Overview
The drive methods for ultrasonic sensors used for transmission are classified as self-excited and externally-excited.
Self-excited drive
A self-excited driver circuit, like a quartz resonator, uses the ultrasonic sensor's own resonant characteristics to produce oscillation near its resonant frequency.
The following figure shows a self-excited transistor oscillator circuit where the ultrasonic sensor frequency is 40 kHz. Figure 9-14(a) is a Colpitts oscillator. The ultrasonic sensor oscillates at an inductive frequency. This oscillation frequency does not coincide with the series resonance frequency due to the influence of the anti-resonance frequency. Specifically, adjustments of C1 and C2 affect the oscillation. Figure 9-14(b) shows a self-excited transistor oscillator with an oscillation control terminal. Because the ground from Figure 9-14(a) is connected to the emitter of transistor VT2, oscillation stops when VT2 is cut off.
Op amp self-excited oscillator
A self-excited op amp oscillator/amplifier has an oscillation frequency close to the series resonance frequency, so its efficiency is significantly higher than that of the self-excited transistor oscillator. The op amp used in the example circuit is the MC34082. In practice, any op amp with a slew rate greater than 10 V/μs can be used.
