Abstract
A design of an on-chip ac-coupling preamplifier for mobile applications is presented. A microphone preamplifier should have a large input impedance to mitigate the effective-gain reduction caused by the microphone’s non-zero output impedance. From a review of previously reported microphone preamplifier structures in terms of input impedance, feasibility of on-chip ac-coupling, and noise performance, we chose an inverting amplifier structure with capacitive feedback. In addition, to provide dc bias path, we used off-state MOSFET switches as pseudo-resistors of very large resistance in the giga-ohm range. The large resistance enables on-chip ac-coupling with sufficient noise performance. A fast start-up is achieved by turning-on the switch for a short period during the preamplifier start-up. The gain of the preamplifier can be programmed from 0 dB to 21 dB with 3 dB steps. A 2-stage pseudo-class-AB amplifier was adopted to reduce power consumption. The proposed preamplifier was implemented using a 28 nm CMOS process and achieves 107 dB dynamic range in a 20 kHz bandwidth under 0 dB gain setting and balanced differential input signal. The preamplifier dissipates a power of 270 μW with a 1.8 V supply.
Highlights
Recent advances of mobile audio devices such as true wireless stereos and cellular phones require microphone read-out integrated circuits (M-ROICs) with wide dynamic range (DR)
The silicon area occupied by the M-ROIC and the area occupied on the printed circuit board (PCB) by the silicon chip and auxiliary passive devices such as ac-coupling capacitors becomes more critical for the mobile applications, because the number of the required audio recording channels is increasing to enhance the audio quality by techniques such as echo cancellation and environmental noise cancellation [1,2]
A preamplifier is usually used in an M-ROIC to handle such obstacle [3,4,5,6,7,8]
Summary
Recent advances of mobile audio devices such as true wireless stereos and cellular phones require microphone read-out integrated circuits (M-ROICs) with wide dynamic range (DR). The silicon area occupied by the M-ROIC and the area occupied on the printed circuit board (PCB) by the silicon chip and auxiliary passive devices such as ac-coupling capacitors becomes more critical for the mobile applications, because the number of the required audio recording channels is increasing to enhance the audio quality by techniques such as echo cancellation and environmental noise cancellation [1,2]. Microphones have non-zero output resistance, which is usually in the kilo-ohms range This dictates that the preamplifier should have a large input impedance to avoid the reduction of the effective gain. This paper presents a high performance programmable-gain microphone preamplifier with on-chip ac-coupling capacitors for driving an audio ADC. The rest of this paper is organized as follows: In Section 2, the optimum preamplifier architecture for the mobile applications is selected after reviewing previously reported architectures, and the method to implement very large resistance needed for on-chip accoupling is proposed.
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