Capacitive Shunt-Feedback LNA for Noise Figure and Linearity

Solution Overview

Problem

Existing low-noise amplifiers face a conflict between noise figure and linearity due to the requirement of high voltage gain in resistive shunt feedback noise canceling techniques, which degrades the compression point and introduces noise degradation.

Innovation Solution

A low-noise amplifier circuit with a first and second amplifier branch, utilizing a common source or common emitter amplifier with a shunt-feedback capacitor, and an output capacitor to cancel drain noise without relying on high voltage gain, employing capacitive shunt feedback to achieve wide-band matching and improved linearity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If resistive shunt feedback noise canceling technique is used to achieve good noise figure, then noise figure is improved, but voltage gain must be increased which degrades linearity and compression point

Engineering Contradiction:

Improvenoise figureVSAvoidlinearity

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The amplifier is divided into two separate branches: a first amplifier branch containing the main amplifier with noise canceling circuitry, and a second amplifier branch that is noise-free. This segmentation allows the noisy branch to be optimized for noise figure while the overall system maintains linearity through the clean second branch.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A noise-free second amplifier branch is introduced as an intermediary to provide a clean signal path that compensates for the noise in the first branch. The outputs of both branches are combined, allowing the system to achieve good noise figure through the first branch while maintaining linearity through the second branch.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If high voltage gain is used to achieve good noise figure in resistive shunt feedback, then noise figure is improved, but compression point is degraded due to clipping

Engineering Contradiction:

Improvenoise figureVSAvoidcompression point

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The amplifier is divided into two separate branches: a first amplifier branch containing the main amplifier with noise canceling circuitry, and a second amplifier branch that is noise-free. This segmentation allows the noisy branch to be optimized for noise figure while the overall system maintains linearity through the clean second branch.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A noise-free second amplifier branch is introduced as an intermediary to provide a clean signal path that compensates for the noise in the first branch. The outputs of both branches are combined, allowing the system to achieve good noise figure through the first branch while maintaining linearity through the second branch.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Adaptability or versatility

If resistive shunt feedback is used to achieve wide-band matching, then input matching bandwidth is improved, but noise figure is degraded due to resistive feedback element

Engineering Contradiction:

Improvematching bandwidthVSAvoidnoise figure

Core Design Contradiction:

Adaptability or versatilityVSMeasurement precision

Solution Approach 1:

The feedback element is changed from resistive to capacitive. The capacitive shunt feedback provides wide-band input matching without the noise figure degradation associated with resistive feedback elements, as capacitors do not introduce thermal noise like resistors do.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The resistive feedback mechanism is replaced with a capacitive feedback mechanism. This substitution eliminates the thermal noise generated by the resistive element while maintaining the wide-band matching capability through the frequency-dependent reactance of the capacitor.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentEP2624447B8Noise canceling low-noise amplifier

Publication Date: 2014.12.10 TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)

AI summary

A noise-canceling LNA circuit (30) for amplifying signals at an operating frequency f in a receiver circuit (10) is disclosed. The LNA circuit (30) comprises a first (42) and a second (46) amplifier branch, each having an input terminal (43, 47) connected to an input terminal (32) of the LNA circuit (30). The first amplifier branch (42) comprises an output terminal (44) for supplying an output current of the first amplifier branch (42) and a common source or common emitter main amplifier. The main amplifier has an input transistor (50) having a first terminal (52), which is a gate or base terminal, operatively connected to the input terminal (43) of the first amplifier branch, a shunt-feedback capacitor (60) operatively connected between the first terminal (52) of the input transistor (50) and a second terminal (54), which is a drain or collector terminal, of the input transistor (50), and an output capacitor (65) operatively connected between the second terminal (54) of the input transistor (50) and the output terminal (44) of the first amplifier branch (42). The second amplifier branch (46) comprises an output terminal (48) for supplying an output current of the second amplifier branch (46). The LNA circuit (30) comprises circuitry (68) for combining the output current of the first amplifier branch (42) and the output current of the second amplifier branch (46), thereby generating a total output current of the LNA circuit (30).