PBL 385 70
amplitude that can be transmitted to the
line undistorted is dependent of R6.
(amplitude limiting).
(d)
(c)
(a)
(b)
PBL 385 70
PBL 385 70
PBL 385 70
17
17
17
(C)
The transmitter gain and frequency re-
sponsearesetbytheRC-networkbetween
thepins11and3. Seefig.10.Thecapacitor
for cutting the high end of frequency band
is best to be placed directly at the
microphone where it also will act as a RFI
suppressor. The input signal source
impedance to the transmitter amplifier in-
put TI should be reasonably low in order to
keep the gain spread down, saying that
R4//R5 (see fig. 4) must be at least a factor
5 lower than the ZTin. Observe that the
capacitor C1 should have a reasonably
good temperature behaviour in order to
keep the impedance rather constant. The
V+C´s influence on the transmitter DC-
characteristic is shown in the fig.7 (DC-
characteristic), therefore the transmitter
gain would change if the transmitted
signal gives reason to an ac-voltage leak
signal across C1 since this is a feedback
point.Ifthetransmitterhasanunacceptable
low sving to the line at low line currents
<≈10mAthefirststep shouldbetoexamine
if the circuits DC characteristic can be
adjusted upwards.
+
+
+
Z
+
+
+
Rx
Rx
Rx
-
Z
18
18
18
(C)
Z > 5k
The capacitor C is optional
Figure 11. Receiver arrangements.
a). Adjustable with the divider R4,R5
for the transmitter and with R12 for the
receiver.
b). The attack point of the regulator is
adjusted with the divider R1,R2 to either
direction , up or down ,on the line current
axis.
c). Theangleofelevationofthecurveis
mainly set by the value of R6. If the DC-
characteristicsisset accordingtotheline
parameters and a correct value for R6 is
chosen the angle is mostly correct but it
canbeadjustedwithR6.Theadjustement
will affect the DC-characteristics aswell
as most of the other parameters. This is
why the DC-characteristic is set early in
the design phase.
parallelacrossitsoutputtolimitthesignalto
the earphone and thus preventing an
acoustical shock. A resistor in series with
theoutputcanverywellbeusedtoincrease
the protection level. Note, that the noise in
the receiver is allways transmitter 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.
Gain regulation.
Boththereceiverandtransmitteraregain
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. 12
shows a typical transmitter or receiver gain
patternversuslinelength.Thefollowingwill
show, what to alter, to change the look of
the curve.
dB
Receiver amplifier
b.
c.
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
a.
High limit
Low limit
I
L
Figure 12. Gain regulation principle.
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
to24dB). Thedrivingcapacityofthepower
stage can be optimized by a resistor on
pin16, an other method is to connect a
resistor series in with the earphone itself.
The balanced earphone amplifier can not
be loaded to full (both current and signal
level ) single ended.The signal would be
Battery feed
Regulation:
R1
R2
R6
48V,2 200Ω
48V,2 400Ω
48V,2 800Ω
∞
∞
∞
47Ω
75Ω
∞
∞
180k
100Ω
No regulation:
Set for low gain
All feedings
Set for high gain
Set for high gain
∞
18k
22k
22k
∞
∞
47 - 100Ω
47Ω
75 - 100Ω
distorded when returned to ground.
A
methodeisshowninfig.11d.howtoconnect
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
8
ERICSSON [ ERICSSON ]
