Rainbow Electronics GM862-QUAD-PY - Manual (Page 53)

GM862-QUAD / PY Hardware User Guide
1vv0300748 Rev. 5 - 20/09/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 53 of 68
9.4 Handset Earphone Design
As seen previously, a 16Ω earpiece can be directly connected to the output pads EAR_MT+ and
EAR_MT- of the GM862-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: speakers 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 GM862-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 GM862-
QUAD / PY which is limited and for some particular applications may not be enough.
For these reasons, when the power output of the GM862-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 GM862-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 GM862-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: