Tunable Linear MOS Resistors Using Quasi-Floating-Gate Techniques

A family of tunable MOS resistors based on quasi-floating-gate (QFG) transistors biased in the triode region is analyzed in this paper. From the study results, a new device that outperforms previous implementations, is presented. By means of a capacitive divider, the ac component of the drain-to-source voltage scaled with a factor alpha les 1 is added to the gate-to-source voltage leading to a cancellation of the nonlinear terms. The effect of alpha on resistor linearity is analytically studied. Simulation results are also provided for different technologies. Finally, a complete transconductor has been built which preserves the linearity of the MOS resistor. Three versions of the transconductor have been fabricated for different values of alpha (alpha = 0, 0.5, and 1) in a 0.5 mum CMOS technology with plusmn1.65-V supply voltage. Experimental results show (for alpha = 1 ) a THD of - 57 dB (HD2=-70 dB) at 1 MHz for 2-V peak-to-peak differential input signal with a nominal ac-transconductance of 200 muA/V and a power consumption of 3.2 mW.