Oscillators Circuit Diagrams

Micropower Crystal Oscillator Schematic Circuit Diagram

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.

Micropower Crystal Oscillator Schematic Circuit Diagram

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.


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