PBL 3853
Connecting the pin 6 to ground disables
the line length regulation in the high gain
mode, whereas setting the pin 6 on 1.2
V dc level or higher disables the line
length regulation in low gain mode.
Transmitter Amplifier
PBL 3853
The transmitter consists of two stages.
The output of the transmitter drives a
signal with adjustable impedance into
the line. The signal at pin 1 (positive line)
is 180 degrees out of phase with the
signal at pin 2 from where the side tone
compensation signal is taken. The
transmitter has two inputs. An input at
pin 5, where both active impedance and
DC-characteristic can be adjusted.
Another input at pin 3, which is the
microphone signal input, where the filter
for the transmitter frequency
11
12
-
a)
M
+
Microphone Amplifier
13
The microphone amplifier is primarily
designed to be used with dynamic or
magnetic microphones but can be used
with buffered elctret microphones as
well. The input of the amplifier is
balanced for good CMRR. The gain of
the amplifier can be regulated with the
line length. The microphone amplifier
has a mute input of its own. Pin 10.
It is possible to use the microphone
amplifier as a limiter (added to the limiter
in the transmitter output stage) of the
transmitted signal. The positive output
swing is then limited by the peak output
current of the microphone amplifier. The
negative swing is limited by the
saturation voltage of the output amplifier.
The output of the amplifier is DC-wise at
internal reference level (1.2 V). The
lowest negative level for the signal is
reference minus one diode and
sat.transistor drop. (1.2-0.6-0.1=0.5 V).
The correct clipping level is found by
determining the composite AC- and DC-
load that gives a maximum symmetrical
unclipped signal at the output. This
signal is then fed into the transmitter
amplifier at a level that renders a
Dynamic mic.
PBL 3853
11
12
-
b)
characteristic is placed. Pin 3 is at the
same time used as a summing point to
all other audio signals that are
transmitted into the line (DTMF tones,
hands free signal...). See figure 4.
M
+
13
Magnetic mic.
PBL 3853
11
Receiver Amplifier
12
c)
-
The receiver gets its input signal from
the summing point where the side tone
cancellation takes place. The output
drives single sided a low Ω (150 Ω)
acoustical transducer. Diodes for
absorbing acoustical shock must be
provided externally. The amplifier has
two inputs. One of them can be
regulated with line length, the other has
fixed unity gain. The latter can be used to
cut off the receiver by forcing the input
positive and is also useful for injecting
the DTMF confidence tone into the
receiver. The receiver amplifier is
powered from the regulated DC-supply
at pin 18.
M
+
13
Extended low voltage/
current operation
2x50k
4
PBL 3853
11
12
13
-
d)
M
+
symmetrical signal clipping on the line
(adjust with ratio R3, R4). The total
transmitter gain can then be adjusted
with the load of the electret microphone
buffer amplifier. See figure 9.
DC
PBL 3853
DC (ref.≈1.2V)
PBL 3853
11
12
ref.-diod≈0.5V
Powerful
cc gen
Valid when no C4 is used.
DC-load=R3+R4 R3+R4>6k
-
C4
R3
e)
AC-load=R3+R4// (R
+Z
) //Z
DTMF DTMF
TI
M
+
+
11
Line
13
DC-
load
R1
R
PBL3853
DTMF
to gain reg.
with line length
4
AC-
Z
Z
R4
(≈ 6dB)
VF
TI
DTMF
Circuit supply
load
I=0.3mA
7.5k
1.2V
13k
15k
+
R
4
Figure 9. Microphone amplifier output
clipping, level.
37.5k
R
C
D
PBL 3853
11
12
6
5
C3
-
45k
R
10k
2.5k
f)
V
X
M
+
+
18
13
8.77k
10
+
C
1.2V
PBL 3853
DTMF in
15k
17
If balanced mic.is used an
additional RC link is recomm.
if pin 4 is used as supply
Figure 8. Fixed gain and line length
regulation.
Figure 10. Common receiver and
transmitter mute arrangement.
Figure 11. Microphone solutions.
7
ERICSSON [ ERICSSON ]
