Analysis and verification of an analog VLSI incremental outer-product learning system

An architecture is described for the microelectronic implementation of arbitrary outer-product learning rules in analog floating-gate CMOS matrix-vector multiplier networks. The weights are stored permanently on floating gates and are updated under uniform UV illumination with a general incremental analog four-quadrant outer-product learning scheme, performed locally on-chip by a single transistor per matrix element on average. From the mechanism of floating gate relaxation under UV radiation, the authors derive the learning parameters and their dependence on the illumination level and circuit parameters. It is shown that the weight increments consists of two parts: one term contains the outer product of two externally applied learning vectors; the other part represents a uniform weight decay, with time constant originating from the floating gate relaxation. The authors address the implementation of supervised and unsupervised learning algorithms with emphasis on the delta rule. Experimental results from a simple implementation of the delta rule on an 8x7 linear network are included.