GC864-QUAD-C2/PY-C2 Hardware User Guide
1vv0300744 Rev.1 - 24/10/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 20 of 27
“Ear_HF” Power Buffers path
•
line
coupling:
AC
•
line
type:
single
ended
•
output load resistance :
≥
14
Ω
•
internal
output
resistance:
4
Ω
( >1,7
Ω
)
•
signal bandwidth:
150 - 4000 Hz @ -3 dB
•
max. single ended output voltage
656 mV
rms
(typ, open circuit)
•
SW
volume
level
step
-
2
dB
•
number of SW volume steps
10
10.2 Microphone Biasing
The electret microphones usually needs a biasing voltage to work properly.
You can use the on-board circuitry on external biasing network.
10.2.1 On-board default biasing
In order to eliminate the noise coming from the power lines, the microphone bias voltage is obtained
from a voltage source filtered by a capacitor multiply circuit (Q101) and connected through
R104/R105
to
line or
R104/R106
to
line.
The voltage source comes from a dedicated voltage regulator (U102), supplied directly by VBATT.
You can use also the inner VAUX1 source; in this case U102 must be removed and R112 inserted.
