Highly linear voltage-controlled CMOS transconductors

A circuit technique for realizing voltage-tunable linear high-frequency CMOS transconductor cells that use two cross-coupled MOS or CMOS transistor pairs operating at saturation is described. The tuning capabilities attained with an adjustable CMOS voltage source are examined. Design trade-offs between linearity and bandwidth are discussed, and a simple operational transconductance amplifier (OTA) example is simulated via SPICE. The simulation results show that the transconductance can be varied by a factor of 8 and that, for a power supply of +or-5 V, the linearity error is less than 0.5% for an input voltage of +or-3.0 V. A cutoff frequency of over 370 MHz is obtained.