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A simple nA/V CMOS transconductor for ultra-low power low-frequency gm-C filters is introduced. Its input transistors are kept in the triode-region to benefit from the lowest gm/ID ratio. In contrast with weak inversion current-controlled techniques that demand an extremely low ID, which is difficult to obtain precisely, the gm is adjusted by a well defined (W/L) and VDS, the latter a replica of the tuning voltage, VTUNE. Since the minimum VDS is still considerably above the equivalent noise of the replica circuit, an improved control of gm is achieved. The resulting design complies with VDD=1.5 V and a 0.35 μm CMOS process. Its gm ranges from 1.1 nA/V to 5.5 nA/V for 10 mV ≤ VTUNE ≤ 50 mV. A set of PSPICE simulations supports theoretical results. A designed bandpass filter has a 5 Hz-center frequency and has a maximum idle dissipation of 17 nW, whereas SNR=59.2 dB for THD <1% @ 150 mV peak value.