Speech instruction is a kind of popular function in a lot of application, also be one of advantages that allow a product to have competition ability of poor dissimilation market. Microphone is the main component with any indispensable systems that are based on speech or speech, and halt extremely body microphone depends on bulk the common choice that small, low cost and high-powered characteristic made this kind of application.
The article around high-powered, cost sensitive model the theme of circuit series article, the be stationed ining that for everybody introduction bulk minimum, cost optimizes extremely the design of preamplifier of microphone of type of system electric capacity. This design uses TLV9061, this is the operation amplifier with the smallest industry (Op Amp) , use 0.8mm×0.8mm to exceed small appearance not to have down-lead (X2SON) encloses a technology. Halt extremely if the graph is shown 1 times,the circuit of body microphone amplifier is configured.
Graph 1: Be stationed in in phase extremely circuit of body microphone amplifier
Great majority is stationed in extremely body microphone uses transistor of knot field effect (JFET) has in-house buffer, JFET is used resistor is pulled to undertake slanting on 2.2kΩ buy. Component of microphone of sound wave shift, the JFET that brings about electric current to flow into microphone interior leaks extremely. JFET leaks extremely electric current produces voltage drop on R2, this voltage drop communicates coupling, slant the IN+ that buy receives operation amplifier repeatedly to intermediate power source cites a base. Operation amplifier configuration is the inphase amplifier circuit that leads the wave that connect filter. The output range that uses expectant input signal n and place to need and answer, you can be calculated the gain of circuit and frequency are answered.
Let us see the design of give typical examples of a circuit, use at + 3.3V power source, the input is 7.93mVRMS, output signal is 1VRMS. 7.93mVRMS correspondence is inputted at having the 0.63Pa sound level of microphone and – 38dB sound pressure level (SPL) sensitivity norms. Bandwidth target is the bandwidth of common speech frequency 300Hz deliver 3kHz.
Formulary the 1 transfer function that displayed to concern between definition VOUT and AC input signal:
The input signal n that 2 basises expect formula and the gain that requires output n calculation wants:
The 10kΩ that chooses a level combats feed block, use formula 3 computation R6:
The attenuation in wanting to want place bandpass falls from 300Hz to 3kHz, will go up please (FH) and below (FL) ends frequency setting requires bandwidth in place besides (formulary 4) :
FL of choice C7 setting ends frequency (formulary 5) :
Choice C6 ends with installing FH frequency (formulary 6) :
Should end input signal frequency is installed enoughly low in order to make low frequency sound wave still can be passed, choose C2 to end in order to realize 30Hz please frequency (FIN)(formulary 7) :
Graph the 2 measurement that showed microphone preamplifier circuit transfer function. Because be connected high filter and low a tape between filter is wide with attenuation, smooth belt gain achieves 41.8dB or 122.5V / V only, slightly under the target.
Graph 2: The transmission function of microphone preamplifier
The X2SON that uses TI encloses a technology to install circuit 6mm diameter to be stationed in extremely the reverse side of body microphone. Because install dimension limitation, need uses very small operation amplifier: Of TLV9061 occupy an area only 0.8mm×0.8mm. In addition, resistor of 0201 small size and capacitance utmost ground reduce printed circuit board (PCB) area, you also can use less resistance to reduce this area further. Printed circuit board if layout pursues 3 show 4 times with the graph.
Graph 3: Installation 6mm diameter is stationed in extremely the microphone preamplifier layout of body microphone the reverse side
Graph 4: The three-dimensional view that PCB devises, showed the different point of view of microphone and PCB
You can adjust afore-mentioned design stage, in order to satisfy different microphone sensitivity requirement. When the small-sized amplifier such as use TLV9061 undertakes designing, notice to consult “ uses TI X2SON to enclose the layout in undertaking design and be productioned please optimal practice ”