Built-in self-test system and method for crystal oscillator amplifier

A built-in self-test system for a crystal oscillator amplifier, the crystal oscillator amplifier comprising a bias current source that sources current from a supply voltage, a base transistor having current terminals coupled between an output node and a supply reference node and having a control terminal coupled to an input node, and a feedback resistor coupled between the input and output nodes, the built-in self-test system comprising:

• current mirror circuitry that can mirror current through the current terminals of the base transistor into a test resistor coupled to a test node during first and second phases of a test mode;
• test control circuitry comprising a test switch controlled by a test mode signal to couple the output node to the bias current source during a normal mode and to couple the output node to the current mirror circuitry during the test mode;
• an analog to digital converter that can convert a self-bias voltage developed on the input node during the normal mode into a digital bias code, that can convert a first test voltage on the test node into a first test code during the first phase of the test mode, and that can convert a second test voltage on the test node into a second test code during the second phase of the test mode;
• the test control circuitry further comprising a test controller that can provide the test mode signal to switch between the normal and test modes, and that can increment and decrement the digital bias code by a delta value to provide an upper digital bias code and a lower digital bias code during the first and second phases, respectively, of the test mode;
• and
• a digital to analog converter that can convert the upper digital bias code to a first bias voltage that is driven onto the input node during the first phase of the test mode, and that can convert the lower digital bias code to a second bias voltage that is driven onto the input node during the second phase of the test mode.