A CMOS three-stage ring oscillator is examined in UMC 0.13
m and 0.18
m technologies. The influence of PMOS transistor and resistor, as inverter feedbacks, on the ring oscillator frequency and the peak-to-peak amplitude is investigated in both technologies. Furthermore, as the ring oscillator usually drives a buffer in pulse generator/transmitter chain, dependence of its Figures of Merit on the buffer feedback is presented in the paper. Simulation results showed that the ring oscillator frequency is strongly dependent on the inverter feedback. The presented techniques can be used to increase (resistive feedback) and control (PMOS transistor feedback) the ring oscillator frequency. As the ring oscillator is a part of an IR-UWB (Impulse Radio Ultra Wide Band) pulse generator, its oscillating frequency determines the spectrum central frequency and has significant effect on spectrum fitting within UWB FCC mask.