Hardware implementations of spiking neurons can be extremely useful for a large variety of applications, ranging from high-speed modeling of large-scale neural systems to real-time behaving systems, to bidirectional brain-machine interfaces. The specific circuit solutions used to implement silicon neurons depend on the application requirements. In this paper we describe the most common building blocks and techniques used to implement these circuits, and present an overview of a wide range of neuromorphic silicon neurons, which implement different computational models, ranging from biophysically realistic and conductance-based Hodgkin-Huxley models to bi-dimensional generalized adaptive integrate and fire models. We compare the different design methodologies used for each silicon neuron design described, and demonstrate their features with experimental results, measured from a wide range of fabricated VLSI chips.

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Neuromorphic Silicon Neuron Circuits

In the paper, an analysis of the state-of-the-art of active elements for analog signal processing is presented which support - in contrast to the conven tional operational amplifiers - not only the voltage-mode but also the current- and mixed-mode operations. Several pro blems are addressed which are associated with the utiliza tion of these elements in linear applications, particularly in frequency filters. A methodology is proposed which generates a number of fundamentally new active elem ents with their potential utilization in various areas o f signal processing.

Active Elements for Analog Signal Processing: Classification, Review, and New Proposals

In this paper a new family of scroll grid attractors is presented. These families are classified into three called 1D-, 2D- and 3D-grid scroll attractors depending on the location of the equilibrium points in state space. The scrolls generated from 1D-, 2D- and 3D-grid scroll attractors are located around the equilibrium points on a line, on a plane or in 3D, respectively. Due to the generalization of the nonlinear characteristics, it is possible to increase the number of scrolls in all state variable directions. A number of strange attractors from the scroll grid attractor families are presented. They have been experimentally verified using current feedback opamps. Also Lur'e representations are given for the scroll grid attractor families.

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Families of scroll grid attractors

The linear range of approximately ±75 mV of traditional subthreshold transconductance amplifiers is too small for certain applications—for example, for filters in electronic cochleas, where it is desirable to handle loud sounds without distortion and to have a large dynamic range. We describe a transconductance amplifier designed for low-power (< 1 µW) subthreshold operation with a wide input linear range. We obtain wide linear range by widening the tanh, or decreasing the ratio of transconductance to bias current, by a combination of four techniques. First, the well terminals of the input differential-pair transistors are used as the amplifier inputs. Then, feedback techniques known as source degeneration (a common technique) and gate degeneration (a new technique) provide further improvements. Finally, a novel bump-linearization technique extends the linear range even further. We present signal-flow diagrams for speedy analysis of such circuit techniques. Our transconductance reduction is achieved in a compact 13-transistor circuit without degrading other characteristics such as dc-input operating 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 construct a follower–integrator with an experimental dynamic range of 65 dB. We show that, if the amplifier‘s noise is predominantly thermal, then an increase in its linear range increases the follower–integrator‘s dynamic range. If the amplifier‘s noise is predominantly 1/f, then an increase in its linear range has no effect on the follower–integrator‘s dynamic range. To preserve follower–integrator bandwidth, power consumption increases proportionately with an increase in the amplifier‘s linear range. We also present data for changes in the subthreshold exponential parameter with current level and with gate-to-bulk voltage that should be of interest to all low-power designers. We have described the use of our amplifier in a silicon cochlea [1, 2].

A Low-Power Wide-Linear-Range Transconductance Amplifier

This paper presents a new configuration for linear MOS voltage-to-current conversion (transconductance). The proposed circuit combines two previously reported linearization methods. The topology achieves 60-dB linearity for a fully balanced input dynamic range up to 1 V/sub pp/ at a 3.3-V supply voltage, with slightly decreasing performance in the unbalanced case. The linearity is preserved during the tuning process for a moderate range of transconductance values. The approach is validated by both computer simulations and experiments.

G. Pearce

Papers

Analogue IC Design: the Current Mode Approach