![]() ![]() This former design idea was published in the Octoissue of EDN magazine.I'm interested in building a preamp for a piezo contact mic like. Step response at the OUTR output of IC1 in Figure 1, while that device is driving a WM-R57A piezoelectric speaker. The network maintains stability by maintaining a minimum high-frequency load of about 10Ω at the device output.įigure 4. In that case, you can isolate the speaker's capacitance from the amplifier by adding a simple inductor/resistor network in series with the speaker (shown within the dotted lines on Figure 1). IC1 remains stable with the speaker shown ( Figure 4), but a speaker with different characteristics might cause instability. THD+N vs frequency for the Figure 1 circuit. output voltage for the Figure 1 circuit.įigure 3. Because the speaker appears to the amplifier as a capacitor, the speaker impedance decreases as frequency increases, drawing larger currents from the amplifier.įigure 2. ![]() Note that THD+N increases as frequency increases in both graphs. The circuit was tested with a Panasonic WM-R57A piezoelectric speaker, yielding the THD+N curves shown in Figures 2-3. To ensure low distortion and good matching, set the left-channel gain using precision resistors. Configured as a unity gain follower, the left channel inverts the output of the right channel and drives the other leg of the speaker. It sets the gain of the device, drives one side of the speaker, and provides a signal to the left channel. In the BTL configuration, IC1's right channel serves as the master amplifier. Configuring IC1 as a bridge-tied load driver (BTL) doubles the maximum swing at the load to 12V P-P. The internal ☓V supply allows each output of IC1 to swing 6V P-P. IC1 (the MAX4410) is a specialized device that combines a stereo headphone driver with an inverting charge pump that derives a negative -3V supply from the positive 3V supply. This bridge-tied-load (BTL) configuration multiplies the amplifiers' voltage-swing capability. One solution is IC1 in Figure 1, which can be configured to drive a piezoelectric speaker with as much as 12V P-P, while operating from a single 3V supply.įigure 1. But they require voltage swings of 8V P-P, which exceeds the supply voltage typically available from conventional battery-driven amplifiers. Low-profile piezoelectric speakers provide compact, quality sound for portable electronics devices. ![]()
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