Philips Semiconductors
Versatile telephone transmission circuit with
dialler interface
Product specification
TEA1065
Automatic gain control
Automatic compensation of line loss is obtained by connecting a resistor (R6) between AGC and VEE. The automatic gain
control varies the gain of the microphone amplifier and receiving amplifier in accordance with the DC line current (see
Fig.12). The control range is 5.9 dB; this corresponds to a line length of 3.5 km of twisted pair cable (see Fig.11). The
DTMF gain is not affected by this feature.
If automatic line loss compensation is not required the AGC pin can be left open-circuit, the amplifiers then give their
maximum gain.
handbook, full pagewidth
34.8 Ω 75 µH
13 nF
34.8 Ω
75 µH
75 µH
24.3 nF
34.8 Ω
24.3 nF
75 µH
34.8 Ω
MBA572
Fig.11 Typical 0.5 km line cell model used for automatic gain control optimization.
handbook, full pagewidth
1
∆Gv 0
(dB)
−1
R6 = ∞
MBA549
−3
−5
R6 = 86.6 kΩ 118 kΩ 147 kΩ
187 kΩ
−7
0
20
40
60
80
Iline (mA)
Fig.12 Variation of gain as a function of line current with R6 as a parameter; R9 = 20 Ω.
March 1994
11