A Constant – Vov/-gm Technique based on a Self-Biased Current Source for MOS Transistors

In this work, a bias technique for generating a temperature stable overdrive voltage $V_{o v}=V_{g s}-V_{t h}$ for MOS transistors is presented by generating a NTAT-current based on a self-biased current source with a temperature stable floating voltage source. First, this is the basis for avoiding headroom problems when operating analog CMOS circuits over large temperature ranges. Second, on the foundation of the proposed bias technique, a further concept with the aim of keeping the small signal slope $g_{m}$ of MOS transistors approximately constant over temperature is presented. By this, in addition to $V_{o v}$ and $g_{m}$, further analog circuit characteristics (eg. $f_{T}$, GBW and current consumption) can also be kept approximately constant over temperature. To demonstrate the proposed constant-$V_{o v}$ bias technique, two ‘square-law’ amplifier designs, each with different $V_{o v}$ in the input transistors and associated bias circuitry, were fabricated in a $0.18 \mu \mathrm{m}$ CMOS technology. The measurement results show that the remaining linearity errors as well as the respective possible dynamic ranges at different $V_{o v}$ for the two fabricated ‘square-law’ arrangements are each temperature independent in the measured temperature range from −35°C to +125°C. Additional simulation results demonstrate an almost temperature stable $g_{m}$ of 284ppm/°C from −35°C to +125°C.