FBDDA Amplifier Bypass Switching for THD and Noise Trade-Off
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Solution Overview
Problem
Fully-balanced differential difference amplifiers (FBDDA) face issues with total harmonic distortion (THD) at high input signal levels due to unbalanced differential pairs, leading to linearity deterioration, and existing solutions either increase noise or require excessive current consumption.
Innovation Solution
The FBDDA amplifier incorporates resistive-degeneration groups coupled with by-pass switches controlled by control signals, which are selectively included only for high input signal levels, allowing the amplifier to operate in different conditions to maintain linearity without excessive noise or current consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If resistive-degeneration groups are continuously coupled to differential-input pairs, then linearity is improved, but noise increases
Solution Approach 1:
The patent applies dynamic switching of the resistive-degeneration groups using by-pass switches controlled by control signals. The switches selectively connect or disconnect the degeneration resistors based on input signal amplitude, transforming a static configuration into a dynamic one that adapts to signal conditions, thereby resolving the contradiction between linearity improvement and noise introduction.
Solution Approach 2:
The patent changes the resistance parameter of the differential-input pairs by selectively connecting degeneration resistors only when input signal exceeds a threshold. This parameter change is controlled by detecting signal amplitude and activating the by-pass switches accordingly, allowing the system to maintain low noise for small signals while improving linearity for large signals.
2Manufacturing precision
If dynamic-biasing circuits are used to improve linearity at high signals, then total harmonic distortion is reduced, but current consumption increases excessively
Solution Approach 1:
The patent uses dynamic switching of resistive-degeneration groups only when needed (at high signal levels) rather than continuous operation. The by-pass switches are controlled by signal amplitude detection, enabling the system to activate linearity improvement mechanisms only during high-signal conditions, thus avoiding excessive current consumption during low-signal operation while still reducing total harmonic distortion when required.
3Device complexity
If the FBDDA amplifier operates without closed-loop configuration, then simplicity is maintained, but total harmonic distortion increases at high input signals
Solution Approach 1:
The patent applies local quality improvement by adding resistive-degeneration groups specifically to the differential-input pairs that require linearity enhancement, rather than implementing a global closed-loop feedback architecture. This localized approach maintains the overall simplicity of the open-loop configuration while improving linearity locally at the input stage where it is most needed.
Data Source
AI summary
A fully balanced differential difference amplifier includes a first differential input stage that receives an input voltage and a second differential input stage that receives a common-mode voltage. A first resistive-degeneration group is coupled to the first differential input and a second resistive-degeneration group is coupled to the second differential input. A differential output stage generates an output voltage. A first switch is coupled in parallel to the first resistive-degeneration group and a second switch is coupled in parallel with the second resistive-degeneration group. The first and second switches are driven into the closed state when the voltage input assumes a first value such that said first input stage operates in the linear region, and are driven into the open state when the voltage input assumes a second value, higher than the first value, such that the first input stage operates in a non-linear region.