Oscillator circuit with signal buffering and start-up circuitry

Abstract

A crystal oscillator circuit includes a crystal driven by a variable current source having a transconductance device with transconductance dependent on current, and a CMOS buffer circuit for receiving a sinusoidal signal from the crystal and providing a square wave output signal. The buffer circuit includes first and second bi-level buffers capacitively coupled to receive sinusoidal signals and operating in a push-pull mode for providing square wave output signals from each of said first and second buffers, and a third buffer driven by output signals from the first and second buffers, whereby duty cycle of the first and second buffers is controlled by bias voltages applied to CMOS transistors in the buffers. The circuit further includes a bias current circuit which provides through the variable current source a bias current, I-bias, which is set to provide an optimum transconductance for oscillation start-up and a reduced regulated bias current once oscillation voltage increases, thereby preventing runaway oscillations.