MOS Output Stage Current Servo for Low-Voltage Amplifiers
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Solution Overview
Problem
Conventional audio amplifiers face inefficiencies due to high supply voltage requirements in the output stage, especially in portable devices, leading to excessive transistor consumption and losses, particularly when amplifying low-level signals, and are sensitive to temperature variations and transistor mismatches.
Innovation Solution
An amplifier design with a servo circuit that controls the current in output transistors to a reference value, using measurement MOS transistors coupled to the output stage transistors and a control branch with a second voltage greater than the supply voltage, allowing the supply voltage to be lowered below the threshold voltage of an MOS transistor, thereby reducing power consumption and minimizing losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the supply voltage of the output stage is reduced to lower power consumption, then energy efficiency improves, but the transistor current control becomes inaccurate and operation reliability deteriorates
Solution Approach 1:
The patent implements a feedback mechanism where the current through the output transistors is measured and used to control their gate voltages. A measurement transistor is connected in parallel with each output transistor, and its current is fed back to a control circuit that adjusts the gate voltage to maintain the desired current level, ensuring reliable operation even at reduced supply voltages
Solution Approach 2:
The patent dynamically adjusts the gate-source voltage of the output transistors based on the measured current. When the transistor current deviates from the reference value, the gate voltage is modified to compensate, allowing the system to maintain proper transistor operation across a wide range of supply voltages including those below the MOS threshold voltage
2Measurement precision
If current-voltage conversion resistors are inserted in series with output transistors to measure current, then current measurement capability is achieved, but additional power losses increase and measurement accuracy decreases
Solution Approach 1:
The patent uses measurement transistors as intermediary devices to measure the current through output transistors without inserting resistors in series. The measurement transistor is connected in parallel with the output transistor, and its gate is connected to the output transistor's drain, allowing current measurement through the relationship between the two transistors without adding series resistance that would cause power losses
3Use of energy by moving object
If the supply voltage is lowered below the threshold voltage of MOS transistors, then power consumption decreases, but conventional amplifier operation becomes impossible
Solution Approach 1:
The patent employs dynamic control of the gate voltages of the output transistors. The gate voltages are not fixed but are continuously adjusted based on feedback from current measurements. This dynamic adjustment allows the transistors to operate correctly even when the supply voltage fluctuates or is lowered below the nominal threshold voltage, maintaining amplifier functionality across varying conditions
Data Source
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AI summary
The invention relates to an amplifier comprising: an output stage (1) of which two first supply terminals (15, 17) are adapted to receive a first voltage defined by first positive (+V2) and negative (-V2) variable potentials ) with respect to a reference potential; and a circuit (2) for slaving the current in the transistors of the output stage to a reference value, in which the output stage comprises a first and a second MOS transistors (MNh, MNl) in series between the two first terminals, the midpoint of this series association defining an output terminal (OUT) of the amplifier; the servo circuit comprises two measurement MOS transistors (N21, N22) whose respective sources and gates are coupled to the respective sources and gates of the first and second transistors of the output stage; at least one control branch (23, 24; 23'), comprising transistors in series between two application terminals of a second voltage, defines nodes (11, 13) connected to the gates of the output transistors, said second voltage being greater than the first.