RF FET Switch Stack Body Bypass for Low Loss and High Roff
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Solution Overview
Problem
Series RF switches with stacked FETs face challenges due to large die surface area, leading to increased resistive and capacitive paths to the substrate, resulting in high insertion loss in the ON-state and inadequate Roff in the OFF-state, along with limitations in body and gate bias resistor 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 gate and body resistor networks, and control circuits to manage biasing 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 resistor bypass circuit dynamically switches between high and low impedance states based on the RF switch state. During ON-state, the bypass circuit provides a low impedance path to shunt body current, reducing insertion loss. During OFF-state, the high impedance path maintains adequate isolation. This dynamic adaptation resolves the contradiction between maintaining high compression point and reducing insertion loss.
Solution Approach 2:
The invention changes the effective body resistance parameter by introducing a bypass path. The body resistor value is effectively reduced during ON-state through the bypass circuit, while maintaining higher values during OFF-state. This parameter modulation allows the system to achieve both high compression point and low insertion loss at different operational phases.
2Power
If a stack of FETs is used to achieve high compression point, then the compression point is improved, but Roff decreases in the OFF-state
Solution Approach 1:
The bypass circuit dynamically adjusts its impedance based on the RF switch state. During OFF-state, the bypass path is disconnected (high impedance), allowing the body resistor to maintain adequate isolation and Roff. During ON-state, the bypass activates (low impedance) to handle body current. This temporal separation allows both high compression point and adequate Roff to be achieved at different times.
Solution Approach 2:
The body resistor bypass circuit operates periodically, activating during ON-state and deactivating during OFF-state. This periodic action ensures that body current is shunted only when needed (during ON-state), while maintaining proper isolation characteristics during OFF-state, thus resolving the contradiction between compression point and Roff.
3Power
If the die surface area is increased to accommodate the FET stack, then high compression point is achieved, but resistive and capacitive paths to substrate increase
Solution Approach 1:
The invention extracts the body current path from the main RF signal path by introducing a separate bypass circuit. This dedicated bypass path for body current removes the harmful interaction between body current and RF signal, allowing large die area to be used for high compression point without proportionally increasing harmful resistive and capacitive effects on the RF path.
Solution Approach 2:
The bypass circuit acts as an intermediary element that handles body current separately from the RF signal path. By providing this intermediate path, the harmful resistive and capacitive effects are isolated from the main RF signal, allowing the FET stack to achieve high compression point without proportionally increasing insertion loss.
4Power
If resistor values in the body bias network are reduced to manage body current, then body current is controlled, but RC time constant increases affecting switching time
Solution Approach 1:
The invention segments the body bias network into two functional parts: the body resistor network for current management and the bypass circuit for speed enhancement. The body resistors can be sized for proper current management while the bypass circuit provides a low impedance path that reduces the effective RC time constant during switching transitions, thus maintaining both body current control and fast switching.
Solution Approach 2:
The bypass circuit dynamically changes the effective resistance in the body bias network. During switching transitions, the bypass provides low impedance to reduce RC time constant and accelerate switching. During steady ON-state, the bypass is inactive and the body resistors maintain proper current management. This dynamic behavior resolves the contradiction between current management and switching speed.
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.