Transformer-Coupled Amplifier Feedback for Load-Independent Linearization
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Solution Overview
Problem
Existing amplifier linearization techniques, such as pre-distortion, feed-forward error correction, and Cartesian feedback, face challenges like sensitivity to drift, stability issues, and limited bandwidth, making them unsuitable for high-frequency applications, and power amplifiers often rely on inherent device linearity which compromises efficiency and linearity.
Innovation Solution
The use of magnetically coupled feedback through a transformer with a primary winding in series with the amplifier output and a secondary winding coupled to the input, providing a feedback loop that is independent of load impedance, allowing for improved linearity and stability by adjusting the coupling factor and turn-ratio of the transformer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If pre-distortion is used to linearize the PA output, then linearity is improved, but the system becomes sensitive to drift and requires initial calibration
Solution Approach 1:
The patent implements feedback-based linearization by capturing a portion of the PA output signal through a coupling network and feeding it back to the input stage. This closed-loop approach continuously corrects for drift and non-linearities, eliminating the need for pre-calibration while maintaining linearity. The feedback mechanism adapts to changing conditions in real-time, resolving the contradiction between linearity and drift sensitivity.
2Reliability
If Cartesian feedback is used to linearize the PA, then drift sensitivity is reduced, but stability issues and limited bandwidth occur
Solution Approach 1:
The patent applies local quality by implementing feedback only at specific critical points in the signal path rather than throughout the entire system. The coupling network selectively feeds back only the necessary portion of the output signal to the input stage, maintaining stability while correcting drift. This localized approach avoids the bandwidth and stability limitations of comprehensive Cartesian feedback systems.
3Manufacturing precision
If linear feedback is used to linearize the amplifier, then linearity performance is improved, but stability becomes difficult to maintain for all load conditions
Solution Approach 1:
The patent uses partial action by implementing a coupling network that captures only a portion of the PA output signal for feedback, rather than the full signal used in traditional linear feedback systems. This partial feedback approach provides sufficient linearity correction while reducing the risk of instability across varying load conditions, as the feedback loop gain is naturally limited by the coupling ratio.
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
This approach enhances amplifier linearity and stability, reduces distortion, and maintains constant DC power dissipation across signal levels, enabling efficient operation closer to compression without degrading linearity, suitable for high-bandwidth applications like WiFi power amplifiers.
Implementation Method 1
The primary winding and the secondary winding are arranged such that a portion of a magnetic field generated by the primary winding couples to the secondary winding through a magnetically coupled feedback loop
Data Source
AI summary
An amplifier circuit includes an amplifier having an amplifier input and an amplifier output. A transformer disposed to provide a signal for driving a load includes a primary winding in series with the amplifier output. A secondary winding of the transformer is coupled to the amplifier input where the primary winding and the secondary winding are arranged such that a portion of a magnetic field generated by the primary winding couples to the secondary winding so as to establish a magnetically coupled feedback loop from the amplifier output to the amplifier input. A loop gain of the magnetically coupled feedback loop is substantially independent of an impedance of the load and is defined at least in part by a coupling factor and turn-ratio of the transformer. The load may be included within an output load arrangement including a balun.