RF Isolation Switch Circuit With Gate-Source Shorting for Linearity
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Solution Overview
Problem
Conventional RF switch circuits introduce unwanted parasitic capacitances and degrade circuit linearity, especially when handling high RF signal voltage swings, which affects the performance of RF transmitters and transceivers.
Innovation Solution
The RF isolation switch circuit employs a gate-to-source shorting circuit and resistive coupling to maintain the gate voltage of the main transistor close to zero volts during high RF signal conditions, and selectively couples the bulk electrode to the source or ground to prevent leakage, using a 65 nanometer CMOS semiconductor fabrication process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional RF switch circuit is used, then the RF signal can be switched between coupled and isolated states, but unwanted parasitic capacitances are introduced and circuit linearity is degraded
Solution Approach 1:
The conventional single-transistor RF switch is segmented into multiple functional components: a main transistor for signal switching, a gate-to-source shorting circuit with controlled transistors for parasitic capacitance management, and bulk control circuits for leakage prevention. This segmentation allows each component to address specific issues independently, reducing overall parasitic capacitance while maintaining switching functionality.
Solution Approach 2:
The gate-to-source shorting circuit acts as an intermediary mechanism that dynamically manages the parasitic capacitance. When the main transistor is turned off, the shorting circuit connects the gate to the source, effectively neutralizing the parasitic capacitance effect. This intermediary action prevents the parasitic capacitance from degrading circuit linearity while preserving the switching function.
2Power
If the transistor size is increased to reduce on-resistance, then the source-to-drain on resistance decreases, but parasitic capacitance increases
Solution Approach 1:
The invention employs dynamic control of the transistor operating state through the gate-to-source shorting circuit. The main transistor can be rapidly switched between on and off states, and the shorting circuit dynamically adjusts the gate-to-source voltage to maintain optimal performance. This dynamic operation allows the transistor to achieve low on-resistance when needed while minimizing parasitic capacitance effects during the off state.
Solution Approach 2:
The invention changes the electrical parameters of the transistor by introducing controlled voltage levels through the gate-to-source shorting circuit. By adjusting the gate-to-source voltage dynamically, the transistor can operate in different regions (linear or saturation) to optimize the balance between on-resistance and parasitic capacitance. The bulk control circuits also adjust the bulk-to-source voltage to further optimize transistor performance parameters.
3Reliability
If a thick gate dielectric transistor is used to handle high RF signal voltage swing, then the transistor can withstand high voltages, but the transistor size must be increased which increases parasitic capacitance
Solution Approach 1:
The voltage handling function is segmented from the signal switching function. The main transistor handles the high voltage swing, while the gate-to-source shorting circuit and bulk control circuits separately manage the parasitic capacitance and leakage issues. This segmentation allows the main transistor to be optimized for voltage handling without being constrained by parasitic capacitance considerations.
Solution Approach 2:
The gate-to-source shorting circuit serves as an intermediary that protects the main transistor from the harmful effects of its own parasitic capacitance. When the main transistor is off, the shorting circuit actively manages the gate-to-source voltage to prevent the parasitic capacitance from causing leakage or distortion, allowing the transistor to handle high voltage swings effectively.
4Productivity
If the gate-to-source voltage is allowed to vary with RF signal amplitude, then the transistor can operate efficiently, but the gate-to-drain junction may become forward biased causing leakage
Solution Approach 1:
The bulk control circuit implements a feedback mechanism that monitors the gate-to-drain voltage condition and adjusts the bulk-to-source voltage accordingly. When the gate-to-drain junction approaches forward bias conditions, the feedback circuit adjusts the bulk voltage to maintain reverse bias, preventing leakage current while allowing efficient switching operation.
Solution Approach 2:
The gate-to-source shorting circuit applies preliminary anti-action by proactively managing the gate-to-source voltage before the gate-to-drain junction can become forward biased. The shorting circuit prepares the transistor state in advance to prevent leakage conditions, ensuring that efficient switching can occur without the harmful effects of forward-biased junctions.
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 solution maintains consistent input impedance and improves linearity by preventing parasitic capacitance and leakage, enhancing the overall performance of RF transmitters and transceivers, particularly in high-frequency applications.
Implementation Method 1
The gate-to-source shorting circuit shorts the gate of the main transistor to the source of the main transistor so the gate-to-source voltage (Vgs) on the main transistor remains close to zero volts
Implementation Method 2
The digital logic high voltage (for example, 1.3 volts) present on the first control input conductor is resistively coupled onto the gate of the main transistor, thereby turning the main transistor on
Data Source
AI summary
In a first aspect, an RF switch includes a main transistor and a gate-to-source shorting circuit. When the RF switch is turned off, the gate-to-source shorting circuit is turned on to short the source and gate of the main transistor together, thereby preventing a Vgs from developing that would cause the main transistor to leak. When the RF switch is turned on, the gate-to-source shorting circuit is turned off to decouple the source from the gate. The gate is supplied with a digital logic high voltage to turn on the main transistor. In a second aspect, an RF switch includes a main transistor that has a bulk terminal. When the RF switch is turned off, the bulk is connected to ground through a high resistance. When the RF switch is turned on, the source and bulk are shorted together thereby reducing the threshold voltage of the main transistor.