MOSFET Switch Circuit With Constant Vgs for Low-THD Signals
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Solution Overview
Problem
MOSFET switches experience varying on-resistance (Ron) with input signal voltage changes, leading to increased total harmonic distortion (THD) in audio signals, as existing solutions like the '874 patent rely on logic signal voltage to maintain constant gate-to-source voltage (Vgs), which can be affected by power supply changes.
Innovation Solution
A signal switch circuit using a field effect transistor (FET) with a charge pump circuit to maintain a constant gate-to-source voltage (Vgs) by coupling a reference voltage to the well of the FET, ensuring minimal distortion and reduced power consumption, even with varying signal voltage levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If logic signal voltage is used to control Vgs level, then the switch can be turned on/off, but Vgs changes with power supply voltage changes, limiting dynamic range
Solution Approach 1:
A dedicated voltage reference circuit is introduced as an intermediary between the power supply and the FET gate control. This reference circuit generates a stable reference voltage that is used to set the Vgs level independently of power supply variations, thereby maintaining constant on-resistance across the full dynamic range while preserving automatic switch control
Solution Approach 2:
The control method transitions from using logic signal voltage directly to control Vgs to using a dedicated reference voltage that maintains a constant voltage difference relationship with the FET well. This parameter change decouples Vgs from power supply voltage changes, enabling both automatic control and full dynamic range operation
2Ease of operation
If Vgs varies with input signal voltage, then the switch can operate, but Ron changes, increasing THD
Solution Approach 1:
The circuit establishes a feedback relationship where the FET well voltage tracks the input signal common-mode voltage, and the reference voltage maintains a constant differential relationship with the well. This feedback mechanism ensures Vgs remains constant despite input signal variations, enabling easy operation while minimizing THD
Solution Approach 2:
The reference voltage is configured to maintain a constant voltage difference with the FET well, creating an equipotential relationship that compensates for signal voltage changes. This ensures the gate-to-source voltage remains constant throughout the signal dynamic range, reducing distortion while maintaining operational ease
Data Source
AI summary
This document discusses, among other things, a signal switch circuit including a first field effect transistor (FET) configured to couple a first node to a second node in an on-state and a charge pump circuit configured to provide a first supply voltage to control the FET, wherein a reference voltage of the charge pump circuit is coupled to a well of the FET to maintain a constant gate to source voltage of the FET during the on-state.