Philips Semiconductors
Low voltage versatile telephone transmission circuit
with dialler interface and transmit level dynamic limiting
Product specification
TEA1064B
Automatic gain control input AGC
Automatic compensation of line loss is obtained by
connecting a resistor (R6) between AGC and VEE1.
This automatic gain control varies the gain of the
microphone amplifier and receiving amplifier in
accordance with the DC line current. The control range is
6.1 dB; this corresponds to a 5 km line of 0.5 dB diameter
copper twisted-pair cable (DC resistance = 176 Ω/km,
average attenuation = 1.2 dB/km). The DTMF gain is not
affected by this feature.
The value of R6 must be chosen with reference to the
exchange supply voltage and its feeding bridge resistance
(see Fig.15 and Table 1). Different values of R6 give the
same line current ratios at the start and the end of the
control range. If automatic line-loss compensation is not
required the AGC pin can be left open-circuit, the
amplifiers then provide their maximum gain.
handbook, full pagewidth
0
∆Avd
(dB)
−2
R6 = ∞
MLB030
−4
R6 = 66.5 kΩ 93.1 kΩ 118 kΩ
−6
0
20
40
60
80
100
120
140
Iline (mA)
Fig.15 Variation of gain as a function of line current with R6 as a parameter; R9 = 20 Ω.
Table 1 Values of R6 giving optimum line-loss compensation at various values of exchange supply voltage (Vexch) and
exchange feeding bridge resistance (Rexch); R9 = 20 Ω
Rexch (Ω)
400
600
800
1000
R6 (kΩ)
35
84.5
66.5
x
x
Vexch
(V)
48
118
93.1
77.8
66.5
60
x
x
97.6
84.5
March 1994
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