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 ID, which is difficult to obtain precisely, the g. 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 is achieved. The resulting design complies with V(DD)=1.5V and a 0.35 mu m CMOS process. Its g. ranges from 1.1 nA/V to 5.5nA/V for 10m V <= V (TUNE) <= 50mV. A set of PSPICE simulations supports theoretical results. A designed bandpass filter has a 5Hz-center frequency and has a maximum idle dissipation of 17nW, whereas SNR=59.2dB for THD < 1% @ 150mV peak value.