User's Manual

Telit GE863-QUAD / GE863-PY
Hardware User guide
1vv0300715, Rev. ISSUE#0, - 21/02/06
Reproduction forbidden without Telit Communication written authorization – All Right reserved – Right of modification reserved page 45 of 71
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/PY 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/PY 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/PY, 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/PY 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/PY 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.