Complementary FET Shunt VVA for High RF Linearity
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Solution Overview
Problem
Existing voltage variable attenuators (VVAs) in RF systems face challenges in achieving high linearity and low distortion, particularly in maintaining signal integrity across varying RF signal levels, which affects the overall performance and linearity metrics such as IP3 and IM3.
Innovation Solution
The implementation of a VVA design that includes multiple shunt arms with series-connected NFETs and PFETs, where the gates of NFETs are controlled by a control voltage and the gates of PFETs are controlled by a complementary control voltage, effectively canceling non-linearity induced by RF signal amplitude variations, and a control circuit generates these voltages based on an analog attenuation control signal to provide fine-tuned attenuation control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional VVA design is used, then the device complexity is low, but the linearity and signal integrity deteriorate
Solution Approach 1:
The VVA is divided into multiple shunt arms (first shunt arm, second shunt arm, third shunt arm) with series-connected FETs in each arm. This segmentation allows independent control of different attenuation ranges, improving overall linearity while managing device complexity through modular architecture
Solution Approach 2:
Different shunt arms are designed with different numbers of FETs (e.g., first shunt arm has two NFETs, second shunt arm has one NFET and one PFET) to optimize performance for specific attenuation ranges. Each arm's FETs are strategically placed to provide complementary non-linearity cancellation, enhancing local linearity characteristics
2Adaptability or versatility
If the attenuation control range is increased, then the adaptability improves, but the linearity deteriorates
Solution Approach 1:
The wide attenuation range is achieved by segmenting the control into multiple shunt arms, each handling a specific attenuation portion. The first shunt arm provides coarse attenuation, while second and third shunt arms provide fine attenuation, maintaining linearity across the entire wide range
Solution Approach 2:
The control voltage is divided into multiple portions (first control voltage, second control voltage, third control voltage) that independently control different shunt arms. This parameter segmentation allows the system to maintain optimal FET operating points across the full attenuation range, preserving linearity
3Manufacturing precision
If complementary FETs are used in parallel, then the linearity improves, but the device complexity increases
Solution Approach 1:
Complementary NFETs and PFETs are merged in parallel within the same shunt arm (e.g., second shunt arm contains both NFET22 and PFET22). This merging enables non-linearity cancellation while sharing common circuit resources, reducing overall complexity compared to completely separate arms
Solution Approach 2:
Complementary FETs are strategically placed in specific shunt arms rather than uniformly distributed. The second shunt arm contains both NFET22 and PFET22 to provide local non-linearity cancellation, while other arms use only one type of FET, optimizing the balance between linearity improvement and complexity
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 configuration enhances the linearity of the VVA, improving IP3 and IM3 performance, ensuring consistent attenuation across a wide range of signal levels and frequencies, thereby improving the overall RF system's linear performance and signal integrity.
Implementation Method 1
The first shunt circuit includes at least one NFET having a gate biased by the control voltage. The second shunt circuit includes at least one PFET having a gate biased by the complementary control voltage
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
Provided herein are apparatus and methods for high linearity voltage variable attenuators (VVAs). In certain configurations, a high linearity VVA includes multiple shunt arms or circuits that operate in parallel with one another between a signal node and a first DC voltage, such as ground. Thus, the shunt arms are in shunt with respect to a signal path of the VVA. The multiple shunt arms include a first shunt arm (352) of one or more n-type field effect transistor, NFETs (402, 404, 406) and a second shunt arm (354) of one or more p-type field effect transistor, PFETs (408, 412, 414). The gates of the NFETs (402, 404, 406) are controlled using a control voltage (Vc), and the gates of the PFETs (408, 412, 414) are controlled using a complementary control voltage (V2-Vc) that changes inversely with respect to the control voltage.