A Low Thermal Sensitivity Subthreshold-Current to Pulse-Frequency Converter for Neuromorphic Chips

To convert a subthreshold current to a pulse frequency efficiently and predictably, we designed a silicon soma that conserves energy with current feedback and lessens thermal sensitivity with voltage feedback. When the input current charges a capacitor close to the inversion point of an inverter, its short-circuit current wastes energy. To shorten this period, existing designs accelerate the charging rate with positive feedback: Either a capacitive divider feeds back voltage or a current mirror feeds back current. Voltage feedback is less effective because it kicks in only at the inversion point. Current feedback is less predictable because its leakage current is exponentially sensitive to temperature variation. By quantifying this thermal sensitivity with an analytic model of the subthreshold MOS transistor, we successfully combined current feedback with voltage feedback to design a silicon soma 10-fold less sensitive to temperature than a previous current-feedback-only design that uses 7.6-fold more silicon area. This advance allowed a mixed-signal neuromorphic chip to be predictably programmed for the first time.