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National Semiconductor New Amplifiers Improve Audio Performance and Sensitivity in Microphones

Amp in a Mic Transforms Market by Replacing Decades-Old Technology

The manufacturer says . . .

ChipCenter's Paul O'Shea says . . .

National Semiconductor Corporation announced at Electronica two new amplifiers that dramatically improve audio performance by replacing existing microphone technology with a revolutionary design solution. The LMV1012 and LMV1014 are the marketıs first "amps in a mic," integrated directly inside 2- and 3-wire electret condenser microphones (ECMs). These amplifiers replace the junction field effect transistor (JFET), for longer battery life and greater noise immunity, resulting in better microphone performance. These products are ideal for use in microphones in mobile handsets, headset accessories and other portable microphone applications. "These new products place National Semiconductor in a leading position in terms of capturing designs, particularly in the handset space," said Susan Inouye, senior industry analyst at Databeans Inc, in Reno, Nevada. "Handset production continues to migrate to lower cost manufacturing regions, increasing the importance of lower component count and simplicity of production." "With National Semiconductorıs amplifiers, which can be integrated directly into the ECM, BSE is able to create higher sensitivity microphones which significantly improve audio performance in applications like mobile phones and other communications devices," said Kingly Bang, corporate marketing manager with BSE Corporation, a leading Korean microphone manufacturer. "To optimize todayıs microphones, careful board design and other filtering components are necessary," said Dennis Swan, marketing manager for Nationalıs amplifier products group. "Also, until recently, the resulting packaged chip size did not allow amplifiers to fit into the increasingly smaller-sized ECMs. National recognized that a combination of innovative circuit design and packaging technology would make significant improvements to properties such as sensitivity, distortion and supply current, enabling ECMs with greatly improved performance and battery life." National Semiconductorıs LMV1012 is an amplifier for 2-wire ECMs. The built-in gain of 17dBV has been optimized for handset microphones. Additional benefits of the LMV1012 are improved RF immunity and low distortion. New ECMs using the LMV1012 offer THD of 0.1%, SNR greater than 55dB and supply current less than 240 micro amps. Applications for high-gain microphones include mobile communications, automotive accessories, cellular phones and PDAs. The LMV1014 is an amplifier for 3-wire, low current and low noise ECMs. This product draws a supply current of less than 40 microamps while providing a low output impedance of 200 Ohms. The LMV1014 provides a high power supply rejection ratio (PSRR) greater than 60dB and an SNR of greater than 55db. This device is designed for applications requiring low supply current and high noise immunity, such as BluetoothTM wireless technology communication products, microphone accessory products, handheld recording devices and PDAs. Both amplifiers are fully compatible with existing ECM standards. National Semiconductor Web Site

The genesis of National Semiconductor's chips, the LMV1012 & LMV1014, began as a way to get components out of the phone and make it smaller. These cell phones use electret condenser microphones (ECM) and National wanted to find a way to reduce the parts count and shrink parts size. The ECM will always be needed for microphones, but its audio input could be tweaked. First, they needed a manufacturing technology that could shrink any new designs to fit within the ECM canister. They found that the micro surface mount device fit the bill and it became a key piece of the puzzle, allowing a new design to fit within the canister limitations. Further integration to replace the junction field effect transistor (JFET) and getting the power supply rejection ratio under control with phantom biasing, was also part of the necessary technology improvements. The JFET has been a great solution for 20 years in the ECM, however, National thinks they have found a way to improve on the design. The ECM is used in almost every consumer and communications audio application with total yearly volume of well over one billion units. The most commonly used ECM consists of an electret microphone canister with a JFET inside acting as a buffer between the capacitive sensor and the output. JFETs are small, three-terminal devices. The small size of the JFETs is one of the main reasons that ECMs have also shrunk. The method of biasing and interfacing the ECM, called phantom biasing, involves setting up a load resistor of a few k-ohms and a series capacitor, and enables a very efficient ECM design with only ground and signal/bias connections. The main benefits of this type of ECM include its small size, low cost, and relatively low noise. However, the big issue is that the JFET canıt provide built-in gain. The combination of a single JFET and low-load resistor results in low gain (typically -3 dBV) and therefore, low sensitivity of the ECM. JFETs are also inherently non-linear (typical THD is 1-10%). The low sensitivity of an ECM results in small output voltages of the sensor (on the order of 0.1-10 mV average, and 100 mV peak). For such a microphone to work in something like a mobile phone, careful board design and additional filtering components are necessary to keep a reasonable signal-to-interference ratio. More complex amplifier circuitry can address the disadvantages of ECMs that use a single JFET - but that adds components. This new design from National makes it possible to apply commonly used amplifier circuit techniques to interface and amplify the signals from an ECM. For example, applying an operational amplifier (that has feedback) with high voltage gain will achieve improvements to sensitivity and linearity. Using an ECM, with integrated pre-amplification in a design will result in a better signal-to-interference ratio and better THD of the audio input. This reduces the design effort and external components of the audio input, thereby reducing development and production cost. National Semi knew that the answer wasn't complete with just a better design - it also needed to make the design easy to implement. That's why a two-wire direct replacement of JFET ECMs was important. The resulting product, the two-wire LMV1012 has about an 18 dBV gain increase over the same ECM that uses a JFET inside. The LMV1014 is a three wire product for applications like Bluetooth and is for extremely low-power and low-current consumption applications. It has three wires to help control noise and consumes only 38 micro amps, which means that the battery can be left on without concern about draining the battery. These products will enable you to remove external op amps used to boost the gain in microphone designs, and you can remove the shunts and RC filters that were used for reducing noise. The large external gain block in the baseband or external op amp to create the gain will also no longer be needed. What about power consumption? Itıs approximately the same as the JFET. The LMV1012 uses 170-200 micro-amps while a similar size canister and JFET has a power consumption in the low 200 micro amp range. The LMV1014 has significantly lower power consumption of 38 micro amps and also has a gain that is about 20% better than a JFET. It is true that manufacturers of JFETs can make units with the same 38 micro amp power consumption as the LMV1014, but they would have to screen for the best products to find them and that process will cost you. Price is also an issue and according to National Semiconductor, a JFET costs about $0.05 to $0.08 cents in lots of one-thousand, compared to the $0.39 to $0.49 cents for the LMV1012 and LMV1014. However, an additional ten to twenty cents for design time and real estate reduction on the PCB can be saved immediately. In the past, designers had to use external components and use some of their precious baseband ASIC (one of the most expensive parts) to increase the JFET microphone gain. Now they donıt need the space on the baseband ASIC and can shrink the size of the baseband, therby reducing system cost and ultimately, the cost to the consumer. The LMV1012 is available now in 4-bump micro SMD packaging and is priced at $0.39. The LMV1014, available now in 4-bump micro SMD packaging, is priced at $0.49. Both prices are in 1000-unit quantities. Product Folders: LMV1012 LMV1014

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