Differential MDS LNA Topology for Low Noise and Input Capacitance
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Solution Overview
Problem
Current Low Noise Amplifiers (LNAs) face challenges in minimizing distortion and noise while maintaining high gain, particularly in cellular telephone receivers, where third-order intermodulation and cross-modulation distortions are difficult to separate from desired signals due to non-linear amplifier characteristics and noise introduction.
Innovation Solution
A differential Modified Derivative Superposition (MDS) LNA topology is employed, utilizing transistors biased in both saturation and sub-threshold regions, with specific circuit configurations and biasing schemes to optimize distortion cancellation and reduce noise, including source degeneration inductors and capacitors to phase-align and cancel distortion signals effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If high gain is used to minimize noise contributions, then noise performance is improved, but distortion (third-order intermodulation and cross-modulation) increases
Solution Approach 1:
The LNA is divided into two separate stages: a first stage using saturation-biased transistors for high gain with low noise, and a second stage using sub-threshold biased transistors for distortion cancellation. This segmentation allows each stage to be optimized for its specific function, resolving the contradiction between noise performance and distortion.
Solution Approach 2:
The second stage acts as an intermediary that generates distortion signals to cancel the distortion produced by the first stage. The sub-threshold biased transistors in the second stage create opposite-phase distortion components that cancel the intermodulation and cross-modulation distortion from the high-gain first stage.
2Object-generated harmful factors
If sub-threshold biased transistors are used for distortion cancellation, then distortion is reduced, but noise is introduced
Solution Approach 1:
The LNA is divided into two separate stages: a first stage using saturation-biased transistors for high gain with low noise, and a second stage using sub-threshold biased transistors for distortion cancellation. This segmentation allows each stage to be optimized for its specific function, resolving the contradiction between noise performance and distortion.
Solution Approach 2:
The noise introduced by sub-threshold biased transistors is converted into a benefit by placing them in the second stage where their noise is masked by the high gain of the first stage. The distortion cancellation benefit outweighs the noise penalty because the first stage's high gain suppresses the second stage's noise contribution.
3Device complexity
If conventional LNA topologies are used, then circuit simplicity is maintained, but input capacitance is large
Solution Approach 1:
The patent changes the biasing parameter of the transistors from conventional saturation region operation to sub-threshold region operation for the second stage. This parameter change reduces the input capacitance while maintaining the distortion cancellation function, as sub-threshold biased transistors have lower gate capacitance.
4Object-affected harmful factors
If saturation region biased transistors are used, then noise is minimized, but distortion cancellation capability is reduced
Solution Approach 1:
The LNA is divided into two separate stages: a first stage using saturation-biased transistors for high gain with low noise, and a second stage using sub-threshold biased transistors for distortion cancellation. This segmentation allows each stage to be optimized for its specific function, resolving the contradiction between noise performance and distortion.
Solution Approach 2:
The second stage acts as an intermediary that generates distortion signals to cancel the distortion produced by the first stage. The sub-threshold biased transistors in the second stage create opposite-phase distortion components that cancel the intermodulation and cross-modulation distortion from the high-gain first stage.
Data Source
AI summary
A differential low noise amplifier (LNA) involves two main amplifying transistors biased in saturation, and two cancel transistors biased in sub-threshold. In one example, the gates of the cancel transistors are coupled to the drains of main transistors, in a symmetrical and cross-coupled fashion. The main transistors are source degenerated. Because the gates of cancel transistors are not coupled to the differential input leads of the LNA, the input capacitance of the LNA is reduced. Noise introduced into the LNA output due to the cancel transistors being biased in the sub-threshold region is reduced because there are two stages. The first stage involves the main transistors, and the second stage involves the cancel transistors. By increasing the gain of the first stage and decreasing the gain of the second stage, overall LNA gain is maintained while reducing the noise that the sub-threshold biased transistors contribute to the LNA output.