Tiny amplifiers improve electrets
By Joshua Israelsohn -- EDN, January 9, 2003
People have been talking to inanimate objects for centuries, but the objects have only recently been listening. Unfortunately, those most eager to do so—mobile phones, PDAs, and laptop computers—have precious little room for the ears. As the electret microphones these devices employ shrink to accommodate smaller and particularly thinner case dimensions, little room remains for the output-buffer devices the capacitive microphones require.
Electrets are among the smallest condenser microphones available, with mounting heights as low as 2 mm. Allowing for the plates that form the acoustic sensor, spacers, and a shell, the package thickness of the internal buffer device—usually a single JFET—needs to be as small as 0.5 mm (Reference 1). Unfortunately, the JFET and the bias resistor that feeds it combine to offer rather lackluster performance: The typical buffer gain is –3 dB, harmonic distortion is usually greater than 1%, and the supply current varies substantially from part to part due to the threshold voltage tolerance.
National Semiconductor's LMV1012 and LMV1014 electret amplifiers improve microphone gain and distortion and result in consistent maximum supply current as low as 325 and 55 µA, respectively, over temperature. Both devices are available in four-bump micro-SMD packages that measure 0.5 mm high and 1.133 mm on a side (Picture). The LM1012 is designed for two-wire electret circuits and serves as a direct replacement for a JFET buffer. It provides 15.6-dB typical gain and 0.09% THD when operating on a 2.2V supply. At such a low operating voltage, the 1012 can accommodate signals as large as 100 mV p-p and offers a –3-dB bandwidth of 65 Hz to 95 kHz and a 60-dB SNR.
The LMV1014 implements a three-pin electret interface that separates the output signal from the supply, thereby allowing a significant reduction in the operating current. The amplifier, which is specified over a 1.7 to 5V range, provides 6-dB gain and handles 300-mV p-p input swings. The typical THD is 0.11%, and the SNR is 58 dB, even under low-line supply conditions. The amplifier's –3-dB bandwidth extends from 70 Hz to 120 kHz.
Both devices offer robustness suitable to tough portable applications, including a –40 to +85°C operating-junction- temperature range and ESD tolerance to 2500V (human-body model) and 250V (machine model). Prices start at 39 cents and 49 cents (1000) for the LMV1012 and LMV1014, respectively.
National Semiconductor, www.national.com.
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