Amplifier Output Stage Bias Control at Sub-Threshold Supply Voltage
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Solution Overview
Problem
Audio amplifiers face inefficiency and harmonic distortion due to high power supply voltage requirements, especially in portable devices, as existing solutions either introduce additional losses or are sensitive to temperature variations and transistor mismatches.
Innovation Solution
An amplifier design with a control circuit that uses measurement MOS transistors to adjust the gate voltage of output stage transistors, allowing the power supply voltage to be reduced below the MOS transistor threshold, while maintaining minimum current for efficient operation, and is less sensitive to temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the power supply voltage is reduced below the MOS transistor threshold voltage, then efficiency and power consumption are improved, but the transistors cannot properly operate without sufficient voltage headroom
Solution Approach 1:
The patent separates the power supply voltage function into two independent parts: a fixed first voltage for the output stage and a variable second voltage for the control circuit. This segmentation allows the output stage to operate with minimal voltage headroom while the control circuit receives adequate voltage for proper transistor operation, resolving the contradiction between low power consumption and reliable transistor operation.
Solution Approach 2:
The patent introduces a new dimension by providing two separate power supply voltages instead of a single voltage source. The first voltage supplies the output stage with minimal overhead, while the second voltage supplies the control circuit with sufficient headroom, effectively adding a voltage dimension to the power supply architecture to simultaneously satisfy conflicting requirements.
2Ease of operation
If current measurement resistors are interposed in series with output transistors to control biasing, then transistor current control is achieved, but additional power losses are introduced
Solution Approach 1:
The patent extracts the current measurement function from the main output path by using dedicated measurement transistors connected in parallel with the output transistors. This extraction allows current sensing without introducing resistive losses in the signal path, as the measurement occurs through a separate branch that does not carry the full output current.
Solution Approach 2:
The patent introduces measurement transistors as intermediary elements that sense the current in output transistors without being part of the main current path. These measurement transistors act as mediators, providing current information to the control circuit while isolating the output path from measurement-related power losses.
3Device complexity
If a single power supply voltage is used for both output stage and control circuit, then device complexity is reduced, but efficiency decreases for low-amplitude signals
Solution Approach 1:
The patent segments the power supply into two distinct voltage sources: a fixed first voltage for the output stage and a variable second voltage for the control circuit. This segmentation enables independent optimization of each section, allowing the control circuit to receive higher voltage for proper operation while the output stage operates with minimal voltage for maximum efficiency during low-amplitude signals.
Solution Approach 2:
The patent makes the power supply configuration dynamic by allowing the second voltage to the control circuit to be adjusted independently of the first voltage to the output stage. This dynamic adjustment enables the system to optimize efficiency for low-amplitude signals while maintaining adequate control voltage when needed.
Data Source
AI summary
An amplifier including: an output stage having two first power supply terminals capable of receiving a first voltage defined by first positive and negative variable potentials with respect to a reference potential; and a circuit for controlling the current in transistors of the output stage with a reference value, wherein the output stage includes a first and a second MOS transistors in series between the first two terminals, the junction point of this series association defining an output terminal of the amplifier; the control circuit includes two measurement MOS transistors having their respective sources and gates coupled to the respective sources and gates of the first and second transistors of the output stage; at least one control branch, comprising transistors in series between two terminals of application of a second voltage, defines nodes connected to the gates of the output transistors, said second voltage being greater than the first one.