Design of a Gm-C low pass filter with low cutoff frequency

Abstract

The design of a low-cutoff-frequency Gm-C low-pass filter is presented, the key part in the filter, the operational transconductance amplifier utilize a folded-cascode structure. As the circuit is desired to operate at low frequency, it is necessary to design a small linear trans-conductance amplifier employing a cross-coupled input stage. To compensate the gain loss, the auxiliary amplifiers are added. The differential pairs are offsetted in the sub-threshold zone, and an appropriate adjustment of the operating point can effectively curve the transconductance as a function of temperature. The filter is fabricated in a 0.18 ¿m CMOS process, operating with a 1.8 V supply and using a bias current of 4 ¿A. The results show that the cutoff frequency of low-pass filter can be 10 kHz, and the maximum deviation arising from variations of temperature is 0.0016 dB/°C. The total power consumption is 225 ¿W with a total capacitance load of 60 pF.