SAN3021

3/5

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6.1.2 Microphone Amplifier

Q5 is a simple “classic” amplifier, which amplifies the single ended signal by

≈10dB. Gain can be adjusted by

varying R21. R23 is the feedback resistor to set a stable DC bias point. R24 and R25 are setting the base voltage.

The transistor type used (in this case BC549) should be a general purpose low noise transistor.

The single ended signal is decoupled by C17. R26 is required to limit the input current to the amplifier during

ringing.

C15 and C16 are EMC capacitors to limit the bandwidth of the amplifier. Frequencies >10kHz are being attenuated.

The amplifier is supplied by (the initial electret microphone supply) R20 and C14. C14 should be increased to

220µF. Since this supply is derived from the line output, there will be some small ripple from the transmitted signal

at C14.

In order not to form a positive feedback loop from LI over the microphone supply, Q5 and the internal microphone

amplifier (M1/M2 to LI) , care must be taken to connect M1/M2 inputs in the correct manner as shown in the

application.

With this connection, a negative feedback loop is formed, rejecting any ripple on C14.

Because of the unbalanced, single ended structure of this amplifier, its input is very sensitive. Therefore it

is recommended to use this application only in one-piece telephones with shortest possible wires

between microphone and amplifier !

6.2 Amplifier for General Purpose Applications (Schematic :B)

This amplifier is similar to the amplifier described in pt. 6.1, however the microphone is balanced (RR23B and

RR26 = 2k

Ω).

Amplifier gain = RR21 / RR23B = 3:1 = 10dB.

This application can directly replace an electret microphone. Because of the differential, balanced structure of the

amplifier input , it is also suitable for longer distances between microphone and amplifier, e.g. corded handsets.