Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1999-09-29
2001-06-05
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S17700V, C332S112000, C332S113000, C327S291000
Reexamination Certificate
active
06242988
ABSTRACT:
RELATED APPLICATIONS
Subject matter relating to that of the present application is presented in U.S. Patent applications filed on the same day as this application: “Edge-Triggered Toggle Flip-Flop Circuit” by R. Herrera and R. Sarpeshkar; and “Spike-Triggered Asynchronous Finite State Machine,” by R. Herrera and R. Sarpeshkar. These applications are assigned to the assignee of the present application and are hereby incorporated by reference in the present application.
FIELD OF THE INVENTION
The present invention relates to electronic circuits, and, more particularly to electronic voltage-controlled and current-controlled oscillator circuits and monostable pulse-generating circuits. Still more particularly, embodiments of the present invention relate to pulse-generating circuits for producing output signals similar to those occurring in neurons in the nervous system of animals.
BACKGROUND OF THE INVENTION
Spiking neuron circuits are inspired by the functioning of neurons in the nervous systems of animals. A simple and effective way of modeling such neurons is as integrate-and-fire units: The neuron integrates input current on a capacitor until the voltage on the capacitor reaches a predetermined threshold voltage. Then, the neuron generates a spike signal and resets the voltage on the capacitor to a reference potential (typically zero voltage, or ground potential). See generally, C. Mead,
Analog VLSI and Neural Systems
, Addison-Wesley, Reading Mass., 1989, especially chapters 4 and 12.
Commercial importance of neuron circuits arises, in part, because they can be used as building blocks in neuronal networks for pattern recognition, as voltage-controlled or current-controlled oscillators, or as monostable pulse-generating circuits. In practice, neuron circuits are often used in arrays, as in U.S. Pat. No. 4,961,002 issued Oct. 2, 1990 to S. M. Tam, et al. It is therefore a useful attribute of neuron circuits that they be fabricated using a small number of devices, and that such devices dissipate a minimum of heat while operating. Yet, many prior implementations of neuron circuits are very complex and consume energy at such levels as to preclude packaging in densities suitable to many tasks.
In operating in an analogous way to biological neurons, neuron circuits often receive input signals from one or more sources, analogous to dendrites in biological contexts. The input signals are typically low-level signals that are often weighted to form logical functions for use in neural networks. A characteristic of most integrate-and-fire circuits is a reset of accumulated input signals upon firing at the output. Prior art neuron circuits typically perform this resetting by involving inputs in a positive feedback loop used in generating the neuron circuit spiking signal. However, such feedback can cause undesirable effects on input signal sources during neuron circuit firing and during a transient period following firing. Such feedback, and the effects of input signals arriving during the output pulse duration, can undesirably alter the charging operations of input capacitors and can adversely affect input signal sources.
Prior art neuron circuits have exhibited limitations respecting the pulse width, shape for neuron circuit outputs and the threshold voltage for neuron circuits. Additionally, it has not proven possible in some neuron circuits to control the refractory period, i.e., the time elapsed between the termination of an output pulse and the generation of a new output pulse.
SUMMARY OF THE INVENTION
Limitations of the prior art are overcome and a technical advance is made in accordance with the present invention described below in illustrative embodiments.
In one illustrative embodiment, a neuron circuit efficiently employs device area by using a small number of transistors and two capacitors. The output pulse width and threshold voltage are adjustable, and the refractory period is illustratively identical to the output spike width. Moreover, during the duration of the spike, the neuron circuit is insensitive to signals appearing at its input.
By judicious adjustment, the illustrative neuron circuit may be used to generate pulse-like waveforms or repetitive sawtooth or triangular waveforms over a very wide range of frequency (typically from rates of a few Hz to hundreds of MHz). For example, the neuron circuit can generate sawtooth waveforms with an adjustable amplitude that vary with the threshold of the neuron circuit.
Importantly, unlike prior topologies, positive feedback responsible for spike firing in the neuron need not directly involve the input in a positive-feedback loop, thereby reducing interference with sensitive analog waveforms at the input of the neuron from noisy output waveforms. A further result of isolation of the noisy output signals from inputs is that it is possible to achieve greater similarity and matching of subthreshold input loads across neurons irrespective of the output state of the neuron.
In an alternative illustrative embodiment input currents are further isolated using a current mirroring arrangement, with the further added advantages that discharge currents need not exceed input currents, thus improving overall energy consumption.
REFERENCES:
patent: 4723114 (1988-02-01), D'Arrigo et al.
patent: 5412350 (1995-05-01), Kim
Lucent Technologies - Inc.
Mis David
Ryan William
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