Telit Wireless Solutions GE863-QUAD - Manual (Page 47)

GE863-QUAD
GE863-PY
1vv0300715 Rev. 1 - 19/09/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 47 of 79
8.4 Handset Earphone Design
As seen previously, a 16Ω earpiece can be directly connected to the output pads EAR_MT+ and
EAR_MT- of the GE863-QUAD / PY.
This solution is often the more cost effective, reducing the components count to a minimum. There are
several limitations to the use of this solution: speaker direct connect imposes the speaker
characteristics to be almost exactly the suggested ones, otherwise the power output may be reduced
(if speaker impedance is bigger than 16Ω) or the GE863-QUAD / PY ear port may be damaged (if
speaker impedance is less than 15Ω).
The other limitation of the speaker direct connection is the power output capability of the GE863-
QUAD / PY which is limited and for some particular applications may not be enough.
For these reasons, when the power output of the GE863-QUAD / PY 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 GE863-QUAD / PY 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 GE863-QUAD / PY should be AC coupled with a ceramic
capacitor of 100nF (or bigger ) .
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: