  Generating an exponential decay function with a time constant on the order of
hundreds of milliseconds is a mainstay for neuromorphic circuits. Usually,
either subthreshold circuits or RC-decays based on transconductance 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 neuromorphic circuits where >100 of these
time constant circuits may be required on a single chip. We present a silicon
verified operational transconductance amplifier that achieves a gm of 5 pS in
only 700 {\mu}m2, a factor of 10-100 less area than current examples. This
allows a high-density integration of time constant circuits in target
appliations such as synaptic learning or as driving circuit for neuromorphic
memristor arrays.
