0.7 pA/V transconductor allowing Gm-C filters in range of mHz using sub-pF capacitors

A new topology of a transconductor suitable for the implementation of transconductance-C (Gm-C) integrators with time constants of tens of seconds using sub-pF capacitors is introduced. The proposed circuit relies on a cascode stage driven backwards, i.e. the input signal is applied to the cascode output node. The key point is taking advantage of the high impedance of the cascode to impose high attenuation to the input signal and to establish transconductances in the order of fA/V. The concept is demonstrated at the simulation level by realising a Gm-C integrator with a time constant of 100 s. The integrator comprises a 0.7 pA/V transconductor and an 800 fF capacitor at its output.