A compact nA/V CMOS triode-transconductor and its application to very-low frequency filters

A simple nA/V CMOS transconductor for ultra-low power low-frequency g_m-C filters is introduced. Its input transistors are kept in the triode-region to benefit from the lowest g_m/I_D ratio. In contrast with weak inversion current-controlled techniques that demand an extremely low I_D, which is difficult to obtain precisely, the g_m is adjusted by a well defined (W/L) and V_DS, the latter a replica of the tuning voltage, V_TUNE. Since the minimum V_DS is still considerably above the equivalent noise of the replica circuit, an improved control of g_m is achieved. The resulting design complies with V_DD=1.5 V and a 0.35 μm CMOS process. Its g_m ranges from 1.1 nA/V to 5.5 nA/V for 10 mV ≤ V_TUNE ≤ 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.