Analysis and design of a 3-stage CMOS current controlled ring oscillator

Abstract

This thesis describes a ring oscillator using 3 stages of an NMOS driving a current load and a capacitor. Its frequency can be shown to increase monotonically with the load current giving rise to the name current controlled oscillator (CCO). The CCO circuit using 180 [micrometre]/0.18 [micrometre] NMOS with 1pF load can be tuned to oscillate from 468 Hz to 305 MHz using load current from 1nA to 1mA. An accurate derivation of the formulae relating the frequency with current has been derived. Its calculation result agrees with the simulation result to a high accuracy within 1 %. Then the effect of the channel length modulation has been attempted to derive using the same method, but it is too complicated to be solved. Original and improved CCO are designed. They have output frequency of 10.02 MHZ and 10MHz respectively. The improved circuit employs additional buffers to make it is considerably more linear than the conventional one.