The Pierce oscillator is a stable variant of the Colpitts oscillator, commonly used as an RF oscillator in RF communication circuits. This blog post explores the working principle of the Pierce crystal oscillator and presents a modified version that utilizes the internal capacitance of a bipolar junction transistor (BJT) for feedback filtering along with the quartz crystal. The oscillator's circuit diagram and a practical implementation using a 2N2222 BJT are provided.
BJT-Based Pierce Crystal Oscillator Circuit
In this design, the traditional capacitors found in a basic Pierce oscillator are omitted, and the internal capacitance of the BJT is leveraged for forming the feedback filter in conjunction with the crystal.
The following picture shows the 2N2222 bipolar transistor based Pierce crystal oscillator constructed on a breadboard.
The circuit diagram below illustrates the BJT-based Pierce crystal oscillator.
The 2N2222 bipolar transistor is employed in this example, although other transistors such as 2N3904 or BC547 can also be used. A 16MHz quartz crystal is utilized, resulting in an oscillation frequency of 16MHz. The feedback network is connected to the crystal resonator in a manner that provides a 180-degree phase shift. This is achieved by connecting the crystal between the base and collector of the BJT. The feedback network is connected between the emitter and the base of the transistor. Additional components include the coupling capacitor Cc1, DC biasing resistors R1, R2, and RE, emitter bypass capacitor CB, and the radio frequency choke RFC.
See for more details How Pierce crystal oscillator Works.
Video Demonstration
Watch the following video to observe the BJT-based Pierce crystal oscillator in action on a breadboard.
Applications of Pierce Crystal Oscillator
Pierce oscillators, including this BJT-based design, find applications in various RF circuits such as AM modulators, AM demodulators, FM circuits, and others where a stable local oscillator signal is required. Additionally, they can be used in beat frequency oscillator (BFO) circuits to generate stable signals.
Conclusion
The BJT-based Pierce crystal oscillator presented in this blog post among other types of oscillators offers a stable RF oscillator solution for communication circuits. By utilizing the internal capacitance of the BJT along with a quartz crystal, it provides reliable oscillation at a specific frequency. Experimentation with different components and crystal frequencies can further expand its versatility in RF applications.
