Ground-Partitioned Power Amplifier Feedback for RF Stability
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Solution Overview
Problem
Power amplifiers (PAs) face instability due to large voltage drops on parasitic inductance paths, leading to positive feedback loops and instability, especially when ground connections between stages interact, which existing solutions like Kelvin connections or segmented ground configurations either fail to address effectively or come with performance penalties.
Innovation Solution
Implementing a controlled amount of negative feedback using ground signal dividers or weighting techniques to neutralize local positive feedback loops, allowing for robust stability and maintaining high RF performance without significant degradation, by strategically connecting ground pins and using existing parasitic inductances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If ground connections of different gain stages are directly interconnected via wires with finite parasitic inductance to save pins and increase voltage drive, then device complexity and pin count are reduced, but voltage drops on parasitic inductance paths create positive feedback loops leading to PA instability
Solution Approach 1:
The ground connection structure is segmented into multiple independent ground paths for different gain stages, with each stage having its own dedicated ground connection to the ground plane. This segmentation eliminates the common parasitic inductance that causes positive feedback loops, thereby maintaining PA stability while managing complexity through structured organization.
Solution Approach 2:
Each gain stage is provided with locally optimized ground connections that are tailored to its specific current requirements and frequency characteristics. The ground path impedance is minimized locally at each stage rather than using a shared path, ensuring that local voltage drops do not create feedback issues while maintaining overall system stability.
2Reliability
If a Kelvin connection of grounds is used for all stages to eliminate common inductance and ensure stability, then PA stability is improved, but a large number of ground pins are required which are not economical or feasible
Solution Approach 1:
The Kelvin connection concept is applied selectively to each gain stage individually rather than requiring all stages to use separate ground pins. Each stage has its own segmented ground path that provides the isolation benefits of Kelvin connections while sharing the ground plane infrastructure, thus achieving stability without proportionally increasing pin count.
Solution Approach 2:
The ground connection strategy moves from a two-dimensional plane view (shared ground plane) to a three-dimensional perspective by creating multiple ground paths that route through different physical dimensions and layers, effectively providing isolation similar to Kelvin connections while utilizing the ground plane more efficiently.
3Reliability
If separate output stage ground inductance is used to prevent ground feedback loops, then PA stability is improved, but transistor drive voltages are reduced due to negative feedback from ground inductance degeneration
Solution Approach 1:
The ground inductance is optimized locally at each stage to provide just enough isolation to prevent feedback loops, rather than using excessive inductance that would cause degeneration. The inductance value is carefully selected to be sufficient for stability but minimal enough to avoid significant voltage drops that would reduce drive capability.
Solution Approach 2:
The ground inductance parameter is adjusted and optimized for each specific gain stage based on its operating conditions, current levels, and frequency range. This parameter optimization ensures that the inductance provides necessary isolation for stability while minimizing its negative impact on drive voltage through precise parameter selection.
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 effectively stabilizes PAs by neutralizing positive feedback loops while maintaining strong drive and RF performance, avoiding the need for additional components and minimizing power consumption, thus achieving robust stability and high efficiency.
Implementation Method 1
Implementing a controlled amount of negative feedback using ground signal dividers or weighting techniques to neutralize local positive feedback loops
Implementation Method 2
The alternating currents (AC) running through the signal path stages of the PAs is very large. This results in large voltage drops on the parasitic inductor paths
Data Source
AI summary
Achievement of robust stability of a power amplifier (PA) that allows the sharing of the ground between the driver stages and the output stage is shown. A controlled amount of negative feedback is used to neutralize the local positive feedback that results from the driver-to-output stage ground sharing in the signal path, for example, a radio frequency (RF) signal path. The solution keeps a strong drive and a good performance of the PA. Exemplary embodiments are shown for the PA positive feedback neutralization. A first embodiment uses a ground signal divider while another embodiment uses a ground signal divider weighting technique.