Rahul Sarpeshkar

White noise in MOS transistors and resistors

The theoretical and experimental results for white noise in the low-power subthreshold region of operation of an MOS transistor are discussed. It is shown that the measurements are consistent with the theoretical predictions. Measurements of noise in photoreceptors-circuits containing a photodiode and an MOS transistor-that are consistent with theory are reported. The photoreceptor noise measurements illustrate the intimate connection of the equipartition theorem of statistical mechanics with noise calculations.<<ETX>>

A Low-Power Wide-Linear-Range Transconductance Amplifier

The linear range of approximately ±75mV of traditional subthreshold transconductance amplifiers istoo small for certain applications—for example, for filtersin electronic cochleas, where it is desirable to handle loudsounds without distortion and to have a large dynamic range.We describe a transconductance amplifier designed for low-power(< 1 µW) subthreshold operation with a wideinput linear range. We obtain wide linear range by widening thetanh, or decreasing the ratio of transconductance to bias current,by a combination of four techniques. First, the well terminalsof the input differential-pair transistors are used as the amplifierinputs. Then, feedback techniques known as source degeneration(a common technique) and gate degeneration (a new technique)provide further improvements. Finally, a novel bump-linearizationtechnique extends the linear range even further. We present signal-flowdiagrams for speedy analysis of such circuit techniques. Ourtransconductance reduction is achieved in a compact 13-transistorcircuit without degrading other characteristics such as dc-inputoperating range. In a standard 2 µm process,we were able to obtain a linear range of ±1.7V.Using our wide-linear-range amplifier and a capacitor, we constructa follower–integrator with an experimental dynamic rangeof 65 dB. We show that, if the amplifiers noise is predominantlythermal, then an increase in its linear range increases thefollower–integratorsdynamic range. If the amplifiers noise is predominantly 1/f,then an increase in its linear range has no effect on thefollower–integratorsdynamic range. To preserve follower–integrator bandwidth,power consumption increases proportionately with an increasein the amplifiers linear range. We also present data for changesin the subthreshold exponential parameter with current leveland with gate-to-bulk voltage that should be of interest to alllow-power designers. We have described the use of our amplifierin a silicon cochlea [1, 2].

Analog VLSI architectures for motion processing: from fundamental limits to system applications

This paper discusses some of the fundamental issues in the design of highly parallel, dense, low-power motion sensors in analog VLSI. Since photoreceptor circuits are an integral part of all visual motion sensors, we discuss how the sizing of photosensitive areas can affect the performance of such systems. We review the classic gradient and correlation algorithms and give a survey of analog motion-sensing architectures inspired by them. We calculate how the measurable speed range scales with signal-to-noise ratio (SNR) for a classic Reichardt sensor with a fixed time constant. We show how this speed range may be improved using a nonlinear filter with an adaptive time constant, constructed out of a diode and a capacitor, and present data from a velocity sensor based on such a filter. Finally, we describe how arrays of such velocity sensors call be employed to compute the heading direction of a moving subject and to estimate the time-to-contact between the sensor and a moving object.

An analog VLSI velocity sensor

An integrated circuit that computes the velocity vector of a visual stimulus in one dimension is presented. The circuit combines optical sensors and associated electronics on a single silicon chip, processed with standard CMOS technology. The velocity is inferred from the time delay of the appearance of an image feature at two fixed locations on the chip. The circuit operates quite robustly for high-contrast stimuli over considerable irradiance and velocity ranges. With lower-contrast stimuli the output signal for a given velocity tends to decrease, while the direction selectivity is still maintained. The individual motion-sensing cells are compact, and they are therefore suited for use in dense 1D or 2D imaging arrays.

An analog VLSI cochlea with new transconductance amplifiers and nonlinear gain control

We show data from a working 45-stage analog VLSI cochlea, built on a 2.2 mm/spl times/2.2 mm tiny chip. The novel architectural features in this cochlea are: (1) The use of a wide-linear-range low-noise subthreshold transconductance amplifier. (2) The use of fuse-like nonlinear positive-feedback amplification in the second-order cochlear filter. Several new circuit techniques used in the design are described here. The fuse nonlinearity shuts off the positive-feedback amplification at large signal levels instead of merely saturating it, like in prior designs, and leads to increased adaptation and improved large-signal stability in the filter. The fuse filter implements a functional model of gain control due to outer hair cells in the biological cochlea. We present data for travelling-wave patterns in our silicon cochlea that reproduce linear and nonlinear effects in the biological cochlea.

Analog VLSI motion discontinuity detectors for image segmentation

Two analog integrated circuits that locate velocity discontinuities in a one-dimensional image are presented. Each circuit combines the image-sensing stage and the motion-processing stage on a single chip. The circuits are compact and operate in real time, so that they are suitable for use in mobile systems to perform image segmentation for navigation purposes. Both chips were tested with high-contrast stimuli and showed good performance.