OTA based 200 GΩ resistance on 700 μm2 in 180 nm CMOS for neuromorphic applications

AbstractGenerating an exponential decay function with a time constant on the order of hundreds of milliseconds is amainstay for neuromorphic circuits. Usually, either subthreshold circuits or RC-decays based on transcon-ductance amplifiers are used. In the latter case, transconductances in the 10 pS range are needed. However,state-of-the-art low-transconductance amplifiers still require too much circuit area to be applicable in neu-romorphic circuits where >100 of these time constant circuits may be required on a single chip. We presenta silicon verified operational transconductance amplifier that achieves a g m of 5 pS in only 700 µm 2 , a factorof 10-100 less area than current examples. This allows a high-density integration of time constant circuitsin target appliations such as synaptic learning or as driving circuit for neuromorphic memristor arrays.1. IntroductionNeuromorphic circuits that operate in real timehaveto exhibit the sametimescale asbiologicalneu-rons [1]. This means a circuit has to generate multi-ple exponential decay functions with time constantsof up to hundreds of milliseconds, employed e.g. asthe membrane time constant, presynaptic adapta-tion or postsynaptic current trace [2].In the form of the gm-C configuration [3], Op-erational Transconductance Amplifiers (OTA) arewidely used to produce exponential decay func-tions. To achieve time constants in the aboverange,large capacitances and low transconductances areneeded. Circuit area limits capacitances to ≤1 pF,requiring transconductances on the order of 10 pS[4, 5].While somepapershavereportedsuchlowvalues,their large circuit area implementations prohibit anintegration of multiple instances of these OTAs for