PBL 3853
is taken from VDC will leave even less
current available from VDC. The speech
functions and with them the VDC output
will die below 16 mA line current. There
is an unintentional effect that might
cause puzzlement. Just below 16 mA
line current the VDC will come into
function and will be operating when a
high signal is on the line. With a high
signal on the line half of the set current,
in this case 6 mA, can be taken out. The
possible available current out from VDC
will increase with increasing line current
and the margin to, that a signal on the
line has the effect of decreasing the
available current out from VDC, will
increase in the same extent as the line
voltage increases.
Figure 4 shows only one of the many
ways to use the circuit to generate the
VDC supply. The conditions for
dimensioning the VDC are set by the
available line voltage and line current,
what voltage respectively current the VDC
supply has to deliver or actually the
difference between the line voltage and
current will go, either through T1 to VDC
or through T2 to ground, has to be
altered. It is done easiest by adding a
shunt regulator between the collector of
T1 and pin 18 on the circuit. The VDC is
still taken at the collector of T1,
Comments to the Reference
Figure for PBL 3853 Test Set
up (fig.4) Regarding the
External DC-Supply (VDC).
This schematic is a specific application
of the PBL 3853, where the main
see fig. 20.
objective is to optimize the usage of the
line current so that of 16 mA line current
12 mA can be taken out to feed auxiliary
functions. Typical line voltage 6.0 V at
20 mA line current (with a transmitter
signal swing of 1.8 Vpeak).
It would be possible to save some
more current (≈ 50mA) by instead of
increasing the DC-characteristic towards
the line with low ohmic Rc and Rd (the
sum 41k) make these larger
In the most simple case the shunt
regulator can be a diode (eventually a
Shottky diode) maybe with an addition of
a resistor between pin 18 and -line to
keep somewhat constant voltage across
the diode. Unfortunable the voltage can
not be increased more than a couple of
hundred millivolts before the function
that is to provent the T1 from saturating
regarding the voltage across it would
disappear and which in is turn would
result that the negative half periodes of
the signal would be clipped with a
massive distortion as a result. What has
to be done at the same time to come
further in this matter is to make the
voltage drop smaller across R13 (also to
use a T1 with low sat.voltage). This
requires that the sense level of which
voltage has to be across R13 is altered.
The voltage across R13 is one diode
drop plus the voltage drop across R12.
What can be done is to substract some
of the diode drop, see fig. 21.
(68k+33k=101k) and lift the DC-char.
with Rb instead (see fig.7). The gain of
this is questionable because the need of
an additional resistor.
It ought to be understood that these
12 mA charge current into VDC out of
16 mA line current can only be acchie-
ved in no signal condition, both transmit-
ter and receiver. In case there is a
transmitted signal above a certain (low)
amplitude, it will cause breaks in the 12 mA
VDC charge current during a part or the
whole time of the negative half periodes
of the signal on the line. This means that
the filtered VDC will have a lower usable
current output. Is the said signal on the
line large, it will cause an absense of the
charge current half of the time and the
useful current will sink to 6 mA.
VDC out respectively line current contra
the possible output current from VDC. It is
possible to minimize these differencies
somewhat but it requires more complex
solutions, on the other hand if the
requirement to keep the voltage and
current difference as small as possible is
not of utmost importance it is possible to
create simpler solutions than what is
shown in the figure 4.
In case there is a need to minimize the
voltage difference between line and VDC
it can be influenced as follows. To start
with, the level that sets which way the
Observe that the circuitry needs a
recalculation of several components
especially R13. How far it is possible to
come, by decreasing the gap between
Vline and VDC depends on spread figures
and temperature requirement.
The difference between the line
current and current out from VDC is
possible to alter but only to minor extent.
(See fig. 15). A similar thing will happen
at receiving as at transmitting but on top
of that, the current to the receiver which
I
I
L
V
1
L
R13
R12
Line with signal
I
DC
9
1
PBL3853
Circuit supply
I=0.3mA
4
to gain reg.
with line length
TR2 control
level
V
F
-
7
8
V
T
2
D
DC- supply ≈ 4.5V
TR1
7.5k
1.2V
+
13k
15k
R1
+
V
x
+
2xV
D
T
1
37.5k
+
-
6
5
TR1 conducting. DC- supply charged constantly.
45k
V
DC
18
16
C
1
10k
2.5k
+
+
8.77k
1.2V
-
-
+
Mic.
12
15k
+
36k
7.5k
A
1
13
1.2V
14
R2
Line with signal
10
11
3
2
17
15
TR2 control
level
4.5V
C
4
V
F
DC- supply ≈ 4.5V
TR1
R
C
5
R3
A
+
C
3
R5
C
2
R
R4
B
TR2 conducts. DC- supply charge
interrupted during these periodes.
Figure 18.
12
Figure 19 . Functional Diagram.
ERICSSON [ ERICSSON ]
