PBL 3853
Expression for DC-characteristics. from figure16, (empirical).
VF
= V
L
− V
L
−1. 3 − V DC 0.07 10−3 + 0.6 10−3
R
1
+R
2
....................................................................................8
(
)
(
)
(
)
VF
− 2.0 / 45 + 0.14
(
)
.......................................................................................................................................9
VX
=
0.18
5.8 V −1
X
R
R
12
13
0.65
(
)
...........................................................................................................................10
IL
=
1+
+
R
5
R
13
If the resistors RA and RB are connected in the circuit as in figure 16, the VX
will have somewhat more complex expression:
V
F
− 2.0 / 45 103 + 0.14 10−3 + Z V
L
/ 2.5 103R
A
(
)
(
)
(
)
................................................................................11
V
X
=
=
2
0.18 10−3 +1/ 2.5 103 − Z 2.5 103
(
)
1
1
1
0.65
+
+
Z
RA
RB
2.5 103
If the resistors RA and RB are set to infinity in this expression it will take the same form as above.
NOTE
Telephone
Line
I
L
I
Load
1. These expressions are
built on the asumption that the
I
1
βof the transistors is infinite
and that the influence of
capacitors except C2 can be
neglected.
R12
R13
γ
-
Z
p
ZLine
V
L
+
2. These expressions are built
on an ideal situation and are
only for help to understand
α
δ
V
+
-
V
R5
Telephone
set side
Line side
Active impedance
section
a
Passive impedance
section
PBL 3853
16
T
R
x
x
Z
Z
1
2
15
2
Figure 16.
R
R
7
b
R
11
C
6
R
R
8
MUTE and Cut-off Function
R
Z
R
10
5
bal
C
6
9
The receiver mute (pin 17) is a cut-off function. The normal
receiver amplifier is cut-off but a second amplifier is activated
to let the DTMF or the payphone signal injected to pin 17 get
through to the earphone. The input level for cut-off:
Figure 14.
IL
V cut-off = Rx output (VDC/2) + 2x V diode ≈ 2.2 V + 1.3 V ≈
3.5 V
V+L
R
R
11
The signal for DTMF confidence tone in the earphone must
be injected to pin 17 at correct DC level or via a capacitor.
The mute signal is taken from pin 18 via a resistor in series
with at diode. The diode is needed in order not to disturb the
DC-level at pin 17 in not muted condition.
R
7
6
Z
V
R
19
TO
Line
Z
in
RI
R
10
R
Z
bal
5
The transmit mute (pin 10) can be taken from the same point
(pin 18) with a series resistor (see figure 10).
Figure 15.
9
ERICSSON [ ERICSSON ]
