RF Switch Bias Swapping for Isolation Without Charge Pumps
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Solution Overview
Problem
Current RF switching circuits face challenges in achieving optimal performance between insertion loss and switch isolation, particularly in switching modes, and often require additional charge pump circuitry to generate voltage levels outside the normal supply voltage range.
Innovation Solution
The RF switching circuit employs a bias swapping circuit that switches bias voltage values between different levels in response to changes in operating mode, mode controller, signal power, supply voltage, and temperature, allowing for improved switch performance without additional charge pump circuitry by varying bias voltages based on these conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single MOSFET is used to implement a simple switch, then the device complexity is reduced, but the switch isolation performance deteriorates due to non-zero capacitance Cds-off from drain to source
Solution Approach 1:
The single MOSFET switch is segmented into a series combination of first and second MOSFETs. The first MOSFET handles the main switching function while the second MOSFET specifically addresses isolation requirements. This segmentation allows each transistor to be optimized for its specific function, improving overall switch isolation without significantly increasing device complexity.
Solution Approach 2:
A bootstrap circuit is introduced as an intermediary mechanism to dynamically control the body terminal voltage of the MOSFETs. This bootstrap circuit uses capacitive coupling to transfer the RF signal voltage to the body terminal, creating a time-varying body bias that reduces the effective drain-source capacitance during the off-state, thereby improving isolation performance.
2Loss of energy
If bootstrap resistors Rg and Rb are added to improve bandwidth and linearity, then the insertion loss is reduced, but the device complexity increases
Solution Approach 1:
The bootstrap resistors for gate (Rg) and body (Rb) are merged into a single shared resistor structure. The same resistor serves dual purposes: bootstrapping the gate terminal to reduce parasitic capacitance effects and bootstrapping the body terminal to improve isolation. This merging reduces the total number of components and simplifies the circuit while maintaining the insertion loss benefits.
Solution Approach 2:
The bootstrap resistor is designed to perform multiple functions simultaneously: it provides gate bootstrapping to reduce parasitic capacitance, provides body bootstrapping to improve isolation, and serves as a current path for the bootstrap capacitor charging. This multi-functionality reduces the need for separate dedicated components for each function.
3Reliability
If a charge pump is included to improve switch isolation by generating negative voltage, then the switch isolation is enhanced, but the device complexity and power consumption increase
Solution Approach 1:
The bootstrap circuit uses the RF signal itself to generate the body bias voltage through capacitive coupling. The RF signal voltage is transferred to the body terminal via the bootstrap capacitor, creating a time-varying body bias that improves isolation. This self-service approach eliminates the need for external charge pumps or negative voltage supplies, reducing device complexity and power consumption while maintaining improved isolation performance.
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 enhances the RF switch's performance by ensuring stronger switching states, reducing stress on the device, and extending its lifetime, while maintaining acceptable switch performance and integrating seamlessly with RF transceivers using CMOS technology.
Implementation Method 1
a first transistor having a first terminal, a second terminal, and a control terminal; wherein the transistor is arranged to switchably couple a RF signal input to a RF signal output
Implementation Method 2
a bias swapping circuit having a bias voltage output coupled to the first terminal and the second terminal; wherein the bias swapping circuit is operable to switch the bias voltage output between a first bias voltage value and a second bias voltage value
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
A RF switching arrangement (400) is described including a bias swap circuit (30). The bias swap circuit switches the bias voltage dependent on the state of the RF switch. This improves the performance of the RF switch without requiring charge pump circuitry.