Inductor-Less LNA Feedback Layout for Low-Frequency Stability

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Solution Overview

Problem

Inductors in low-noise amplifiers (LNAs) increase die area and are prone to magnetic coupling, leading to low frequency instability and instability at low frequencies due to unintended positive feedback loops.

Innovation Solution

Implement an inductor-less LNA design with a coupling capacitor located outside the RF feedback loop, splitting RF feedback into separate loops for PMOS and NMOS transistors, and using bias circuits to set DC bias points, thereby weakening the positive feedback loop and improving low frequency stability.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If inductors are used in LNA design, then RF signal amplification is achieved, but die area increases and magnetic coupling causes low frequency instability

Engineering Contradiction:

Improvelow frequency stabilityVSAvoiddie area

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

The patent removes inductors from the LNA design entirely, extracting the problematic magnetic coupling element while maintaining RF amplification functionality through an inductor-less topology with coupling capacitors positioned outside the RF feedback loop

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces coupling capacitors as intermediary elements that perform the function of signal coupling without creating magnetic coupling issues, placing them outside the RF feedback loop to prevent low frequency instability

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If inductors are used in LNA design, then RF signal amplification is achieved, but magnetic coupling creates unintended positive feedback loops

Engineering Contradiction:

Improvefeedback loop stabilityVSAvoidcircuit structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent extracts inductors from the circuit, eliminating the source of magnetic coupling and unintended positive feedback loops while maintaining RF amplification through an alternative inductor-less architecture

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent uses coupling capacitors positioned outside the RF feedback loop as intermediary elements to couple RF signals without creating magnetic coupling paths that would generate unintended positive feedback

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS20250385651A1Jammer resilient inductor-less low-noise amplifier for high frequencies

Publication Date: 2025.12.18 QUALCOMM INC

AI summary

A system for wireless communications. The system includes a low-noise amplifier (LNA). The LNA includes an inverter, a radio frequency (RF) feedback circuit coupled between an output of the inverter and an input of the inverter to provide an RF feedback loop, and a coupling capacitor coupled between an input of the LNA and the input of the inverter, wherein the coupling capacitor is located outside of the RF feedback loop. The system also includes a bias circuit coupled between the output of the inverter and the input of the inverter.