A CMOS "Soft-Switched" transconductor and its application in gain control and filters

This paper presents a transconductor suitable for implementation in submicron CMOS technology. The transconductor is nearly insensitive for the second-order effects of the MOS transistors, which become more and more prevalent in today's submicron processes. The transconductor relies on a differential pair with variable degeneration resistance, while the degeneration resistors are “soft-switched” by means of MOS transistors. The transconductance is continuously tunable. A transconductor, using a device in which the degeneration resistors and “soft switches” are merged, is optimized for a maximum tuning range and can be used in variable gain stages like in an automatic gain control (AGC) circuit. Besides, a third-order 5.5 MHz low-pass filter has been realized in a 0.5-μm CMOS process using the “soft-switched” transconductor. At a 3.3 V supply voltage the filter dissipates 12 mW and the dynamic range equals 62 dB where the total harmonic distortion (THD) is -48 dB for an input voltage of 1 Vpp