RF Switch Circuit Using Bypass FET for Faster Gate Transitions
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Solution Overview
Problem
RF switching circuitry faces challenges in achieving fast switching times while maintaining low insertion loss and high power handling capability, as existing solutions either compromise on switching speed or increase insertion loss by reducing resistor sizes or FET sizes.
Innovation Solution
The implementation of a multi-level drive signal using the built-in gate capacitance of a bypass FET, which provides an overvoltage above the positive power supply voltage to maintain the bypass FET in an on state during transitions, bypassing the common resistor and avoiding the need for extra circuitry like charge pumps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the size of the common resistor and gate resistors is reduced to increase switching speed, then the switching speed improves, but the insertion loss increases
Solution Approach 1:
The bypass FET is activated in advance during the transition period before the main FETs switch states. This preliminary action provides a low-impedance discharge path for the gate capacitance, accelerating the switching speed without requiring permanent reduction of resistor sizes, thereby maintaining low insertion loss during steady-state operation.
Solution Approach 2:
The circuit dynamically changes the effective resistance seen by the gate capacitance during transitions. The bypass FET temporarily shorts the common resistor during switching transitions, creating a time-varying resistance profile: low resistance during transitions (fast switching) and high resistance during steady state (low insertion loss).
2Speed
If the size of the FETs is reduced to decrease gate capacitance and increase switching speed, then the switching speed improves, but the power handling capability decreases
Solution Approach 1:
The bypass FET provides preliminary discharge of gate capacitance during transitions, enabling faster switching without reducing main FET size. The main FETs retain their full size and power handling capability while the bypass FET temporarily assists during transitions, decoupling switching speed from power handling requirements.
3Speed
If a charge pump is added to generate overvoltage for multi-level drive signal, then the switching speed improves, but the device complexity increases
Solution Approach 1:
The bypass FET's own gate capacitance is utilized to generate the multi-level drive signal. When the bypass FET switches on, its gate capacitance charges to the overvoltage level, which then automatically drives the gate of the main FETs. This self-service mechanism eliminates the need for external charge pumps or complex voltage generation circuitry.
Solution Approach 2:
The bypass FET acts as an intermediary element that mediates between the control signal and the main FETs. It temporarily stores energy in its gate capacitance and uses this stored energy to provide the multi-level drive signal, simplifying the overall circuit architecture while achieving fast switching.
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 significantly enhances switching speed without increasing insertion loss or reducing power handling capability, leveraging the built-in capacitance of the bypass FET to generate the multi-level drive signal using only the positive and negative power supply voltages.
Implementation Method 1
leveraging the built-in capacitance of the bypass FET to generate the multi-level drive signal
Data Source
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AI summary
RF switching circuitry includes a plurality of FETs coupled between an input node, an output node, and a gate drive node. When a positive power supply voltage is provided at the gate drive node, the plurality of FETs turn on and provide a low impedance path between the input node and the output node. When a negative power supply voltage is provided at the gate drive node, the plurality of FETs turn off and provide a high impedance path between the input node and the output node. Switch acceleration circuitry in the RF switching circuitry includes a bypass FET and multi-level driver circuitry. The bypass FET selectively bypasses the common resistor in response to a multi-level drive signal. The multi-level driver circuitry uses a built-in gate to capacitance of the bypass FET to provide the multi-level drive signal at an overvoltage that is above the positive power supply voltage.