RF FET Switch Stack Body Bias Bypass for Insertion Loss Control
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Solution Overview
Problem
Series RF switches with a stack of FETs face challenges with insertion loss in the ON-state and impedance to ground in both ON and OFF states due to large die surface area, and limitations in resistor values for body and gate bias networks under high power conditions.
Innovation Solution
The implementation of a body resistor bypass in the bias feed network for RF switch stacks, allowing resistance to be introduced or bypassed during specific steady states and transitions, using a stacked arrangement of FET switches with state-dependent and transition-dependent resistor networks to manage body and gate bias voltages effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a stack of FETs is used to achieve high compression point, then the compression point is improved, but insertion loss increases in the ON-state
Solution Approach 1:
The body bias network is segmented into multiple body resistors (Rb1, Rb2, Rb3) connected in series, each controlling a portion of the FET stack. This segmentation allows selective bypassing of individual resistors during transitions to reduce overall body resistance, thereby reducing insertion loss while maintaining the stacked FET architecture for high compression point.
Solution Approach 2:
The body resistors are made dynamic through the inclusion of bypass switches (Mb1, Mb2, Mb3) that can selectively short each resistor during transition periods. This dynamic configuration allows the body resistance to be high during steady states (maintaining compression point) and low during transitions (reducing insertion loss), resolving the contradiction between these two parameters.
2Power
If the die surface area is increased to accommodate the FET stack, then the compression point is improved, but impedance to ground increases in both ON and OFF states
Solution Approach 1:
The body bias network is divided into multiple segmented resistors (Rb1, Rb2, Rb3) with associated bypass switches. This segmentation allows selective activation of bypass paths during transitions, dynamically controlling the impedance to ground without requiring a smaller die area, thus maintaining high compression point while improving impedance management.
Solution Approach 2:
The body resistance parameter is dynamically changed during operation. During steady states, the full resistance value is maintained for high compression point. During transitions, the resistance is reduced by engaging bypass switches, which changes the impedance parameter to ground temporarily, resolving the contradiction between compression point and impedance control.
3Speed
If resistor values in the body bias network are reduced to meet switching time specifications, then switching speed is improved, but voltage drop across body resistors increases under high power conditions
Solution Approach 1:
The body resistors are made dynamically bypassable during transitions. During steady states, the full resistor values are maintained to minimize voltage drop under high power conditions. During transitions, the bypass switches are activated to short the resistors, providing a low-impedance path that enables fast switching without excessive voltage drop, thus resolving the contradiction between switching speed and voltage drop.
Solution Approach 2:
The bypass switches are activated in advance during transition periods to prepare the body bias network for rapid switching. This preliminary action of engaging the bypass path before the actual switching event ensures that the full voltage drop does not occur during critical transition moments, while maintaining proper bias during steady states.
Data Source
AI summary
A FET switch stack and a method to operate a FET switch stack. The FET switch stack includes a stacked arrangement of body bypass FET switches connected across respective common body resistors. The body bypass FET switches bypass the respective common body resistors during the OFF steady state of the FET switch stack and do not bypass the respective common body resistors during the ON steady state.