Speaker And Microphone; External Amplification - Sony Ericsson Speakerphone/car kit hands free design Application Note

3.1 Speaker and Microphone

The speaker and microphone tested by Sony Ericsson for this Application
Note have the characteristics detailed in Table 1 and Table 2 below. In
addition, there is a 10nF ceramic, surface mount capacitor soldered
directly across the microphone terminals inside the microphone housing.
This helps prevent RF coupling into the microphone and demodulating the
transmitter bursts, which can cause an audible pulsating buzz.
Microphone Details
Sensitivity
Output impedance
Directivity
Current
consumption
S/N ratio

3.2 External Amplification

The diagram of Figure 1 provides a sample circuit suggestion using a 2W
Class-D Audio Power Amplifier (from MAXIM Integrated Products, part
MAX4295).
For most applications, sufficient audio volume will be achieved with a 2W
speaker driver into a 4Ω speaker. If louder volumes are required it is likely
that the GR47 internal echo cancelling algorithms will not be sufficient to
cancel speech echo (unless the microphone gain is reduced by an equal
amount). In addition, a 2W drive allows for supply voltages as low as 5V.
In order to achieve a small circuit footprint the sample circuit uses a 2W
Class-D power amplifier and a switch-mode supply. The switching nature
LZT 123 7895 R1A
Speakerphone/car kit hands free design
Speaker Details
Impedance
Speaker 110 x 65 x 56 mm
Dimensions
Table 1 - Speaker Details
-33 dB +/- 3 dB
680 ohms max.
-8 dB min
500 uA max.
40 dB min
Table 2 - Microphone Details
4 ohms
Conditions:
re. 1 V at 1 Pa, 1 kHz
1Pa, 1 kHz
<90 degree cone angle
0.1 Pa, A curve
6