Telit GM862-QUAD/-QUAD-PY
Hardware User Guide
1vv0300692, Rev. ISSUE#0, - 04/10/2005
Reproduction forbidden without DAI Telecom written authorization – All Right reserved – Right of modification reserved
page 37 of 69
(if speaker impedance is bigger than 16
Ω
) or the GM862-QUAD ear port may be damaged (if
speaker impedance is less than 15
Ω
).
The other limitation of the speaker direct connection is the power output capability of the GM862-
QUAD which is limited and for some particular applications may not be enough.
For these reasons, when the power output of the GM862-QUAD is not enough or if the speaker
characteristics are different from the suggested, then it is preferable to use an amplifier to increase
the power and current output capabilities.
Again the output from the GM862-QUAD is bridged and both lines should be used, where possible,
as inputs to the power amplifier. This ensures a higher common mode rejection ratio, reducing the
GSM current busts noise on the speaker output.
In this case the EAR_MT lines from the GM862-QUAD should be AC coupled with a capacitor of
100nF.
It is always desirable to have a mute control on the amplifier, in order to turn it off while the device
is not sending signal to the output, in this manner the amplifier background noise which may be
audible during idle conditions is cut off.
A principle schematic may be:
The resulting gain and high pass cut can be obtained with the formula:
2
3
R
R
Gain
=
4
*
3
*
2
1
.
C
R
freq
π
=
[Hz]
