Typically, crystal oscillator in digital circuits are constructed as Pierce oscillators utilizing an inverter. This inverter acts as a linear amplifier, necessitating additional current. However, it’s possible to create a crystal oscillator using an operational amplifier, commonly referred to as an op-amp! Particularly for very low frequencies, like 32.768 kHz often used in clocks, a relatively ‘slow’ micropower op amp suffices. In the provided circuit example, a readily available TLC271 is employed. Pin 8 offers the flexibility to set the ‘bias mode,’ providing three options ranging from fast operation with higher current consumption to slower operation at low current.
Choosing the Right Bias Setting:
For our clock crystal, the middle bias setting is the perfect choice. So we connect Pin 8 to the voltage divider R1/R2. Remarkably, the entire circuit exhibits impressively modest current consumption, merely 56 μA at 5 V! Even at 3.3 V, the oscillator performs exceptionally well, with the current dropping to a more battery-friendly 41 μA. A prototype constructed in the Elektor Labs displayed slightly higher values than those indicated in the circuit diagram. While the output signal from this circuit doesn’t exactly resemble a square wave, a bit of fine-tuning is in order, and that’s where the Schmitt trigger comes into play. To conserve current, a CMOS device like the 74HC14 is employed.
TLC271 Operational Amplifier:
The TLC271 operational amplifier stands out due to its diverse range of input offset voltage grades, low offset voltage drift, and high input impedance. Moreover, the TLC271 offers a bias-select mode, allowing users to choose the optimal balance between power dissipation and AC performance for specific applications. Through the bias-select feature, these devices can be configured to suit a wide variety of applications. Three offset voltage grades are available, spanning from the economical TLC271 (10mV) to the low-offset version TLC271B (2mV). These devices are available in both commercial and industrial operating temperature ranges.
Low-Voltage Single-Supply Operation:
In addition to their versatile features, these devices operate efficiently on low-voltage single supplies, with a common-mode input voltage range that encompasses the negative rail.