Level-Shift Common-Mode Feedback for High-Gain Gm Amplifiers
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Solution Overview
Problem
Conventional Gm amplifiers with common-mode feedback circuits often experience reduced gain and increased input offset voltage due to the high impedance of the current source and the presence of resistors in the feedback circuit, which affect the operation and performance of the amplifier.
Innovation Solution
The implementation of a common-mode feedback circuit without resistors, utilizing level shift circuits connected to the transistors to control the DC operating point of the Gm amplifier, ensuring that the feedback circuit does not act as an output load and thereby maintaining the amplifier's gain and reducing input offset voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If resistors are used in the common-mode feedback circuit to determine the DC operating point, then the DC operating point can be controlled, but the gain of the Gm amplifier is reduced due to the resistors acting as output loads
Solution Approach 1:
The patent extracts and removes the resistors from the common-mode feedback circuit that were causing gain reduction. Instead of using resistors to set the DC operating point, the invention uses a different configuration where the common-mode feedback is achieved through the interaction of transistors and current sources without introducing resistive loading at the output, thereby eliminating the trade-off between DC control and gain.
Solution Approach 2:
The patent merges the common-mode feedback function with the existing transistor and current source elements already present in the differential amplifier structure. By utilizing the inherent properties of these components and their existing connections, the DC operating point control is achieved without adding separate resistive elements that would load the output and reduce gain.
2Ease of operation
If resistors are connected to the output of the differential amplifier circuit, then the common-mode feedback can be implemented, but the input offset voltage increases
Solution Approach 1:
The patent removes the resistors from the feedback path that were causing input offset voltage increase. The common-mode feedback is achieved through alternative means using the transistor gate terminals and current sources, eliminating the source of the offset voltage problem while maintaining feedback functionality.
Solution Approach 2:
The patent introduces an intermediary approach where the common-mode feedback is achieved through the gate terminals of the transistors and the current sources rather than direct resistive connections. This intermediary mechanism transfers the feedback control function without introducing the harmful resistive loading that causes offset voltage.
3Power
If a current source with high impedance is used in the differential amplifier, then the differential mode gain is high, but the common-mode feedback circuit cannot be properly implemented without affecting amplifier operation
Solution Approach 1:
The patent merges the high-impedance current source functionality with the common-mode feedback requirement by utilizing the existing current sources already present in the differential amplifier structure. The feedback is achieved through the interaction of these current sources with the transistor gates, allowing both high differential gain and proper common-mode feedback without conflict.
Solution Approach 2:
The patent implements a feedback mechanism that is compatible with high-impedance current sources by using the gate terminals of the transistors as feedback nodes. The common-mode voltage is sensed and fed back through the current sources to the gates, creating a feedback loop that works harmoniously with the high-impedance architecture rather than conflicting with it.
Data Source
AI summary
Provided is a transconductance amplifier including a common-mode feedback circuit that does not affect an operation of the transconductance amplifier. The transconductance amplifier has a transconductance amplifier circuit configured to generate an output current based on an input voltage and a common-mode feedback circuit configured to determine a DC operating point of an output of the transconductance amplifier circuit. The common-mode feedback circuit has a plurality of level shift circuits configured to shift levels of input voltages to output the voltages, and are connected to control terminals of a plurality of transistors.