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.

Circuit diagram :

Micropower Crystal Oscillator-Circuit Diagram

Micropower Crystal Oscillator 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