PBL 385 82
signal to the earphone and thus preventing
an acoustical shock. A resistor in series
with the output can very well be used to
increase the protection level. Note, that
the noise in the receiver is allways trans-
mitter noise that has been more or less
well balanced out by the side tone network.
The RC - network (optional) at the
output is to stabilize against the inductive
load that an earphone represents.
V
16
14
12
10
8
6
4
2
V telephone line
V line
V pin 4
V pin 2
PBL 38 582
17
+
+
Rx
-
(C)
Z
I
20
40
60
80
100
120
L
18
Z
(C)
Z > 5k
mA
The capacitor C is optional
Figure 7. DC - characteristics.( R6 = 75
Ω
)
Figure 8. Unbalanced Rx loading.
Transmitter amplifier
The transmitter amplifier in PBL38582
consists of three stages. The first stage is
an amplitude limiter for the input signal at
TI, in order to prevent the transmitted sig-
nal to exceed a certain set level and cause
distortion. The second stage amplifies
further the signal from the first and adds it
to a DC level from an internal DC-regulation
loop in order to give the required DC
characteristic to the telephone set. The
output of this stage is TO. The third stage
is a current generator that presents a high
impedance towards the line and has its
gain from TO to +L. The gain of this
amplifier is ZL/R6 where ZL is the
impedance across the telephone line.
Hence, the absolute maximum signal
amplitude that can be transmitted to the
line undistorted is dependent of R6.
(amplitude limiting)
The transmitter gain is set by the
analog (transmitter) signal from the pass-
band circuit and the frequency response is
set by the capacitors at input circuit at pin
3, the low end being influenced by C3 and
the high end by C6. The input signal
source impedance to the transmitter
amplifier input TI should be reasonably low
in order to keep the gain spread down.
Receiver amplifier
The receiver amplifier consists of three
stages, the first stage being an input buffer
that renders the input a high impedance.
The second stage is a gain regulated diffe-
rential amplifier and the third stage a
balanced power amplifier. The power
amplifier has a differential output with low
DC- offset voltage, therefore a series
capacitor with the load is normally not
necessary. The receiver amplifier uses at
max. swing 4-6 mA peak. This current is
drawn from the +Line. The gain and
frequency response is set at the input RI
with a RC-network. The receiver gain can
be regulated.The range of regulation from
the input to the output is 5
±
2 dB (19 to
24dB). The balanced earphone amplifie
can not be loaded to full (both current and
signal level ) single ended.The signal would
be distorded when returned to ground.
A methode is shown in fig.8
how to
connect a light load (5k ac. or DC wise) to
the output. It is preferred that both outputs
are loaded the same. The receiver has, as
a principal protection, two series diodes
anti parallel across its output to limit the
Gain regulation.
The receiver is gain regulated (line
loss compensated).
There is a fixed default compensation
on the chip that can be adjusted or or set to
constant high or low gain mode. The input
impedance at the gain regulation pin 6 is
5.5k
±
20%. The default regulation pattern
is valid when the input is left open. Fig. 9
shows a typical receiver gain pattern ver-
sus line length. The following will show,
what to alter, to change the look of the
curve.
a). Adjustable with R12 for the recei-
ver.
b). The attack point of the regulator-
can be adjusted with resistors R13 or R14
to either direction, up or down, on the line
current axis.
c). The angle of elevation of the curve
is mainly set by the value of R6. If the DC-
characteristics is set according to the line
parameters and a correct value for R6 is
chosen the angle is mostly correct but it
can be adjusted with R6. The adjustement
will affect the DC-characteristics as well as
most of the other parameters. This is why
the DC- characteristic is set early in the
design phase.
6
ERICSSON [ ERICSSON ]
