Keng Hoong Wee

An Electronically Tunable Linear or Nonlinear MOS Resistor

We present a bidirectional MOS resistor circuit that is electronically tunable and has zero dc offset. For a given I-V characteristic, the circuit senses the source-to-drain potential across an MOS device and automatically generates an appropriate bias for the gate terminal to implement the characteristic via negative feedback. We show that the I-V characteristic of the resistor can be designed to be linear, compressive or expansive by using appropriate translinear current mode circuits for the feedback biasing. Our technique does not require the MOS transistor to operate in the triode region and is valid in both weak and strong inversion. Experimental results from a CMOS process show that a square-root, linear, or square resistor can be implemented as examples of our topology. The linear version was tunable over a resistance range of 1 MOmega-100 GOmega in our particular implementation and exhibited proportional-to-absolute temperature (PTAT) behavior. The measured excess noise of the resistor agrees with theoretical predictions.

An Analog Integrated-Circuit Vocal Tract

We present the first experimental integrated-circuit vocal tract by mapping fluid volume velocity to current, fluid pressure to voltage, and linear and nonlinear mechanical impedances to linear and nonlinear electrical impedances. The 275 muW analog vocal tract chip includes a 16-stage cascade of two-port pi-elements that forms a tunable transmission line, electronically variable impedances, and a current source as the glottal source. A nonlinear resistor models laminar and turbulent flow in the vocal tract. The measured SNR at the output of the analog vocal tract is 64, 66, and 63 dB for the first three formant resonances of a vocal tract with uniform cross-sectional area. The analog vocal tract can be used with auditory processors in a feedback speech locked loop-analogous to a phase locked loop-to implement speech recognition that is potentially robust in noise. Our use of a physiological model of the human vocal tract enables the analog vocal tract chip to synthesize speech signals of interest, using articulatory parameters that are intrinsically compact and linearly interpolatable.

An Articulatory Silicon Vocal Tract for Speech and Hearing Prostheses

We describe the concept of a bioinspired feedback loop that combines a cochlear processor with an integrated-circuit vocal tract to create what we call a speech-locked loop. We discuss how the speech-locked loop can be applied in hearing prostheses, such as cochlear implants, to help improve speech recognition in noise. We also investigate speech-coding strategies for brain-machine-interface-based speech prostheses and present an articulatory speech-synthesis system by using an integrated-circuit vocal tract that models the human vocal tract. Our articulatory silicon vocal tract makes the transmission of low bit-rate speech-coding parameters feasible over a bandwidth-constrained body sensor network. To the best of our knowledge, this is the first articulatory speech-prosthesis system reported to date. We also present a speech-prosthesis simulator as a means to generate realistic articulatory parameter sequences.

Biasing techniques for subthreshold MOS resistive grids

A classic resistive network implemented using MOS transistors suffers from nonlinearity in the subthreshold exponential parameter /spl kappa/ that arises due to varying V/sub GB/ and V/sub BS/. We show two biasing techniques that alleviate these effects. The first technique always uses transistors with constant gate-to-bulk voltage. The second technique uses a novel bulk-to-source biasing scheme to ensure zero bulk-to-source voltage. We propose a PMOS spatial filtering circuit that employs this scheme to extend the range of linearity of subthreshold resistive networks. Measured experimental results from a 1.5um CMOS process show that our spatial filtering circuit has less than 5% variation in space-constant over a measured 94dB (100fA-5nA) dynamic range as opposed to a conventional spatial filtering circuit, which for the same variation has a measured dynamic range of less than 80dB (100fA-1nA). Our techniques should be useful in translinear MOS circuits where linear operation over a wide dynamic range of input currents is important.

An analog bionic vocal tract

We present the first experimental integrated-circuit vocal tract. The 275 muW analog vocal tract chip can be used for real-time speech production in bionic speech-prosthesis systems where low power is critical. We also describe how our vocal tract can be used with auditory processors in a feedback speech locked loop to implement speech recognition that is potentially robust in noise.