Differential Op-Amp Biasing for Common-Mode Ripple Control
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Solution Overview
Problem
Class-D amplifiers in battery-powered equipment face high energy consumption due to large current requirements for low-noise operational amplifiers, exacerbated by common-mode ripple and drift, leading to increased power supply voltage needs.
Innovation Solution
The implementation of fully-differential operational amplifiers with common-mode control and class AB biasing, featuring low-frequency feedback and high-frequency feed-forward paths, reduces energy consumption by controlling common-mode voltage and optimizing biasing for reduced power supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If operational amplifiers with low input-referred noise are used, then noise performance is improved, but energy consumption increases
Solution Approach 1:
The patent segments the operational amplifier into multiple fully-differential stages, each with independent common-mode control. This allows separate optimization of noise performance in the signal path while reducing power consumption through selective biasing in each stage, resolving the contradiction between low-noise requirements and energy efficiency
Solution Approach 2:
The patent implements dynamic common-mode voltage control that adapts to signal conditions. The common-mode control circuit dynamically adjusts bias voltages based on operating conditions, enabling the amplifier to maintain low noise when needed while reducing power consumption during normal operation, thus resolving the static trade-off between noise performance and energy use
2Reliability
If power supply voltage is increased to provide required dynamic range, then common-mode ripple and drift are controlled, but energy consumption increases
Solution Approach 1:
The patent changes the operating parameters by implementing class AB biasing instead of traditional class A biasing. This parameter change allows the operational amplifier to maintain adequate common-mode control and dynamic range while significantly reducing the quiescent current and overall power consumption, resolving the contradiction between reliability and energy consumption
3Measurement precision
If class A bias is used at DC, then low-noise performance is maintained, but power consumption is high
Solution Approach 1:
The patent implements periodic switching between class A and class AB biasing modes based on signal presence and frequency. During DC and low-frequency operation, class A biasing maintains low noise performance. During high-frequency switching operation, the system transitions to class AB biasing, reducing power consumption while maintaining adequate performance through the high-frequency feed-forward path
Data Source
AI summary
An operational amplifier with one or more fully-differential amplifier stages has a common-mode control input. A low-frequency feedback control path is coupled between an output of the fully-differential amplifier stages and the common-mode control input to control low-frequency drift of the common-mode voltage of the output of the stages. A high-frequency feed-forward control path couples a pair of inputs of the stages to control high-frequency ripple of a common-mode voltage of the inputs of the stages. One or more of the differential amplifier stages may have a bias input that controls a direct-current (DC) bias voltage of gates of pull-up transistors of the stage that is both DC and capacitively coupled to the gates so that the stage operates with class A bias at DC and with class AB bias at high frequencies.