Comparative study of two LDOs for supplying a 2.5GHz rail-to-rail VCO

This paper presents a comparative study of two low dropout voltage regulators (LDO) for supplying a 2.5GHz rail-to-rail voltage controlled oscillator (VCO). The effect of the two LDOs noise on the phase noise of the VCO is presented while keeping the supply ripple the same. Both LDO structures are implemented by using the same error amplifier and the same pass transistor, but the classical LDO uses a large off chip decoupling capacitor while the capacitorless LDO employs a current amplifier which forms high bandwidth around the pass transistors to reduce the supply ripple. Post layout simulation results show a -88dBc/Hz and -83dBc/Hz VCO phase noise at 1MHz for the classical and capacitorless LDO. The LDOs and the VCO were designed in 0.18μm CMOS process.