A Tunable CMOS Resistor with Wide Tuning Range for Low Pass Filter Application

In this paper, a new tunable CMOS resistor is proposed. The resistance is inversely proportional to bias current, to provide the resistor with a wider tuning range. And transistors, composing the active resistor, work at saturation region to achieve very large resistance within a small area. As an example, a low pass RC filter using tunable CMOS resistor realized in 0.5-mum CMOS technology is reported. The measured result shows that the cutoff frequency of low pass filter can be widely tuned from 5 kHz to 1.9 MHz. The total harmonic distortion (THD) stays lower than -40 dB for an input signal up to 100 mV Vp-p at 500-kHz input frequency. The die area of the low-pass filter is 0.25 times 0.13 mm² and the maximum power consumption (at fc=1.9 MHZ) is 5.7 mW with single 3.3-V power supply.