Variable-Gain LNA Branch Switching for Input Matching and Linearity
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Solution Overview
Problem
Existing variable gain LNAs face challenges in maintaining input matching and input linearity when switching between high and low gain modes, with additional amplifiers introducing noise and complexity, and tracking between amplifiers being difficult.
Innovation Solution
A variable gain LNA architecture using first and second transistors with coupled degeneration inductors, where the second branch can be turned off to control gain, optimizing input matching and linearity across all gain gears without additional noise or cost.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the LNA is optimized under high gain mode condition, then the gain performance is improved, but the input matching and input linearity will degrade when the LNA operates in low gain mode
Solution Approach 1:
The LNA is divided into two separate branches: a first branch optimized for high gain mode and a second branch optimized for low gain mode. Each branch has its own transistor and degeneration inductor with parameters specifically tuned for its operating mode, allowing both modes to operate at their optimized conditions simultaneously without compromising input matching or linearity.
2Power
If an auxiliary amplifier is added to optimize low gain mode, then the low gain performance is improved, but the noise figure increases due to extra loading
Solution Approach 1:
Instead of adding a separate auxiliary amplifier that would introduce additional loading and noise, the patent merges the low gain optimization directly into the second branch of the existing differential architecture. The second transistor and its degeneration inductor are configured to provide low gain mode optimization without requiring an external auxiliary amplifier, thereby avoiding the noise figure degradation caused by extra loading.
3Adaptability or versatility
If two independent amplifiers are used for high and low gain modes, then the gain range is extended, but the tracking between amplifiers becomes challenging
Solution Approach 1:
The differential LNA architecture provides a universal platform where both high gain and low gain modes are achieved within a single unified circuit structure. By using a differential pair with two branches that share common biasing and output nodes, the design achieves multi-functionality (both gain modes) without requiring independent amplifiers, thereby eliminating tracking complexity while maintaining extended gain range.
Data Source
AI summary
A variable gain low noise amplifier (LNA) and a method for controlling a gain of the variable gain LNA are provided. The variable gain LNA may include a first transistor, a first degeneration inductor, a second transistor and a second degeneration inductor, wherein the first degeneration inductor is coupled to a source terminal of the first transistor, and the second degeneration inductor is coupled to a source terminal of the second transistor. Gate terminals of the first transistor and the second transistor are configured to receive an input signal. The first transistor and the first degeneration inductor belong to a first branch of the variable gain LNA, and the second transistor and the second degeneration inductor belong to a second branch of the variable gain LNA. More particularly, a gain of the variable gain LNA is determined by controlling whether to turn off the second branch.