Two-Stage CMFB Circuit With Supply-Scaled Reference Voltages
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Solution Overview
Problem
Conventional common-mode feedback circuits in fully differential operational amplifiers require large design margins due to significant variations in amplifier current when the power supply voltage changes, leading to complex design challenges.
Innovation Solution
A two-stage common-mode feedback circuit is introduced, featuring first and second stages with respective CMFB components that use operation voltage-dependent reference signals to regulate common-mode voltages, reducing current variations by keeping source-gate voltages independent of the power supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If conventional CMFB circuits are used to regulate output common-mode voltage to a fixed level, then the common-mode voltage stability is improved, but the amplifier current varies significantly when power supply voltage changes, requiring large design margins and increasing design complexity
Solution Approach 1:
The patent transitions from a static fixed reference voltage to a dynamic reference voltage that automatically adapts to power supply voltage changes. The dynamic reference voltage is generated by scaling the power supply voltage through a voltage divider network, allowing the CMFB circuit to maintain stable common-mode voltage while accommodating voltage variations without requiring large design margins
Solution Approach 2:
The patent changes the reference voltage parameter from a fixed value to a variable value that scales with the power supply voltage. By making the reference voltage dependent on the power supply voltage through resistive division, the system maintains proper common-mode voltage regulation across different operating conditions, reducing the need for oversized design margins
2Device complexity
If conventional CMFB circuits use fixed reference voltage, then the circuit structure is simple, but large design margins are required to secure performance within specification across power supply voltage ranges
Solution Approach 1:
The patent modifies the reference voltage parameter to be proportional to the power supply voltage rather than fixed. This parameter change allows the circuit to maintain reliable performance across different voltage conditions without requiring excessive design margins, as the reference voltage automatically scales with the supply voltage
3Reliability
If design margins are increased to accommodate current variations, then performance reliability is improved, but the circuit design becomes more complex with additional components and larger margins
Solution Approach 1:
The patent changes the reference voltage parameter to scale with power supply voltage, which directly reduces current variations. This parameter modification improves performance reliability without requiring additional design margins or complex compensation circuits, as the fundamental cause of current variation is addressed at the reference voltage level
Data Source
AI summary
A two-stage common-mode feedback (CMFB) circuit and a fully differential operational amplifier are provided. The two-stage CMFB circuit includes a first CMFB circuit and a second CMFB circuit. The first CMFB circuit includes a first CMFB component that receives a first differential pair of output signals of the first amplifier and a first reference signal. The first CMFB component generates a first control signal to regulate a first common-mode voltage of the first amplifier to a first reference voltage of the first reference signal. The second CMFB circuit includes a second CMFB component that receives a second differential pair of output signals of the second amplifier and a second reference signal. The second CMFB component generates a second control signal according to a second reference signal, so as to regulate a second common-mode voltage of the second amplifier to a second reference voltage of the second reference signal.