L3281 STMicroelectronics, L3281 Datasheet - Page 7

• Manufacturer Part Number: L3281
• Description: LOW VOLTAGE TELEPHONE SPEECH CIRCUITS
• Manufacturer: STMicroelectronics

AC CHARACTERISTIC
The AC Impedance measured at line terminals is
equal to:
The value of the capacitor C1 must be In the
range of 22 F to 100 F.
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 K to allow
Figure 8: Equivalent Transmitting Circuit.
Figure 9: Equivalent Receiving Circuit.
Zm
R1
jwC1
1
/ /
R2 R3 Zb
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.
L3281
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