Voltage controlled oscillator and phase-locked loop using it

Abstract

A voltage controlled oscillator is provided which includes a pair of gain stages constituting a positive feedback path, a pair of buffer stages in cross connection with the gain stages, a pair of loads connected with the corresponding gain stages, each having a parallel connection of an active device resistor and a clamping diode, and a pair of voltage controlled current sources connected with the corresponding gain stages, for supplying constant currents to the gain stages. A timing capacitor is connected with the input sides of both voltage controlled current sources, and is charged or discharged by the constant currents from the current sources. The oscillator further includes a pair of variable bias generating means, connected with the corresponding buffer stages, for generating variable bias voltages are provided, each of which varies the bias voltage to be applied to the corresponding active device in accordance with the output of the corresponding buffer stage in such a manner that the bias voltage is properly set in accordance with the constant current value of said constant current source so as to set the on-resistance of said active device to a suitable value corresponding to the constant current value. With this arrangement the loop gain of the VCO is made constant regardless of the constant current value.