Linearisation technique for low-voltage tuneable Nauta's transconductor in Gm C filter design

Linearisation technique for low-voltage tuneable Nauta's transconductor in Gm C filter design

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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 Gm characteristic with an increasing counterpart to obtain an overall flat Gm characteristic. The non-ideal decreasing Gm 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-Gm 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.

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