--- title: "Piezoelectric oscillator for ultrasonic waves" subject: "Engineering Physics" chapter: 3 key: "semester_1/engineering_physics:piezoelectric-oscillator-ultrasonic" source: https://www.bctengineer.com/q/semester_1/engineering_physics/piezoelectric-oscillator-ultrasonic site: BCT Engineering --- # Piezoelectric oscillator for ultrasonic waves Engineering Physics · Chapter 3: Acoustics - Asked in: 2074 Asoj, 5 marks - Video: [LECTURE 7 PIEZO ELECTRIC OSCILLATOR](https://www.youtube.com/watch?v=E92fVEiYX1M) — Sugandhi Muthukrishnan - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_1/engineering_physics/piezoelectric-oscillator-ultrasonic - Whole chapter: https://www.bctengineer.com/md/semester_1/engineering_physics/chapter-3 ## Question What is piezoelectric effect? Describe the construction of a piezoelectric oscillator for the production of ultrasonic waves. ## Short answer The piezoelectric effect is the development of opposite charges on certain crystals under mechanical stress (direct) and their deformation under an alternating electric field (inverse). A piezoelectric oscillator uses a quartz plate between metal plates connected across the L1-C tank circuit of a transistor oscillator; L2 provides positive feedback. Tuning C to match the crystal’s natural frequency produces resonance and ultrasonic waves. ## Answer #### Piezoelectric effect Certain crystals (quartz, Rochelle salt, tourmaline) develop opposite electric charges on opposite faces when they are compressed or stretched along a certain direction (direct effect). Conversely, when an alternating electric field is applied across them, the crystal alternately expands and contracts (inverse effect). If the frequency of the field equals the natural frequency of the crystal, it vibrates with large amplitude (resonance) and produces ultrasonic waves. Natural frequency of a quartz plate of thickness t: f = (n/2t) √(Y/ρ), n = 1, 2, 3 …, where Y is Young's modulus and ρ the density. A thin plate gives a high frequency. #### Piezoelectric oscillator (construction) A quartz plate is cut perpendicular to the electric (X) axis and held between two metal plates, forming a capacitor C1. This is connected in parallel with the coil L1 in the collector (tank) circuit of a tuned-collector transistor (or valve) oscillator. The coil L2 in the base circuit is coupled to L1 for positive feedback. ```text +Vcc | +--- L1 ---+--- C (variable) ---+ | | | | collector [quartz between transistor two metal plates, C1] | | | base <-- L2 (coupled to L1) | |__________________________ ____| ``` Tank circuit = L1 in parallel with the variable capacitor C and the quartz capacitor C1. #### Working 1. When the supply is switched on, the L1-C tank circuit oscillates at f = 1/(2π√(L1C)). L2, coupled to L1 by mutual induction, feeds back energy to the base and keeps the oscillation going. Since the crystal plates are connected across L1, an alternating voltage of the same frequency is applied to the crystal. 2. The crystal contracts and expands at this frequency. C is varied until the tank frequency equals the natural frequency of the crystal; resonance then occurs and the crystal vibrates with maximum amplitude. 3. The vibrating faces send ultrasonic waves of the same frequency into the surrounding medium (air or oil). 4. Feedback keeps the oscillation steady. Frequencies range from about 20 kHz up to several MHz. Advantages: stable frequency, easy to adjust, high-frequency output. Used for sonar, flaw detection and ultrasonic cleaning. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the inverse piezoelectric effect, an alternating electric field applied across a crystal makes it A. develop a steady charge B. alternately expand and contract C. melt D. become magnetic Answer: B. The crystal vibrates mechanically at the frequency of the applied field. (Key point: Inverse piezoelectric effect) 2. When does the crystal vibrate with maximum amplitude in the oscillator? A. When the tank frequency equals the natural frequency of the crystal B. When the supply is turned off C. When the capacitor is removed D. When the crystal is thick Answer: A. Variable C is adjusted until 1/(2π√(L1C)) matches the crystal's natural frequency, giving resonance. (Key point: Resonance condition) 3. What is the role of coil L2 coupled to L1? A. To heat the crystal B. To supply positive feedback and an alternating emf to the crystal plates C. To store ultrasonic waves D. To reduce the frequency Answer: B. Mutual induction feeds an alternating emf of the same frequency to the crystal and sustains oscillations. (Key point: Role of L2) 4. From f = (n/2t)√(Y/ρ), a thinner quartz plate gives A. a lower frequency B. no change in frequency C. a higher frequency D. no vibration Answer: C. Frequency is inversely proportional to thickness t. (Key point: Frequency vs thickness) ## Related in other subjects - [Piezoelectric materials and piezoelectric effect](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Apiezoelectric-effect-polarization) — Electrical Engineering Material, chapter 3, asked 2× - [Bunching, field effect and high power oscillator](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Abunching-field-effect-oscillator) — RF and Microwave Engineering, chapter 5 - [Bunching effect and klystron oscillator](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Abunching-klystron-oscillator) — RF and Microwave Engineering, chapter 5 - [Bunching effect and backward wave oscillator](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Abunching-effect-bwo) — RF and Microwave Engineering, chapter 5