Self-Noise Reduction for Capacitive Sensors via Photoelectric DC Servo: Application to Condenser Microphones
The self-noise of capacitive sensors, primarily caused by thermal noise from the gate-bias resistor in the preamplifier, imposes a fundamental limit on measurement sensitivity. In electret condenser microphones (ECMs), this resistor simultaneously determines the noise low-pass cutoff frequency and the signal high-pass cutoff frequency through a single RC time constant, creating a trade-off between noise reduction and signal bandwidth. This paper proposes PDS-Amp (Photoelectric DC Servo Amplifier), a circuit technique that replaces the gate-bias resistor with a photoelectric element functioning as an ultra-high-impedance current source. A DC servo loop using lag-lead compensation feeds back the preamplifier output through an LED to control the photocurrent, thereby stabilizing the gate bias while decoupling the noise and signal cutoff frequencies. A custom photosensor based on the external photoelectric effect of a zinc photocathode was fabricated to achieve sub-picoampere dark current, overcoming the limitations of commercial semiconductor photodiodes. Combined with a cascode JFET preamplifier that minimizes input capacitance through bootstrap action, PDS-Amp achieved a self-noise of 11 dBA with a 12 pF dummy microphone. Despite using a small-diameter ECM capsule, this performance is comparable to that of large-diaphragm condenser microphones costing several thousand dollars. Recording experiments with an actual ECM capsule qualitatively confirmed a significant reduction in background noise. The proposed technique is applicable not only to microphones but broadly to capacitive sensors including accelerometers, pressure sensors, and pyroelectric sensors.