“Sweet Spots” in Moderate Inversion for MOSFET Squarer Transconductors

Two "sweet spots" are analytically identified to enable MOSFET squarer differential transconductors to efficiently operate in moderate inversion. Measurements at low frequencies demonstrate the practicality of using moderate inversion when input signals are limited to about 200 mV_pk-pk. At the first sweet spot, very low distortion of significantly less than 0.5% total harmonic distortion is possible. For comparison, the sweet spot for the third harmonic is shown for the linear differential transconductor. This brief also discusses the effect of the variations in threshold voltage on the location of the sweet spots, and demonstrates that this sensitivity can be greatly reduced using an appropriate biasing circuit. The proposed approach is of particular interest for low voltage and low current squarer transconductance applications.