LNA Gain-Tuning Circuit With Switched Feedback and Source Degeneration
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Solution Overview
Problem
Existing RF amplifier circuits face challenges in improving noise figure in high-gain modes while maintaining good input-referenced third intercept point in low-gain modes, particularly in CMOS-based low-noise amplifiers used in modern electronic systems.
Innovation Solution
A novel gain switching architecture for low-noise amplifiers that includes a degeneration circuit with a two-port inductor and a bypass switch, coupled with a switched feedback circuit between the common-source FET gate and the amplifier output signal path, allowing for impedance matching and linearity improvement across different gain modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a conventional degeneration inductor is used to improve noise figure in high-gain mode, then noise figure is improved, but device complexity increases due to multi-port coil requirements
Solution Approach 1:
The degeneration inductor is segmented into two separate one-port inductors (L1 and L2) that are coupled through magnetic coupling. This segmentation allows each inductor to be simpler in structure while achieving the same noise figure improvement through the coupled configuration, eliminating the need for complex multi-port coils.
Solution Approach 2:
Magnetic coupling acts as an intermediary mechanism between the two one-port inductors, enabling them to work together to improve noise figure without requiring direct complex multi-port connections. The magnetic field serves as the mediator that transfers energy and achieves the desired noise performance.
2Reliability
If feedback circuit is added to improve linearity in low-gain mode, then input-referenced third intercept point is improved, but device complexity increases
Solution Approach 1:
The feedback circuit is merged with the existing degeneration inductor structure by using the coupled inductors L1 and L2. The feedback path is implemented through the magnetic coupling mechanism already present, combining linearity improvement functionality with the noise figure improvement structure rather than adding completely separate components.
Solution Approach 2:
The coupled inductor structure serves multiple functions: it improves noise figure in high-gain mode and provides feedback for linearity improvement in low-gain mode. This multi-functionality reduces the need for separate dedicated components for each function, thereby reducing overall device complexity.
3Adaptability or versatility
If complex multi-port inductors are used to provide multiple gain modes, then adaptability is improved, but IC area increases
Solution Approach 1:
The multi-port inductor is segmented into multiple one-port inductors with magnetic coupling. This allows independent sizing and placement of each inductor, optimizing space utilization and reducing total IC area compared to a single complex multi-port inductor structure.
Solution Approach 2:
The inductors are arranged in a coupled configuration that utilizes spatial dimensions efficiently. The magnetic coupling allows functional connectivity without requiring direct physical overlap or complex multi-terminal structures, reducing the footprint on the IC substrate.
Data Source
AI summary
Circuits and methods for improving the noise figure (NF) of an amplifier, particularly an LNA, in high-gain modes while improving the IIP3 of the amplifier in low-gain modes. The source of an amplifier common-source FET is coupled to circuit ground thorough a degeneration circuit comprising a two-port inductor and a bypass switch coupled in parallel with the inductor. A switched feedback circuit is coupled between the gate of the common-source FET and a feedback node in the amplifier output signal path. During a low gain mode, the inductor is entirely bypassed and the enabled feedback circuit lowers the input impedance of the common-source FET and reduces the gain of the amplifier circuit, essentially eliminating the need for a degeneration inductor. During a high gain mode, the source of the common-source FET is coupled to circuit ground through the inductor and the feedback circuit is disabled. Other gain modes are supported.