductor, a first resistor, and a differential amplifier. A first input terminal of the comparator is coupled to the input terminal of the circuit, and a second input terminal of the comparator is coupled to an output terminal of the comparator. The first and second transistors are coupled to the output terminal of the comparator. The inductor is coupled to the first and second transistors. The first resistor is coupled between the inductor and the output terminal of the circuit. The differential amplifier includes a first input terminal coupled to a first terminal of the first resistor, a second input terminal coupled to a second terminal to the first resistor, and an output terminal coupled to the first input terminal of the comparator.