RF Power Amplifier Log-Feedback Circuit for AM/AM Distortion

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Solution Overview

Problem

Power amplifiers in RF transmitters suffer from non-linear characteristics leading to intermodulation distortion, particularly due to amplitude modulation/amplitude modulation (AM/AM) effects, which traditional envelope feedback schemes struggle to consistently correct, especially at higher symbol rates and varying peak-to-average ratios.

Innovation Solution

An electronic device with a distortion correction circuit comprising a variable gain amplifier and an amplitude correction control loop using matched logarithmic converters to control the gain, independent of the signal envelope, along with a reflected power protection control loop for high VSWR conditions, allowing the power amplifier to operate more efficiently and reducing distortion.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If traditional envelope feedback schemes are used to correct distortion in power amplifier output, then some distortion correction is achieved, but the feedback gain becomes insufficient at higher symbol rates and varying peak-to-average ratios

Engineering Contradiction:

Improvedistortion correction accuracyVSAvoidperformance consistency across different symbol rates and peak-to-average ratios

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

The patent transforms the feedback control from amplitude-domain to logarithmic-domain by applying logarithmic conversion to both the reference signal and feedback signal. This parameter transformation enables the feedback gain to remain consistent across varying signal conditions, symbol rates, and peak-to-average ratios, resolving the adaptability issue of traditional envelope feedback schemes

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent replaces the traditional linear envelope feedback mechanism with a logarithmic domain feedback system. By substituting the linear amplitude comparison with logarithmic conversion and subtraction, the system achieves envelope-independent feedback gain that maintains consistent distortion correction performance across different operating conditions

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Measurement precision

If power amplifier operates in linear regions to avoid signal distortion, then signal quality is maintained, but power efficiency and heat generation are compromised

Engineering Contradiction:

Improvesignal qualityVSAvoidpower efficiency

Core Design Contradiction:

Measurement precisionVSLoss of energy

Solution Approach 1:

The patent applies predistortion to the reference signal before it enters the power amplifier. By pre-compensating for the expected non-linear distortion in the logarithmic domain, the system enables the power amplifier to operate in more efficient non-linear regions while the predistorted signal ensures the final output remains linear and undistorted after amplification

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent converts the harmful non-linear characteristics of the power amplifier into a benefit by operating the amplifier in its non-linear region (where it is more efficient) and using logarithmic predistortion to cancel out the resulting distortion. The non-linearity that would normally cause distortion is now exploited for improved power efficiency while maintaining signal quality through mathematical compensation

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution effectively corrects non-linearity in the output signal amplitude, maintaining consistently high feedback gain and achieving precise automatic transmit gain control, while allowing the power amplifier to operate in a more non-linear mode for improved efficiency in power consumption and heat generation, and reduces out-of-band emissions.

Implementation Method 1

a first logarithmic converter having an input coupled to the RF modulator

Methodology Applied

Scientific EffectLogarithmic conversion:

Implementation Method 2

a second logarithmic converter having an input coupled to an output of the power amplifier

Methodology Applied

Scientific EffectLogarithmic conversion:

Implementation Method 3

a variable gain amplifier coupled between the RF modulator and the power amplifier

Methodology Applied

Scientific EffectSignal amplification:

Data Source

PatentUS7970360B2Electronic device with a distortion correction circuit for a power amplifier, and associated methods

Publication Date: 2011.06.28 L3HARRIS GLOBAL COMMUNICATIONS INC

AI summary

An electronic device includes a radio frequency (RF) modulator, a power amplifier, and a distortion correction circuit coupled between the RF modulator and the power amplifier. The distortion correction circuit includes a variable gain amplifier coupled between the RF modulator and the power amplifier, and an amplitude correction control loop. The amplitude correction control loop includes a first logarithmic converter having an input coupled to the RF modulator, a second logarithmic converter having an input coupled to an output of the power amplifier, and a difference circuit coupled to outputs of the first and second logarithmic converters for controlling the gain of the variable gain amplifier for correcting distortion in an output signal from the power amplifier.