Zhineng Zhu

Date of Award

5-2007

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

Advisor

David E. Kotecki

Second Committee Member

Donald M. Hummels

Third Committee Member

Rosemary Smith

Abstract

A nanopore-based gene sequencer generates a modulated current signal with the expected magnitude to be in the range of several pico-amps to a nano-amp, and the bandwidth up to 10MHz. To detect such a weak signal, an ultra-low noise, low offset, high precision CMOS operational amplifier is designed in a 0.35/itm CMOS process. Most of the literature on low noise amplifier design emphasizes flicker noise reduction. This research analyzed and optimized a proposed differential pair and operational amplifier, with both the flicker noise and the thermal noise minimized simultaneously. The size of each MOSFET is optimized, making the input pair the only dominant noise source. The input pair's bias current is maximized to reduce the thermal noise. Also this bias current is rationed between the active current source load and the current mirror load. This ration of bias current makes the optimization of the amplifier's noise performance and DC gain separated, which is often a trade-off in normal operational amplifier design. An operational amplifier with the input-referred voltage noise Power Spectral Density (PSD) of less than 2nVj\[Wz for frequencies above 1MHz is realized. The DC gain of this amplifier is up to 120dB. The extra bias current and the cascode structure also result in a high speed design: the unity gain bandwidth of this operational amplifier is more than 200MHz with 20pF loading, providing a sufficient close loop gain in the MHz range. CMOS operational amplifiers generally have a higher offset voltage than bipolar ones. In this design, a digital offset trimming method is studied and used to cover lOmV input-referred offset. Its effect on noise performance is minimized. This method is suitable for low voltage and continuous mode applications. Power Supply Rejection Ratio (PSRR) is an important parameter for the low noise operational amplifier. The PSRR limitation of the taped-output operational amplifier with common source output stage is analyzed. An operational amplifier with cascode compensation scheme is studied, which shows the potential improvement in PSRR.

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