A 4.7μW Switched-Bias MEMS Microphone Preamplifier for Ultra-Low-Power Voice Interfaces

Abstract This paper presents a switched-bias MEMS microphone preamplifier for an ultra-low-power voice interface. A switched-MOSFET periodically changes the MOSFET between strong inversion and accumulation to inherently reduce 1/f noise. In addition, a proposed coupling capacitor allows the microphone to benefit from a high bias voltage while the preamp can use a low VDD. The preamp achieves 7 -weighted) with 3.4 oving NEF by 4.5×. Acoustic testing with the preamp and MEMS microphone shows 61.3dBA SNR at 94dB SPL. Introduction Voice interfaces are expected to play a dominant role in wireless sensor nodes and IoT, where conventional interfaces such as touch are limited because of small size. Recent microsystems as small as 6mm3 [1] have been reported with battery capacity of <10uAh. Enabling audio interfaces in a power-constrained system requires low noise and moderate dynamic range interface circuits to achieve high far-field audio quality and clarity. One key challenge lies in the low-power microphone preamplifier implementation; this component boosts audio signal to line level and is the most noise/power sensitive block in the entire signal chain. The human audible frequency range is 20-20kHz and microphone preamp noise is therefore severely impacted by 1/f noise. Chopper amplifiers, which are often used to remove 1/f noise, cannot be used with high impedance input sources such as MEMS microphones (with base capacitance of several pF) since the associated switching current causes high voltage output noise [2], [3]. Therefore, large input transistors must be used, increasing gate capacitance and noise gain, and thus degrading current noise efficiency (NEF=14.9) [4]. Another approach provides excitation signals for capacitive sensors to remove 1/f noise [5]. However, the DC capacitance canceling path to remove the carrier signal doubles noise gain and NEF. Furthermore, the excitation uses the relatively low preamp supply voltage, limiting microphone sensitivity ( 35dBV/Pa) and power (2.5mW). To overcome these limitations, we propose a low-power preamp with switched biasing and a modified coupling capacitor feedback. Together, these techniques enable the preamp to achieve 7.3uVrms noise with 3.4uA, improving over previous work in the audio amplifier space by 14.7× in power and 4.5× in noise efficiency factor. Proposed MEMS Microphone Preamplifier Periodic on/off switching of a MOSFET between strong inversion and accumulation has been shown to reduce 1/f noise [6], [7]. 1/f noise is caused by the trapping/de-trapping process of carriers in the gate oxide. The trapping-detrapping process occurs with a wide range of time constants, including very long timeframes. By turning the device on and off, the long-term memory effect of 1/f noise is essentially reset and low-frequency 1/f noise is inherently reduced. Fig. 1 shows the implementation of the proposed switched-bias preamp. The left or right path is alternately used according to the phase