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.
Circuit diagram :
In the sample circuit shown a widely avail-able TLC271 is used. On pin 8 we have the opportunity to set the ‘bias mode’, with three choices ranging between fast operation with higher current consumption and slower operation at low current. For our clock crystal the middle setting will suit us fine. Pin 8 is there-fore connected to the voltage divider R1/R2. The current consumption of the entire circuit is impressively modest and at 5 V this is just 56 µA! The oscillator also functions astoundingly well at 3.3 V. At the same time the cur-rent drops to a more battery-friendly 41 µA. A prototype built in the Elektor Labs produced the slightly higher values indicated in the circuit diagram.
The output signal delivered by this circuit has admittedly scant similarity to a square wave. Nevertheless some cosmetic surgery will tidy this up, with treatment in the Schmitt trigger following. To save current (naturally) we use a CMOS device such as the 74HC14.
Author :Rainer Reusch - Copyright :Elektor