L3281 데이터 시트보기 (PDF) - STMicroelectronics

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L3281

LOW VOLTAGE TELEPHONE SPEECH CIRCUITS

STMicroelectronics 

AC CHARACTERISTIC

The AC Impedance measured at line terminals is

equal to:

Zm

=

(R1

+

1

jwC1

)

/

/

(

R2 +

R3

+

Zb

)

The value of the capacitor C1 must be In the

range of 22 µF to 100 µF.

The external resistor R1 can be replaced by a re-

sistor/capacitor network in order to realize a com-

plex Impedance Zm.

TRANSMITTING CIRCUIT

The first block of the TX stage is basically a differ-

ential amplifier which converts voltage to current.

The inputs are internally polarized at 300 mVdc.

The differential Input impedance is 60 KΩ to allow

Figure 8: Equivalent Transmitting Circuit.

L3281

a good matching to microphone. The AGC in TX

is function of voltage at pin 14 in order to de-

crease to max gain of 5.5dB to 6.0dB when the

line current increases.

RECEIVING CIRCUIT

Fig.9 shows the equivalent receiving circuit. The

differential input of RX signal across R2+R3 is

transferred to the AGC block when the mute sig-

nal (pin 10) is not active.

The AGC in RX is a function of the voltage at pin

14 and decreases the gain when the line current

increases (5.5dB to 6.0dB).

The final stage is a single ended amplifier with

low output impedance optimized to drive mag-

netic/dynamic transducers.

Figure 9: Equivalent Receiving Circuit.

7/10

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