Auxiliary FET Gate Shunt for RF Switching Speed
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Solution Overview
Problem
Existing RF phase and attenuator circuits using large Field Effect Transistors (FETs) face slow switching times due to high RC time constants, which affect performance, and reducing series resistance leads to higher losses and inadequate gate isolation, hindering speed improvements without degrading RF performance.
Innovation Solution
Incorporating smaller FETs directly coupled to the gate of larger FETs to reduce the effective RC time constant, with multiple smaller FETs in series to enhance switching speed while maintaining RF performance by effectively open-circuiting them when off, and utilizing depletion-mode devices with significantly smaller gate widths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If larger FETs are used to provide matching and low insertion loss, then RF performance is improved, but switching time increases due to higher capacitance
Solution Approach 1:
The patent divides the gate control function into two separate FETs: a large RF FET for maintaining RF performance and a small switching FET for fast switching. The small FET's gate is coupled to the RF FET's gate through a resistive element, allowing independent control of switching speed and RF performance characteristics.
2Speed
If series resistance is reduced to increase switching speed, then switching time decreases, but circuit losses increase negatively impacting RF performance
Solution Approach 1:
The patent separates the switching function from the RF signal path by using a small FET controlled through a resistor. The resistor limits current to the small FET's gate without being in the main RF signal path, avoiding the trade-off between switching speed and RF loss.
3Speed
If transistor size is reduced to decrease capacitance and increase switching speed, then switching time decreases, but phase shift performance deteriorates
Solution Approach 1:
The patent uses a large RF FET to maintain the required phase shift performance and a small auxiliary FET for fast switching control. The size segmentation allows each FET to be optimized for its specific function without compromise.
4Speed
If parallel diodes are added to provide low-impedance path for faster turn-off, then switching speed increases, but gate isolation deteriorates affecting RF signal performance
Solution Approach 1:
The patent uses a resistive element as an intermediary between the control signal and the RF FET gate. This resistor provides the necessary isolation while still allowing the small FET to control the switching action, maintaining both speed and isolation requirements.
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 speeds up the switching time of RF FETs without degrading RF performance, allowing for faster on-to-off transitions and improved phase shift capabilities while minimizing circuit losses.
Implementation Method 1
the effective RC time constant of the RF FET is changed, i.e., reduced
Implementation Method 2
each of the first and second FETs is a depletion-mode device
Data Source
AI summary
In order to increase the switching speed of an RF FET in an RF shunt circuit, a second, smaller, FET, with respect to the size of the RF FET, is connected directly to the gate of the RF FET to shunt the gate to ground quickly when switched from the off-state to the on-state. The smaller FET switches faster, due to being smaller than the larger RF FET, but it is effectively open-circuited in terms of RF performance when off because it is so small.