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GM862-GPS Hardware User Guide
1vv0300728 Rev. 0 - 27/04/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 44 of 55
6.6 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 GM862-GPS 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 GM862-GPS,
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 GM862-GPS “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 GM862-GPS 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 GM862-GPS 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 GM862-GPS internal audio path from the external audio
path.
Example of car kit amplifier schematic.