User's Manual

GC864 Hardware User Guide

1vv0300733 Rev. 0 - 12/06/06

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 GC864 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 GC864, 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 GC864 “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 GC864 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 GC864 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 GC864 internal audio path from the external audio path.

Example of car kit amplifier schematic.