High PSRR and Low Output Impedance Pre-Amplifier Circuit for Plugin-Powered Electret Condenser Microphone

This paper presents a noise suppression circuit for electret condenser microphones (ECMs). In ECMs, electrical noise from the power supply and transmission line tends to be superimposed on the ECM’s peripheral circuits. This overlap can make recordings sound harsh, depending on environmental conditions. Consequently, low-noise peripheral circuits for ECMs are essential to harness the inherent performance of the microphone. In this paper, transimpedance amplifier circuits based on a variable shunt regulator (referred to as SR-TIA) are proposed to suppress noise in the peripheral circuits of ECMs. The proposed SR-TIAs are designed to operate using plug-in power supplied by audio interfaces. SR-TIAs enhance the power supply rejection ratio of the ECM and dramatically reduce the output impedance, thereby suppressing noise from the power supply and transmission line. For example, the 4-transistor type SR-TIA achieved a power supply rejection ratio (PSRR) of 70.6 dB and an output impedance of 0.45 Ω. Four SR-TIAs were developed, and their characteristics and advantages compared to conventional circuits are clarified. The performance of the SR-TIAs was also verified through experiments in real usage scenarios. The results suggest that the SR-TIAs can enable practical and low-noise ECMs with a simple circuit configuration.