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Telit GE863-QUAD / GE863-PY
Hardware User guide
1vv0300697, Rev. ISSUE#0, - 21/11/05
Reproduction forbidden without Telit Communication written authorization – All Right reserved – Right of modification reserved
page 32 of 69
6.4 Microphone buffering
As seen previously, a microphone shall be connected to the input pads of the GE863-QUAD
through a buffer amplifier that boosts the signal level to the required value.
Again the buffered microphone circuitry can be balanced or unbalanced. Where possible it is
always preferable a balanced mic. solution. The buffering circuit shall be placed close to the
microphone or close to the microphone wire connector.
6.4.1
Buffered Balanced Mic.
A sample circuit can be:
This circuit has a gain of 15 times (+24 dB), and is therefore suited for the internal mic. input,
having a microphone with a sensitivity close to the suggested one (-45 dBVrms/Pa), if the used
microphone has a different sensitivity, or if the buffer is connected to the external mic. input,
then a gain adjustment shall be done by changing resistors R604-R605 and R606-R607 and as a
consequence the capacitors C636 and C637 to maintain the bandwidth 150-8000 (-3dB).
The buffer gain is given by the formula:
607
606
605
604
R
R
R
R
Gain
=
=
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