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/sub m/-C filters is introduced. Its input transistors are kept in the triode-region to benefit from the lowest g/sub m//I/sub D/ ratio. In contrast with weak inversion current-controlled techniques that demand an extremely low I/sub D/, which is difficult to obtain precisely, the g/sub m/ is adjusted by a well defined (W/L) and V/sub DS/, the latter a replica of the tuning voltage, V/sub TUNE/. Since the minimum V/sub DS/ is still considerably above the equivalent noise of the replica circuit, an improved control of g/sub m/ is achieved. The resulting design complies with V/sub DD/=1.5 V and a 0.35 /spl mu/m CMOS process. Its g/sub m/ ranges from 1.1 nA/V to 5.5 nA/V for 10 mV /spl les/ V/sub TUNE/ /spl les/ 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.