RF Switch Biasing With Floating Resistors for Isolation Control
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Solution Overview
Problem
Switches in wireless communication devices, particularly those implemented with MOS transistors, face issues due to parasitic capacitances that lead to signal loss and reliability concerns, especially in RF applications where high signal swings can exceed transistor breakdown voltages.
Innovation Solution
The implementation of switches with stacked MOS transistors and additional RF floating resistors to reduce signal loss and improve reliability by distributing the signal swing evenly across multiple transistors, enhancing isolation and reducing the impact of parasitic capacitances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If switches are implemented with MOS transistors to enable signal switching functionality, then the switch can pass or block input signals based on control signals, but parasitic capacitances between terminals cause signal loss and adversely impact performance
Solution Approach 1:
The switch is divided into multiple MOS transistors connected in series between the first and second nodes. Each transistor has its gate connected to a common control node, allowing the signal switching function to be segmented across multiple devices. This segmentation reduces the parasitic capacitance impact on any single transistor, thereby reducing overall signal loss while maintaining the signal passing or blocking functionality.
2Productivity
If high signal swings are used in RF applications to improve signal transmission, then the signal transmission capability is enhanced, but the signal swing may exceed transistor breakdown voltages causing reliability concerns
Solution Approach 1:
By dividing the switch into multiple transistors in series, the voltage swing is distributed across each transistor. If N transistors are used in series, each transistor experiences approximately 1/N of the total voltage swing, keeping individual transistor voltages within safe operating limits even when the total signal swing is high, thus maintaining reliability while preserving signal transmission capability.
Solution Approach 2:
The series connection of multiple transistors acts as a protective structure that beforehand limits the voltage stress on each transistor. This configuration cushioning prevents any single transistor from experiencing excessive voltage that would cause breakdown, thereby protecting the circuit reliability before the harmful effect can occur.
3Reliability
If additional RF floating resistors are added to distribute signal swing across transistors to improve reliability, then the signal swing distribution and isolation are improved, but the device complexity increases
Solution Approach 1:
The control node serves multiple functions: it controls the switching state of all transistors simultaneously and distributes the voltage swing across them. This multi-functionality achieves reliable signal swing distribution without requiring separate control circuits for each transistor, thereby limiting the increase in device complexity.
Solution Approach 2:
Multiple transistors are merged in series with shared control connection, creating a unified switch structure. This merging achieves the reliability benefit of distributed voltage handling while using a compact configuration that minimizes the increase in overall device complexity compared to using separate control circuits for each transistor.
Data Source
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Figure 3A~3B
AI summary
Switches with improved biasing and having better isolation and reliability are described. In an exemplary design, a switch (50D) is implemented with a set of transistors (510a-k), a set of resistors (520a-k), and an additional resistor (530). The set of transistors (510a-k) is coupled in a stacked configuration, receives an input signal (Vout), and provides an output signal. The set of resistors (520a-k) is coupled to the gates of the set of transistors (510a-k). The additional resistor (530) is coupled to the set of resistors (520a-k) and receives a control signal (vcontrol) for the set of transistors (510a-k). The resistors reduce signal loss through parasitic capacitances of the transistors when they are turned on. The resistors also help split the signal swing of the input signal approximately evenly across the transistors when they are turned off, which may improve reliability of the transistors. The switch (50D) may be used in a switchplexer, a power amplifier (PA) module, etc.