Telit Wireless Solutions GE863-QUAD - Manual (Page 49)

GE863-QUAD
GE863-PY
1vv0300715 Rev. 1 - 19/09/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 49 of 79
8.6 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 “Ear_HF” audio path 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 “Ear_HF” audio output lines.
• 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 stabilized with a low ESR (e.g. a tantalum) capacitor of
adequate value.
NOTE: The GE863-QUAD / PY audio path connected to the car kit hands-free amplifier MUST be
“Ear_HF” one, otherwise the echo cancellation will not be done due to the difference in the
echo canceller characteristics of the GE863-QUAD / PY internal audio path from the external
audio path.
Example of car kit amplifier schematic.