Vito Fabbrizio

Low power, low voltage conductance-mode CMOS analog neuron

Analog implementations of neural networks have been used for a wide variety of tasks especially in the area of image processing. Typically, implementations of analog neural networks have been based on the use of either current or charge as the variable of computation. This work introduces a new class of analog neural network circuits based on the concept of conductance-mode computation. In this class of circuits, accumulated weighted inputs are represented as conductances, and a conductance-mode neuron is used to apply nonlinearity and produce an output. The advantages of this class of circuits are twofold: firstly, conductance-mode computation is fast-we have developed circuits based on these principles which compute at 5-10 MHz; secondly, because conductance-mode computation requires the minimum charge necessary to compare two conductances, its energy-consumption is self-scaling depending on the difficulty of the decision to be made-we have a working prototype which consumes 166 fJ per connection. The computing precision of these circuits is high: test results on a small test structure indicate an intrinsic precision of 8-9 bits. We have developed a larger test circuit which is able to perform computation with 1056 binary-valued inputs. Initial measurements in this large test structure indicate a more limited computing precision of 6+ to 8+ bits depending on the common mode of the input signal.