A PVT independent subthreshold constant-Gm stage for very low frequency applications

This paper presents the design of an ultra low power sub-threshold G_m stage for Gm-C filters with very low cut-off frequency. The sub-threshold region of operation provides a robust trans-conductance performance against temperature variations. We propose a new topology for a PVT (process, voltage & temperature) independent g_m stage with constant g_m biasing circuit operating in sub-threshold region. The significant variation to temperature due to channel length modulation is corrected using a parallel combination of two g_m blocks of different value. The circuit is made immune to process variations by using replica-biasing circuit to track the changes in process parameters. The circuit performance also remains nearly constant for a supply voltage range of 1.1V to 1.7V. A general biquad filter has been implemented and simulations show about 1% variation in the 3 dB bandwidth (in the range of few hertz) for a temperature range of -30degC to 110degC. The circuit has been implemented using 0.5mum CMOS technology with a nominal supply voltage of 1.2 V and power consumption for each g_m block is 55.3 nW.