New electroacoustic product interface technology

Today's major electroacoustic products include high-performance microphone/handle microphones and balanced armature receivers and silicon microphone/ultrasonic sonic sensors.

Half of the annual sales of microphones (also known as microphones or transducers) are very inexpensive low-end microphones for the toy market and other applications where size and performance parameters are less stringent. The other half is the portable, high-end application market, such as mobile phones, mobile phones, digital cameras, laptops and so on. Mobile phones are seen as the fastest growing segment of the microphone market. The size of mobile phones is getting smaller and smaller, and the functions are getting more and more, so the requirements for the performance of next-generation microphones are constantly increasing.

Microphone products should have the following characteristics: small size makes the product design more special; waterproof, splash-proof products are designed for outdoor use, adapt to high humidity environment; wide frequency range products (including ultrasonic frequency band), can be used for voice, music and detection Equipment product design; superior anti-noise design (including waterproof) can be used in high noise environment; low power consumption products solve power consumption problems; balanced armature microphone (BJ series) provides its own signal source; piezoelectric ceramic microphone adapts to different Harsh environment, high sensitivity to vibration.

It can be said in general that the biggest feature of modern microphone products is the placement (SMD) and digitization and miniaturization of MEMS (Micro Electro Mechanical System) technology.

The following introduces the new electroacoustic product interface technology such as electret condenser microphone, MEMS (micro electro mechanical system) SMD silicon crystal microphone, digital microphone and ultrasonic acoustic patch sensor.

Electret condenser microphone

Electret condenser microphones (ECMs) are commonly used in electronics and communications products. ECM provides good acoustic performance and reliability under non-rigid environmental conditions. Its main features: high-performance electrical specifications; industry standard size; omnidirectional, single-point and noise-cancelling (bidirectional); integrated capacitors for RF filtering; terminal diversity - non-welded, Pin and pad types; For a wide range of applications, including wired/wireless headsets, corded/cordless phones, PDAs, notebooks and mobile phones.

This microphone consists of a diaphragm, a backing plate and an electret layer. The movable diaphragm and the fixed back plate form the two plates of the variable capacitor. The electret layer stores a fixed charge equivalent to a capacitor voltage of approximately 100V. The sound pressure causes the diaphragm to vibrate, thereby changing the capacitance of the microphone. Since the number of charges distributed across the capacitor is constant, the voltage across the capacitor changes as the capacitance changes, according to the following capacitor charge formula:

Where Q is the charge, C is the capacitance, and V is the voltage. As the sound pressure changes, the capacitance increases or decreases slightly (ΔC), thereby causing the voltage to decrease or increase proportionally (ΔV).

Microphones in mobile applications are very small, usually 3mm~4mm in diameter and lmm~1.5mm in thickness. Therefore, their capacitance is also quite small, typically 3pF to 5pF, and in some cases even as small as 1pF.

Knowles AcousTIcs' MB3015 is the smallest ECM microphone in this class. Suitable for design applications that require ultra-compacting. The internal structure and external schematic diagram are shown in Figure 1.

Figure 1 Schematic diagram of the internal structure and external connection of the electret condenser microphone

If the capacitive microphone produces insufficient signal drive capability, a buffer or amplifier is required before further processing of the signal. Traditionally, a simple junction field effect transistor (JFET) input amplifier has been used to achieve preamplification of such a microphone. With the improvement of the ECM micromechanical process, the microphone volume is getting smaller and smaller, and the capacitance is also decreasing. Because standard JFET amplifiers have considerable input capacitance, causing significant loss of signal from the microphone unit, JFET amplifiers are no longer suitable for microphone requirements.

Improvements in amplifier circuits have been driven by improvements in CMOS manufacturing processes. There are many benefits to replacing JFET amplifiers with CMOS analog and digital circuits. Compared to traditional JFET amplifiers, preamplifiers implemented in modern sub-micron CMOS processes offer several advantages: reduced harmonic distortion, easier gain settings, and versatile modes, including low-power sleep mode, analog-to-digital conversion, Enables the microphone to directly output digital signals, greatly improving the quality of the sound.

Improved anti-interference ability.

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