Amplifier Circuit Bias Control for Capacitive Transducer Settling
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Solution Overview
Problem
Prior art amplifier circuits for capacitive transducers face a trade-off between low noise and fast settling time, with large input impedance necessary for noise performance but resulting in long settling times and instability during high-level acoustical events, rendering miniature microphones non-operational for extended periods.
Innovation Solution
An amplifier circuit with a non-linear device and differential servo amplifier that uses a reference setting means to rapidly set and maintain the DC operating point of the preamplifier, allowing it to achieve high input resistance while minimizing settling time, utilizing a non-linear device such as cross-coupled diodes or diode-coupled transistors to control impedance and ensure noise performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a very large input impedance is used to optimise noise performance, then noise performance is improved, but settling time increases significantly
Solution Approach 1:
The patent applies dynamics by making the input impedance time-variant through a bias control circuit that adjusts the bias current. During power-on, a higher bias current is applied to reduce settling time by decreasing the time constant. After settling, the bias current is reduced to achieve the target high input impedance for optimal noise performance. This dynamic adjustment resolves the contradiction between fast settling and low noise.
Solution Approach 2:
The patent changes the parameter of input impedance from static to time-variant by controlling the bias current. The bias control circuit transitions the input impedance from a lower value during settling to a very high value (e.g., >100 GΩ) during normal operation. This parameter change allows the system to achieve both fast settling and low noise performance at different time periods.
2Measurement precision
If a very large input impedance is used to optimise noise performance, then noise performance is improved, but the amplifier becomes vulnerable to overload from high-level acoustical signals
Solution Approach 1:
The patent applies dynamics by making the input impedance adaptive through bias control. During normal operation, the high input impedance provides optimal noise performance. During overload events (e.g., door slams, mechanical shocks), the bias control circuit increases the bias current, which lowers the input impedance and allows the amplifier to withstand high-level signals without being driven into saturation or rail conditions, thus maintaining reliability.
Solution Approach 2:
The bias control circuit provides preliminary anti-action by detecting overload conditions and preemptively adjusting the bias current to prevent the amplifier from entering a non-operational state. This proactive adjustment protects the system against the harmful effects of high-level acoustical transients.
3Measurement precision
If the input impedance is increased to reduce noise, then noise performance is improved, but the time constant of the transducer-amplifier system increases to 10-30 seconds or minutes
Solution Approach 1:
The patent applies dynamics by making the input impedance time-variant through bias control. During power-on, a higher bias current is applied to reduce settling time by decreasing the time constant. After settling, the bias current is reduced to achieve the target high input impedance for optimal noise performance. This dynamic adjustment resolves the contradiction between fast settling and low noise.
Solution Approach 2:
The patent changes the parameter of input impedance from static to time-variant by controlling the bias current. The bias control circuit transitions the input impedance from a lower value during settling to a very high value (e.g., >100 GΩ) during normal operation. This parameter change allows the system to achieve both fast settling and low noise performance at different time periods.
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
The solution enables an input resistance greater than 100 GΩ with settling times of less than 2 seconds, ensuring optimal noise performance and rapid recovery from transient overloads, maintaining the microphone's operational state during high-level acoustical events.
Implementation Method 1
utilizing a non-linear device such as cross-coupled diodes or diode-coupled transistors to control impedance
Implementation Method 2
differential servo amplifier that uses a reference setting means to rapidly set and maintain the DC operating point of the preamplifier
Data Source
AI summary
An amplifier circuit for capacitive transducers, such as miniature electret or condenser microphones, wherein the amplifier circuit comprises bias control means adapted to improve settling of the amplifier circuit. Another aspect of the invention relates to a miniature condenser microphone and a monolithic integrated circuit comprising an amplifier circuit according to the present invention. The present invention provides amplifier circuits of improved performance by resolving traditionally conflicting requirements of maintaining a large input resistance of the amplifier circuit to optimize its noise performance and provide fast settling of the amplifier circuit.