Transistor Crystal Oscillator

Figure: Circuit diagram of Transistor crystal oscillator

• Figure shows the transistor crystal oscillator. The crystal will act as parallel –tuned circuit. At parallel resonance, the impedance of the crystal is maximum. This means that there is a maximum voltage drop across C₂. This in turn will allow the maximum energy transfer through the feedback network.

• The feedback is +ve. A phase shift of 180⁰ is produced by the transistor. A further phase shift of 180⁰ is produced by the capacitor voltage divider. This oscillator will oscillate only at f_p.

Where f_p = parallel resonant frequency ie the frequency at which the vibrating crystal behaves as a parallel resonant circuit.

Advantages

1. Higher order of frequency stability
2. The Q-factor of the crystal is very high.

Disadvantages

1.     Can be used in low power circuits.

2.     The frequency of oscillations cannot be changed appreciably.