For applications that require high frequency stability, such as carrier sources in transmitters, clock generators in computers, and tuning systems in receivers, a common solution is to use a piezoelectric crystal. These crystals have a natural resonant frequency, which varies based on the size and shape of the crystal. At this frequency, the crystal acts as a high Q tuned circuit. To utilize the crystal in an oscillator circuit, it is typically mounted between two metal plates that provide electrical contact.
There are various ways to connect a
crystal into an oscillator circuit, and one such method is shown below.
In this example, the crystal is connected between the collector and base of a bipolar transistor to form a Colpitts oscillator. The internal capacitances between the collector-base and base-emitter provide positive feedback. The collector circuit does not need to be tuned, and the secondary winding of the transformer provides a convenient output point.
The resistors R1 and R2 forms a voltage divider that biases the base of the transistor such that it is higher than the base to emitter junction voltage which is typically 0.7V. The resistor R3 is also used to bias the emitter of the bipolar junction transistor and provide a negative feedback path so that the transistor operating point or the bias point is stable.
References:
- Crystal Oscillator with 2N2222 Transistors
- Crystal Oscillator with Colpitts Configuration
