Nonlinear distortion is not important in these applications. When you drive a piezoelectric load, its natural resonance removes any frequency components other than the fundamental. This circuit combines a modulator and an amplifier into a single stage. The compactness of the circuit makes it appropriate for portable-system applications.

The load is in series with two switches (Figure 1). The input signal controls S₂, S₃ controls S₁, and the modulating signal controls S₃. This circuit’s modulation operation is similar to that of an AND gate. The switches must have internal resistance to dissipate the harmonics that the resonant load reflects. This circuit uses transistors Q₁ and Q₂ as switches, although they operate in the active region (Figure 2). Their operation resembles that of controlled resistors, and they perform voltage and current amplification. You drive Q₂ with a 42-kHz signal that matches the load’s resonance. You modulate the Q₃ transistor with a periodic low-frequency impulse signal. These impulses open Q₃, which drives Q₁ and Q₂ toward saturation. When Q₃ opens, it drops the voltage across the base of Q₁, blocking the state of Q₂. Q₁ and Q₂ operate conjointly; Q₁ conducts only when Q₂ is conducting. You can view this scheme as a differential amplifier in which the signal in one branch controls the load of another branch.

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Q₂ and Q₃ operate over large signals yet remain in the active region most of the time. The resistor values in the base and collector of Q₁ are critical. When the frequency of the signal is higher than the load’s resonant frequency, D₁ protects Q₁ from the effects of L₁ and of harmonics on the LC circuit. The collector voltage has a spectrum rich in harmonics due to the nonlinear behavior of transistors. This characteristic is not a serious disadvantage because the resonant load removes the harmonics.