In this note, we are discuss about the Transistor Crystal Oscillator. About the Circuit Diagram, Circuit Operation, Advantages, and Disadvantages also. Poly Notes Hub happy to share this note with all of you.
Author Name: Arun Paul.
About Transistor Crystal Oscillator
Fig shows Shows the Transistor Crystal Oscillator. Note that it is a Colpitts Oscillator modified to act as a crystal oscillator. The only change is the addition of the crystal in the feedback network. The crystal will act as a parallel-tuned circuit.
Circuit Operation of Crystal Oscillator
As you can see in this circuit instead of resonance caused by L and (C1 + C2), we have the parallel resonance of the crystal. At parallel resonance, the impedance of the crystal is maximum. This means that there is a maximum voltage drop across C1. This in turn will allow the maximum energy transfer through the feedback network at fp.
Note that feedback is positive. The transistor produces a phase shift of 180°. The capacitor voltage divider produces a further phase shift of 180°. This oscillator will oscillate only at fp. Even the smallest deviation from fp will cause the oscillator to act as an effective short. Consequently, we have an extremely stable oscillator.
Advantages of Crystal Oscillator
- They have a high order of frequency stability.
- The quality factor(Q) of the crystal is very high. The Q factor of the crystal may be as high as 10000 compared to about 100 of the L-C tank.
Disadvantages of Crystal Oscillator
- They are fragile and consequently can only be used in low-power circuits.
- The frequency of Oscillations cannot be changed appreciably.
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