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’ micropower op amp. In the sample circuit shown a widely available 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 therefore 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 current 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.
The TLC271 operational amplifier combines a wide range of input offset voltage grades with low offset voltage drift and high input impedance. In addition, the TLC271 offers a bias-select mode that allows the user to select the best combination of power dissipation and AC performance for a particular application Using the bias-select option, these devices can be programmed to fit a wide range of applications. Three offset voltage grades are available, ranging from the low-cost TLC271 (10mV) to the TLC271B (2mV) low-offset version. The devices are offered in both commercial and industrial operating temperature ranges
The devices also feature low-voltage single-supply operation with a common-mode input voltage range which includes the negative rail.
