User's Manual

GC864 Hardware User Guide

1vv0300733 Rev. 0 - 12/06/06

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 GC864.

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 GC864 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 GC864,

which is limited, and for some particular applications may not be enough.

For these reasons, when the power output of the GC864 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 GC864 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 GC864 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 that may be audible

during idle conditions is cut off.

A principle schematic may be: