Complementary FET Shunt VVA for High RF Linearity
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Solution Overview
Problem
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 n-type and p-type field effect transistors (FETs), where control voltages and complementary control voltages are used to bias the FETs, enhancing linearity by canceling non-linear effects from RF signal amplitude variations, and utilizing a control circuit to generate these voltages based on an analog attenuation control signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single shunt arm with FETs is used for attenuation, then the device complexity is reduced, but the linearity and signal integrity deteriorate due to non-linear effects from RF signal amplitude variations
Solution Approach 1:
The attenuator is segmented into multiple shunt arms (first shunt arm with NFETs, second shunt arm with PFETs) instead of using a single shunt arm. Each shunt arm processes a portion of the attenuation function, and their combined effect cancels non-linearities while maintaining overall attenuation control. This segmentation resolves the contradiction by distributing the attenuation function across multiple parallel paths.
Solution Approach 2:
The patent employs complementary FETs (NFETs and PFETs) in parallel shunt arms that act as counterweights to each other's non-linear effects. When one type of FET introduces non-linearity, the complementary type introduces an opposing non-linearity that cancels it out, thereby improving linearity without significantly increasing overall device complexity.
2Reliability
If multiple shunt arms with complementary FETs are used to cancel non-linear effects, then linearity improves, but the device complexity increases
Solution Approach 1:
The patent merges the attenuation functions of complementary FETs (NFETs and PFETs) in parallel shunt arms into a unified attenuation control system. Both shunt arms are controlled by complementary control voltages derived from the same attenuation control signal, allowing them to work together as an integrated system rather than separate components, thus managing complexity while achieving linearity improvement.
Solution Approach 2:
The multiple shunt arms with complementary FETs serve multiple functions simultaneously: they provide attenuation control, cancel non-linear effects, and maintain signal integrity across varying RF signal levels. This multi-functionality justifies the increased device complexity by delivering multiple performance benefits from a single structural configuration.
3Device complexity
If attenuation control is achieved using a single control voltage, then the control circuit complexity is reduced, but the ability to cancel non-linear effects deteriorates
Solution Approach 1:
The control circuit generates complementary control voltages for the NFETs and PFETs based on feedback from the RF signal amplitude variations. This feedback mechanism allows the control circuit to dynamically adjust the control voltages to optimize the cancellation of non-linear effects, achieving better linearity without requiring excessive control circuit complexity.
Solution Approach 2:
The control circuit changes the parameters of the control voltages (amplitude, phase) to complementary values for NFETs and PFETs respectively. By transforming a single control voltage into two complementary control voltages with specific parameter relationships, the system enables non-linear effect cancellation while keeping the control circuit complexity manageable through systematic parameter transformation.
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 design achieves high linearity and improved IP3 and IM3 performance, ensuring consistent attenuation across a wide range of RF signal levels, thereby enhancing the overall RF system's linear performance and signal integrity.
Implementation Method 1
The first shunt circuit includes at least one n-type field effect transistor (NFET) having a gate biased by the first control voltage. The second shunt circuit includes at least one p-type field effect transistor (PFET) having a gate biased by the first complementary control voltage.
Implementation Method 2
The control circuit is configured to control an amount of attenuation along a signal path through the VVA between the input terminal and the output terminal
Data Source
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 of one or more n-type field effect transistor (NFETs) and a second shunt arm of one or more p-type field effect transistor (PFETs). The gates of the NFETs are controlled using a control voltage, and the gates of the PFETs are controlled using a complementary control voltage that changes inversely with respect to the control voltage.