GE863 Datasheet, PDF (42/69 Pages) List of Unclassifed Manufacturers – Telit GE863-QUAD / GE863-PY Hardware User guide

Telit GE863-QUAD / GE863-PY
Hardware User guide
1vv0300697, Rev. ISSUE#0, - 21/11/05
Some amplifier require a low impedance load at high frequency in order to avoid auto
oscillation, this can be made with a capacitor (100nF) in series with a resistor (15Ω).
When designing your application, remember to provide an adequate bypass capacitor to the
amplifier and place it close to the power input pin of the IC, keeping the traces as short as
possible.
7.4 Hands Free earphone (low power) design
The same design considerations made for the handset are valid for the hands free earphone, the
only difference is that the external ear audio output level from the GE863-QUAD is 6dB lower
than the internal one, so the gain of the amplifier must be 6dB higher to provide the same audio
level.
7.5 Car Kit speakerphone design
For the car kit speaker phone function the power output requirement is usually at least 4W,
therefore an amplifier is needed to boost the GE863-QUAD output.
The design of the amplifier shall comply with the following guidelines:
• The input to the amplifier MUST be taken from the external audio path (EAR_HF+,
EAR_HF-) of the GE863-QUAD, because of its echo canceller parameters suited to a car
cabin use.
• The amplifier shall have a gain of 30-40 times ( 29-32 dB) to provide the desired output
power of 5-10W with the signal from the GE863-QUAD external audio output lines
(EAR_HF).
• If the amplifier has a fixed gain then it can be adjusted to the desired value by reducing the
input signal with a resistor divider network.
• The amplifier shall have a mute control to be used while not in conversation. This results in
two benefits: eliminating the background noise when not in conversation and saving power.
• The power to the amplifier should be decoupled as much as possible from the GE863-QUAD
power supply, by either keeping separate wires and placing bypass capacitors of adequate
value close to the amplifier power input pads.
• The biasing voltage of the amplifier shall be stabilised with a low ESR (e.g. a tantalum)
capacitor of adequate value.
NOTE: The GE863-QUAD audio path connected to the car kit hands free amplifier MUST be the
external one (EAR_HF), otherwise the echo cancellation will not be done due to the difference in the
echo canceller characteristics of the GE863-QUAD internal audio path from the external audio path.
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