Variable-Gain LNA With Switched Degeneration for Stable Impedance
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Solution Overview
Problem
Current low-noise amplifiers (LNAs) face challenges in maintaining high gain, linearity, and low noise figure while operating in various bias modes without significant changes in input and output impedance, leading to inefficiencies and distortion, especially when reducing bias current for lower gain modes.
Innovation Solution
The LNA is designed with split amplifier branches of binary-weighted, thermometer-weighted, or hybrid-coded configurations, maintaining constant current density and impedance by using degeneration inductors with taps and switchable capacitors to adjust impedance and gain, ensuring consistent performance across different bias modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If bias current is reduced for lower gain modes, then power consumption is reduced, but input and output impedance matching deteriorates
Solution Approach 1:
The LNA is divided into multiple parallel amplifier branches with different bias current levels. Each branch is designed to operate optimally at specific impedance conditions. By selectively activating appropriate branches based on the desired gain mode, the system maintains proper impedance matching across all operating points without requiring global reconfiguration of bias conditions.
Solution Approach 2:
The system dynamically switches between different amplifier branches and adjusts bias conditions in real-time based on the required gain mode. This dynamic reconfiguration allows the LNA to adapt its impedance characteristics to match the optimal conditions for each operating point, preventing impedance degradation when transitioning between high and low gain modes.
2Use of energy by stationary object
If gain is reduced by reducing bias current, then power consumption is reduced, but linearity deteriorates
Solution Approach 1:
The amplifier is segmented into multiple branches, each optimized for specific linearity requirements. High-linearity branches with higher bias currents are available when needed, while lower-power branches can be activated when linearity requirements are less stringent. This segmentation allows independent optimization of linearity and power consumption for different operating scenarios.
Solution Approach 2:
The system changes operating parameters (bias current levels) of individual amplifier branches to achieve the desired trade-off between power consumption and linearity. By adjusting which branches are active and at what bias levels, the system can maintain acceptable linearity performance across different power consumption states without requiring all branches to operate at high bias currents.
3Adaptability or versatility
If multiple gain modes are implemented with different bias currents, then adaptability is improved, but maintaining consistent input and output impedance becomes more difficult
Solution Approach 1:
The LNA is segmented into multiple parallel branches, each designed with specific impedance characteristics optimized for particular gain modes. By activating appropriate combinations of branches, the system presents a consistent overall impedance to the input and output regardless of which internal branches are active. The segmentation allows each branch to be independently optimized while contributing to a unified impedance profile.
Solution Approach 2:
The amplifier structure is designed with universal impedance matching networks that function across all gain modes. These matching networks are configured to maintain proper impedance transformation regardless of which amplifier branches are active. The multi-functional design ensures that the input and output impedance characteristics remain stable across the full range of gain modes without requiring mode-specific matching circuits.
Data Source
AI summary
A receiver front-end capable of receiving RF inputs having a broad range of levels. The receiver comprises a low-noise amplifier (LNA) operating in a variety of bias modes that cover a large gain range. Branches of the amplifier can be turned on in various combinations to allow selection of different bias modes. A degeneration inductor coupled to the source of the common source FET of each branch has a plurality of taps that are coupled to degeneration switches that can ground the tap to effectively shorten the degeneration inductor and reduce the amount of degeneration inductance. The degeneration inductor and associated switches can be fabricated using one of several physical layouts. Operating the degeneration switches to select the length of the degeneration inductor to match the bias mode reduces changes in the input impedance as different bias modes are selected.