Study of Speed Enhancement of a CMOS ring VCO

Abstract

Positive feedback has been added to a fully differential CMOS amplifier. With the introduction of positive feedback, the delay time reduces and the speed of operation increases. An even or odd number of stages can be cross coupled or directly coupled to form a ring Voltage Controlled Oscillator or Current Controlled Oscillator (VCO/CCO). Pspice simulation shows an improvement (increase) of speed of around 167 %. Instead of using positive feedback if we use negative feedback then also the speed improves but it requires a higher value of feedback resistance, which is not favoured in integrated circuit fabrication. Here the typical supply voltage requirement is ±2 volt. Using this amplifier some applications have also been discussed.