MOSFET Switch Constant Vgs Circuit for Low Audio Distortion
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Solution Overview
Problem
MOSFET switches experience varying on resistance (Ron) with changes in input signal voltage levels, affecting audio fidelity by introducing harmonic distortion, as existing solutions like maintaining constant gate-to-source voltage (Vgs) are limited by power supply and logic signal variations.
Innovation Solution
A constant Vgs is maintained across the MOSFET switch by generating a reference voltage that is independent of input signal and power supply levels, ensuring Ron remains constant through the use of precision resistors and an op-amp circuit to sum currents proportionally related to the input signal and reference voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If logic signal voltage is used to determine Vgs level, then the switch can be turned on/off, but Vgs changes with power supply voltage and logic signal loads, causing Ron to vary
Solution Approach 1:
The patent introduces an intermediary circuit between the logic signal and the MOSFET gate that converts varying logic signal voltages into a constant Vgs. This intermediary includes resistors that scale the logic signal voltage and a reference voltage source that provides a stable bias, ensuring Vgs remains constant regardless of logic signal variations or power supply changes.
2Reliability
If Vgs is kept constant to maintain constant Ron, then audio fidelity is improved, but the useful dynamic range is limited
Solution Approach 1:
The patent changes the parameter relationship by making Vgs independent of the input signal voltage. Instead of Vgs varying with input signal level, the circuit uses scaled logic signal voltage plus a reference voltage to maintain constant Vgs across the entire dynamic range, thus preserving audio fidelity without limiting adaptability.
3Device complexity
If Ron is allowed to change with input signal voltage, then the circuit is simpler, but harmonic distortion increases
Solution Approach 1:
The patent introduces an intermediary voltage generation circuit that combines scaled logic signal voltage and reference voltage to produce a constant Vgs. This intermediary circuit, while adding some complexity, eliminates harmonic distortion by ensuring Ron remains constant throughout the audio signal range.
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 reduces Ron variability (Rflatness) by maintaining a constant Vgs, thereby preserving audio fidelity with minimal harmonic distortion, as demonstrated by a 98% improvement in Rflatness compared to prior art.
Implementation Method 1
An input signal voltage, Vin, generates a current proportional to the input signal voltage by impressing Vin across a precision resistor. A reference voltage, Vref, generates a current proportional to the reference voltage by impressing Vref across a second precise resistor equal to the first.
Data Source
AI summary
A MOSFET switch is disclosed that is driven on by a circuit that provides a constant gate to source voltage, Vgs, that is independent of the input voltage, the power supply and any logic signals. The constant Vgs is derived from a reference voltage and biases the MOSFET switch such that Ron is constant, or Rflatness is minimized. A minimized Rflatness provides a higher fidelity transfer of audio signals compared to prior art switches where Rflatness is greater.