Ceramic and piezoelectric speakers feature very low dissipation losses and compact size. They're an attractive alternative to moving-coil speakers in applications ranging from flat-screen TVs and monitors to cell phones. Their low dissipation arises from the nature of the load they represent, which is essentially capacitive. Typical values (depending on speaker size) range from 20 to 3000 nF.
Driving a piezoelectric speaker requires an amplifier that can handle such loads without going into oscillation. Single-ended drive is one possible solution. But the more common bridge-tied load (BTL) gets four times the output power of a single-ended driver from a single power-supply rail.
A BTL amplifier consists of two single-ended amplifiers, one connected to each end of the load (see the figure). One sets the gain, and the other acts as a unity-gain inverter. The ratio of RF to RIN sets the closed-loop gain of the first amplifier, while the other two resistors fix the second amplifier's gain.
The output from amplifier A serves as the input to amplifier B, so the signals applied to the speaker are identical in magnitude, but out of phase by 1808. Also, because the outputs are biased at half the supply voltage, there is no net dc voltage across the load. This eliminates the need for an output coupling capacitor.
One catch is that in contrast to moving-coil speakers, piezoelectric speakers are voltage-driven rather than current-driven. Generally, 10- or 30-V p-p signals drive piezoelectric speakers, an inconsistency with battery operation. Even in ac-powered applications, such as big-screen TVs, it implies a separate supply.
But that's not necessarily the case. National Semiconductor's LM4960, LM4961, and LM4802B single-chip devices integrate a boost converter with an audio power amplifier to meet the need for high driving voltages. The LM4802B and LM4961 audio speaker drivers deliver 12 V p-p for ceramic speaker applications. The LM4960 provides 24 V p-p for piezoelectric speakers, and it does so with with less than 1% total harmonic distortion plus noise while operating on a 3-V battery supply.
The three amplifiers have a low-power shutdown mode, an internal thermal shutdown protection mechanism, and slow turn-on/off "pop and click" circuitry. The LM4961 for ceramic speakers adds a low-power mode for earpieces and a high-power mode to drive the ringtones and hands-free speakers. The LM4802B goes for $1.65, the LM4961 for $2.10, and the LM4960 for $2.50, all in 1000-unit lots.
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