Impedance Matched Variable Gain LNA Using Shunt Feedback

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Solution Overview

Problem

Impedance-matched variable gain amplifiers face limitations in dynamic range and input impedance when using feedback, as the input impedance varies with gain, restricting practical gain variation to a few dBs.

Innovation Solution

A switchable feedback mechanism that disengages feedback for low gain applications, using a switch to short the feedback resistor to signal-ground, maintaining stable input impedance and allowing feedback for higher gain settings to enhance dynamic range.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If feedback is used in impedance-matched variable gain amplifier, then dynamic range is improved, but input impedance varies with gain limiting variable gain range

Engineering Contradiction:

Improvedynamic rangeVSAvoidvariable gain range

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The feedback mechanism is made dynamically switchable based on gain settings. The controller activates feedback for high gain settings to improve dynamic range, and disables feedback for low gain settings to maintain stable input impedance, allowing the system to adapt its behavior to different operating conditions

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The feedback resistance is changed based on gain settings by switching it in and out of the circuit. This parameter change allows the amplifier to achieve different input impedance characteristics corresponding to different gain ranges, resolving the contradiction between dynamic range improvement and input impedance stability

Inventive Principle:

Principle #35Parameter changes

2Reliability

If feedback resistance is used for all gain settings, then intercept point is improved, but input impedance grows excessively at low gain levels

Engineering Contradiction:

Improveintercept pointVSAvoidinput impedance stability

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The feedback resistance is dynamically switched in and out based on gain settings. At low gain levels, the feedback resistance is switched out to prevent excessive input impedance growth, while at high gain levels it is switched in to improve intercept point and dynamic range

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The feedback resistance is extracted from the circuit for low gain settings where it would cause harmful effects (excessive input impedance). The controller selectively removes the feedback path when gain is low, eliminating the harmful effect while preserving it when beneficial

Inventive Principle:

Principle #2Taking out (Extraction)

Data Source

PatentUS7301394B2Impedance matched variable gain low noise amplifier using shunt feed-back

Publication Date: 2007.11.27 AVAGO TECHNOLOGIES INTERNATIONAL SALES PTE LTD

AI summary

The variable gain amplifier includes a forward path that provides the amplifier variable gain, and a feedback path. The feedback path uses a switch that is turned on at low gain levels. The switch taps into the feedback resistor, shunting it to signal-ground and eliminating the feedback mechanism. This ensures that the input impedance seen at the input port does not grow excessively, using part of the feedback resistor as a passive termination at low gain levels. In this way variable gain ranges in excess of 30 dB can be achieved.