User Manual

GE863-PRO

Hardware User Guide

1vv0300773a Rev. 1 DRAFT -

24/04/08

9.6 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-PRO3 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-PRO3,

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-PRO3 “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-PRO3

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-PRO3 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-PRO3 internal audio path from the external audio

path.

Example of car kit amplifier schematic.