Open-Loop Adaptive Bias Circuit for Wideband PA Linearity
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Solution Overview
Problem
Power amplifiers in modern communication devices face a challenge in simultaneously achieving high linearity and efficiency over a large bandwidth, as operating near saturation for efficiency typically results in poor linearity, and existing feedback-based linearization techniques are limited to small bandwidths.
Innovation Solution
An adaptive biasing system for power amplifiers that uses a gain expansion circuit and a gain compression circuit to set the bias based on the input RF signal, operating in open-loop mode without feedback, and includes a BALUN transformer with a switchable coupling ratio for impedance matching, allowing the power amplifier to operate in Class-AB mode and adjust bias dynamically.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the power amplifier operates near saturation to improve efficiency, then energy efficiency is improved, but linearity deteriorates
Solution Approach 1:
The patent applies dynamics by making the bias point of the power amplifier dynamic rather than fixed. The bias circuit continuously adjusts the bias voltage based on the instantaneous amplitude of the input signal, allowing the amplifier to operate at optimal efficiency points across varying signal conditions while maintaining linearity through real-time adaptation.
Solution Approach 2:
The patent changes the bias parameter dynamically in response to signal conditions. By detecting the input signal amplitude and adjusting the bias voltage accordingly, the system transitions from a static bias configuration to a dynamic one, enabling the amplifier to achieve high efficiency near saturation while compensating for linearity degradation through bias modulation.
2Manufacturing precision
If feedback-based linearization techniques are used to improve linearity, then signal linearity is improved, but bandwidth is limited
Solution Approach 1:
The patent applies preliminary action by pre-distorting the input signal or pre-adjusting the bias point before the signal reaches the power amplifier's nonlinear region. The bias circuit anticipates the signal amplitude and adjusts the bias in advance, compensating for expected nonlinearity before it occurs, thereby achieving linearization without requiring feedback that would limit bandwidth.
Solution Approach 2:
The patent extracts the linearization function from a feedback loop and implements it through open-loop bias adjustment. By taking out the need for output feedback and instead using input signal detection to control bias, the system achieves linearization while maintaining wide bandwidth, as the solution does not depend on feedback signal processing.
3Manufacturing precision
If adaptive biasing circuits are added to improve linearity and efficiency, then performance is improved, but circuit complexity increases
Solution Approach 1:
The patent merges the biasing function with the signal detection function. The same circuit elements that detect the input signal amplitude are used to generate the bias control voltage, combining multiple functions into a unified structure. This integration reduces the number of separate components and simplifies the overall circuit while achieving adaptive biasing for improved linearity and efficiency.
Solution Approach 2:
The patent implements multi-functionality by designing the bias circuit to simultaneously perform signal envelope detection, bias voltage generation, and linearity compensation. This universal approach allows a single circuit block to accomplish multiple tasks that would traditionally require separate dedicated circuits, thereby improving performance without proportionally increasing complexity.
Data Source
AI summary
A power amplification system includes a Power Amplifier (PA) for amplifying an input RF signal. An adaptive bias circuit is configured to adaptively set a bias of the PA. The adaptive biasing circuit includes a gain expansion circuit, a gain compression circuit and a biasing circuit. The gain expansion circuit derives a gain-expansion control signal from the input RF signal. For a first sub-range of the input RF signal, the gain-expansion control signal has a larger dynamic range than the input RF signal. The gain compression circuit derives a gain-compression control signal from the input RF signal. For a second sub-range of the input RF signal having higher power levels than the first sub-range, the gain-compression control signal has a smaller dynamic range than the input RF signal. The biasing circuit sets the bias of the PA responsively to the gain-expansion control signal and the gain-compression control signal.