Micropower Crystal Oscillator

Crystal oscillators for digital circuits are normally built as Pierce oscillators with an inverter. The inverter operates as a linear amplifier and thus requires extra current. But you can also build a crystal oscillator using an operational amplifier (op-amp for short)! If a very low frequency is involved, for instance 32.768 kHz (commonly used for clocks), you can get away with a comparatively ‘slow’ micro power op-amp.