A crystal driver integrated circuit with external oscillation signal amplitude control including an amplifier core, an input pin and an output pin, an adjustable capacitor, and a controller. The controller operates the amplifier core in any one of multiple operating modes including an oscillator mode and a bypass mode. During the bypass mode, the controller disables the amplifier core and adjusts the adjustable capacitor so that an amplitude of an oscillation signal received via the input pin from an external oscillator has a target amplitude. The external oscillation signal may be capacitively coupled for capacitive voltage division or directly coupled for impedance attenuation. An available voltage may be provided as a source voltage to the external oscillator via the output pin. An internal voltage regulator and/or switch may be included to re-provision the output pin to provide the source voltage during the bypass mode.