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SAN3022

SA2531/2 Single Chip Telephone: Using dynamic microphone as tone ringer

South African Micro Electronic Systems 

SAN3021

5 General description

Dynamic handset microphones have been widely replaced by electret microphones, mainly because of higher

output level, low EMC sensitivity and price issues. Therefore, the standard Single Chip Telephone application,

AN1500 is based on using an electret handset microphone.

However, there are situations where usage of a dynamic microphone is still desired, e.g.

• when updating an existing design , using a dynamic handset microphone

• when taking advantage of the higher speech quality of dynamic microphones

• when using the dynamic microphone as tone ringer

6 Hardware configuration

Generally, because of the lower output level of a dynamic microphone (≈10dB less than an electret microphone)

the microphone signal must be amplified by a simple 10dB amplifier before it can be fed into the microphone

inputs, M1 and M2 of the SA2531/2.

In the following schematic, two basic considerations when using dynamic microphones are shown:

1. A single ended circuit, which uses the dynamic microphone for both microphone and tone ringer.

2. A differential circuit, which is fully balanced, to replace an electret microphone.

6.1 Using the Dynamic Microphone as Tone Ringer (Schematic :A)

6.1.1 Ringer part

The piezo tone ringer used in AN1500 can be replaced by a dynamic microphone, when close attention is paid to

the following considerations :

Since a maximum sound pressure is required in ringing mode, the maximum DC current and AC level

for the type of dynamic microphone used must not be exceeded.

To limit the output of the ringer amplifier Q4, the zener diode D4 should be reduced to 15V . R18 limits the output

current, it can also be used to set the ringer volume.

Since the microphone (= the ringer) is connected single ended to VDD, any noise on VDD would be amplified by the

microphone amplifier, causing higher noise for the transmitted signal. Therefore, D6 and C10 are added to

reference the microphone amplifier input to VSS (over C10) but at the same time keep the DC level above VDD in

ringing mode.

The ringer cannot be referenced to VSS, because it would discharge VDD during ringing!

With the application shown, the anode of D6 will be VDD+0.7V, because during ringing VDD is supplied over the

ringing capacitor. (not shown).

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