Hello, I am currently involved with some hearing research and need to amplify a signal from the tiny Knowles FG-3329 microphone capsule (placed in the ear canal).
We have the capsule, a (very good and clean) studio preamp and a AD converter. However, the output impedance of the mic is way too high for a pro-audio preamp, with a nominal value of 4.4kOhm (2.8 - 6.8 kOhm).
As such, I am now into Day 3 of my electronics career (!), having blitz-read Tom Duncan’s electronics text and scoured forums and datasheets for hours. I really need some advice on selecting the appropriate TI opamp and designing this schematic.
So here’s the summary of where things stand.
Options for solution:
Since I’m new to all this, I feel might be safer to take route 1 with a battery to keep my scope for error as small as possible. However, route 2 is attractive as the mic would be self contained, small with provide invariant amplification.
I’ve managed to find the circuit diagram of the capsule IC which is just a MOSFET. The IC designer recommends having Vdd = 3V and adding another 22kOhm load resistor to -3V to get the required dynamic range.
My first instinct was to build a circuit with 3V coin cell, add the extra resistor to the microphone negative and use a TI opamp to build a simple non-inverting opamp buffer. If I was capable, I would love to have built a (fixed gain?) phantom powered amp using a high quality audio IC amp but I fear this may be beyond me and/or give inferior sonic quality.
Could I ask the community for any recommendations of opamps/ICs, circuits and how they might need adjusting to suit my needs?
Mic data: http://www.knowles.com/search/prods_pdf/FG-23329-C05.pdf
Mic IC schematic: http://www.draftonline.co.uk/downloads/FG-internal-1.PDF
Many thanks for reading!
Wow! That's pretty good after just three days!
There are always a lot of trade-offs in this stuff, I suggest you forge ahead, breadboard something up, and see if it suits your needs.
Audio Applications Engineering Manager
Dallas, TX USA
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