State the piezoelectric effect. With neat circuit diagram explain the principle and working of piezoelectric oscillator .
The piezoelectric crystals e.g., quarts , tourmaline etc have a very special characteristics. Thin slices of these crystals develop a potential difference across the two opposite faces when subjected to a mechanical stress in a perpendicular direction. This is known as DIRECT PIEZOELECTRIC EFFECT. If the direction of the mechanical stress is reversed the potential difference changes its polarity as:-
⦁ Piezoelectric oscillator consists of two circuits interacting with each other by means of mutual inductance between inductors L, L1L1, and L2L2.
⦁ A DC supply is connected to a tank circuit consisting of a variable capacitor (C) and an inductor L2L2.
⦁ The tank circuit is connected to collector terminal of an NPN transistor.
⦁ The base terminal of the transistor is connected to another coil L2L2, which is connected to ground on the other end along with emitter terminal of transistor and negative terminal of the DC supply.
⦁ A variable capacitor (C) and an inductor (L2)(L2) form a tank circuit.
⦁ The frequency of the oscillations can be changed by changing the value of capacitance.
⦁ When the circuit is closed, the current flows through the circuit and charges the capacitor.
⦁ Then the capacitor starts discharging through the inductor, thus the electric energy is stored in the form of electric and magnetic field of capacitor and inductor respectively.
⦁ Thus electric oscillations are produced in the tank circuit.
⦁ The frequency of this oscillating electricity is given by f=12πL2C√f=12πL2C
⦁ With the help of the other electronic components including a transistor, electrical oscillations are produced continuously.
⦁ This is fed to the secondary circuit connected to Quartz crystal (Q).
⦁ The oscillating electric field is converted to mechanical vibration of crystal owing to the piezoelectric effect.
⦁ This vibration produces sound wave of the frequency equal to the frequency of vibration, which is the frequency of electric oscillations.
⦁ In this way ultrasonic sound waves can be produced.
⦁ Natural frequency of crystal is given by
`f = k/(2t) √Y/ ρ`
⦁ t = Thickness of crystal slab
⦁ Y = Young's Modulus
⦁ ρ = Density
⦁ k = 1, 2, 3, ... (Integer Multiple)
⦁ When the frequency of electric oscillations is equal to that of natural frequency of the crystal, resonance is achieved and the sound waves of maximum amplitude are produced.