Linearisation technique for low‐voltage tuneable Nauta's transconductor in G m − C filter design

Abstract

A low-distortion, low-voltage transconductor based on Nauta's inverter-type transconductor is proposed. The transconductor's core MOSFETs are pushed into a strong inversion region under a low-voltage supply utilizing a level shifter consisting of a linear resistor and a MOSFET current source. The transconductor's linearization relies on summing a decreasing G m characteristic with an increasing counterpart to obtain an overall flat G m characteristic. The non-ideal decreasing G m is exploited from a non-linear behaviour of the triode-MOS current source that restricts a | V GS | increment of the core strongly-inverted MOSFET quartet while its increasing- G m counterpart found in another weakly-inverted auxiliary MOSFET quartet. The MOSFET current source plays significant role in the linearization process where it has to be in a triode mode of either a strong, weak or moderate inversion region. Simulation results are provided to verify the feasibility of the proposed transconductor with a 5th-order Chebyshev lowpass filter in a 0.18 μm CMOS process. The filter operates under a 0.5 V supply (the ratio V DD / V TH = 1.19) with a continuous bandwidth tuning from 500 kHz to 2.8 MHz. The proposed filter with a nominal 1.4 MHz bandwidth and a 430 mW power consumption renders the two-tone SFDR of 64.9 dB.