Page 66 - Semester-IV-Electronics
P. 66
Piezoelectric Frequency Control
In transistor oscillators, the frequency of oscillations cannot remain
constant due to variations in temperature, biasing voltage or current.
Stability of frequency of oscillator also depends on quality factor Q
of the L-C circuit.
The value of Q of the L-C circuits used in Hartley and Colpitts
oscillator is about 40-50.
To obtain oscillations of constant frequency from the oscillators,
piezoelectric crystals are used in tuned circuits because their Q is
about 10,000 or more.
Rochelle salt and quartz crystals show piezoelectric effect.
When an alternating voltage is applied across their opposite faces,
mechanical vibrations take place on the opposite faces.
Oppositely, when pressure is applied on the opposite faces, charges of
opposite nature appear on the other opposite faces. Hence, in
piezoelectric crystals electromechanical coupling happens. 66
