Constant-Vgs MOSFET Switch Circuit for Flat Ron in Audio Paths
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Solution Overview
Problem
MOSFET switches used in audio applications face challenges in maintaining constant on-resistance (Ron) across varying input signal voltage levels, leading to distortion and potential gate oxide degradation due to changes in gate-to-source voltage (Vgs) caused by power supply fluctuations.
Innovation Solution
A constant gate-to-source voltage (Vgs) is maintained using a summing circuit with an amplifier and resistors to stabilize the voltage between the gate and source of a MOSFET passgate, independent of input signal polarity and voltage variations, thereby reducing distortion and extending the voltage range before gate oxide degradation occurs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the logic signal voltage value is used to determine the Vgs voltage level, then the switch can be turned on/off, but the Vgs voltage level changes as the power supply voltage changes, causing Ron to change and limiting the useful dynamic range
Solution Approach 1:
The patent employs a feedback mechanism where the actual Vgs voltage is monitored and used to adjust the gate control signal. This ensures that despite power supply fluctuations, the Vgs remains stable at the desired level, preventing Ron variations and maintaining consistent switch performance across the dynamic range.
Solution Approach 2:
The patent transforms the control approach by changing from direct logic signal control to indirect control through Vgs stabilization. By introducing an additional control loop that adjusts the gate voltage based on the actual Vgs measurement, the system maintains stable operating parameters despite external voltage variations.
2Device complexity
If Vgs varies with power supply voltage, then the switch operates with simple logic control, but Ron changes which limits the useful dynamic range and affects audio fidelity
Solution Approach 1:
A feedback loop continuously monitors Vgs and adjusts the gate control signal accordingly. This feedback mechanism compensates for power supply voltage changes, maintaining stable Vgs and thus stable Ron, which ensures high audio signal fidelity without requiring overly complex external control circuits.
Solution Approach 2:
The control circuit automatically adjusts itself to maintain stable Vgs without requiring external intervention or complex external circuitry. The system self-regulates by using its own output characteristics to control its input, creating a self-stabilizing mechanism that maintains audio fidelity.
3Adaptability or versatility
If large negative signal swings are present, then the switch can handle high dynamic range signals, but the MOSFET's gate oxide degrades due to excessive Vgs voltage
Solution Approach 1:
The patent changes the control parameter from direct logic voltage to stabilized Vgs voltage. By maintaining Vgs within safe limits regardless of the signal voltage swing magnitude, the system can handle large dynamic range signals without exposing the gate oxide to damaging voltage stress.
Solution Approach 2:
The stabilized Vgs control circuit acts as an intermediary between the logic control signal and the MOSFET gate. This intermediary layer ensures that even when large signal swings are present, the actual Vgs applied to the gate oxide remains within safe operating limits, protecting the device while allowing high dynamic range operation.
Data Source
AI summary
Apparatus and methods for a switch circuit to provide a substantially constant gate-to source voltage to a passgate are provided. In an example, a switch circuit includes a summing circuit having an output configured to couple to the gate of a passgate, the summing circuit can be configured to maintain a substantially constant voltage between the gate and the source of the pass gate.