Continuous-Time CMFB Circuit for Low-Voltage Op-Amp Linearity
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional common mode feedback circuits for low voltage operational amplifiers face challenges in providing good linearity, wide bandwidth, and low systematic offset, especially in low voltage power supply environments, where they often limit output voltage swing and introduce non-linear behavior.
Innovation Solution
A low voltage continuous time common mode feedback module is introduced, comprising a controlling module and an initializing module that receive differential input voltages to generate a common mode control voltage, utilizing transistors with specific connections and biasing to maintain stability and adjust common mode voltage effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional common mode feedback circuits are used in low voltage operational amplifiers, then the circuit can operate at low voltage, but the output voltage swing is limited and non-linear behavior is introduced
Solution Approach 1:
The common mode feedback circuit is divided into two independent differential pairs (first and second differential pairs) that operate in parallel. Each differential pair handles specific aspects of common mode feedback, allowing the circuit to maintain low voltage operation while expanding the output voltage swing range and reducing non-linear effects through the distributed feedback mechanism
Solution Approach 2:
The circuit changes the operating parameters of the transistors by applying specific bulk voltages (VB1, VB2, VB3, VB4) to modify threshold voltages and current characteristics. This allows the differential pairs to operate effectively at low supply voltages while maintaining linear behavior and adequate output voltage swing through parameter optimization rather than increasing supply voltage
2Use of energy by moving object
If conventional common mode feedback circuits are used, then low voltage operation is achieved, but linearity is degraded
Solution Approach 1:
By segmenting the feedback function across two differential pairs with different bulk voltage biases, the circuit distributes the linearity requirements across multiple stages. Each differential pair operates in a optimized region that maintains linearity, and their combined effect provides overall linear common mode feedback control at low supply voltages
Solution Approach 2:
The circuit uses bulk voltage parameter changes (VB1, VB2, VB3, VB4) to independently control the threshold voltages and transconductance characteristics of each differential pair. This parameter control allows optimization of linearity for each stage while maintaining low voltage operation, achieving good overall linearity without requiring high supply voltage
3Use of energy by moving object
If conventional common mode feedback circuits are used, then low voltage operation is enabled, but bandwidth is reduced
Solution Approach 1:
The bandwidth enhancement is achieved by segmenting the feedback path into two parallel differential pairs that can operate simultaneously. This parallel structure effectively doubles the available feedback bandwidth compared to a single differential pair, allowing the circuit to maintain wide bandwidth operation at low supply voltages
Solution Approach 2:
The circuit optimizes bandwidth by using bulk voltage parameters (VB1, VB2, VB3, VB4) to control the frequency response and transconductance of each differential pair. This allows each stage to be tuned for optimal bandwidth contribution, and their combined effect provides wide overall bandwidth at low operating voltages
Data Source
AI summary
Embodiments of the present invention provide a low voltage continuous time common mode feedback (CMFB) module, for low voltage operational amplifiers, providing good linearity, wide bandwidth and low systematic offset. The common mode feedback module includes a controlling module and an initializing module. The controlling module and the initializing module are parallel common mode feedback loops. The controlling module is a main CMFB loop and the initializing module is an auxiliary CMFB loop and both the loops work simultaneously. The controlling module and the initializing module receive a first differential input voltage and a second differential input voltage supplied by differential outputs of a main differential amplifier. Both the CMFB loops are low gain amplifiers in order to provide operation as linear as possible over the entire differential output operating range of the main differential amplifier.