PBL3852 Datasheet, PDF (8/24 Pages) Ericsson – Universal Transmission Circuit

PBL 3852
Transmitter amplifier
The transmitter amplifier in PBL 3852
consists of three stages. The first stage is
an amplitude limiter for the input signal at
TI, in order to prevent the transmitted
signal 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 for 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/R7 where
ZL is the impedance across the tele-
phone line. Hence, the absolute maxi-
mum signal amplitude that can be
transmitted to the line undistorted is
dependent of R7.(amplitude limiting) The
figure 20 shows the range for the
amplitude limiter dependent of the
operating point on the DC characteristic.
The transmitter gain and frequency
response are set by the RC-network
between the pins MO and TI (See fig. 11).
The capacitor for cutting the high
frequency end is best to be placed directly
at the microphone where it will also act as
a RFI suppressor. The input signal source
impedance to the transmitter amplifier
input TI should be reasonably low in order
to keep the gain spread down, saying that
R4//R5//R6 (see fig. 32) must be at least a
factor of 5 lower than the ZTin. Observe
that the capacitor C9 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. 8 therefore the transmitter gain
would change if the transmitted signal
gives reason to an ac-voltage leak signal
across C9, this being a feedback point. If
the transmitter has an unacceptable low
sving to the line at low line currents
<≈10mA the first should be to examine if
the circuits DC- characteristic can be
adjusted upwards and first secondly make
use of the linear PD.
11
3
(a)
R_A
C_A
no attn.
11
3
(b)
R_A
C_A
no attn.
C_B
11
3
(c)
R_A
C_A
R_B
attenuation
(a),(c), (d)
11
3
C_C
(d)
R_A
C_A
11
3
(e)
R_A
C_A
11
3
(f)
C_C
R_A
C_A
R_B
R_B
C_B
R_B
C_B
attn.without dc.
attenuation
attn.without dc.
Figure 11. Different possible types of networks between microphone amplifier and transmitter.
(b),(e)
big CA
small CA
(f)
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 differential amplifier and the
third stage a balanced power amplifier.
The power amplifier has a differential
output that does not need a series
capacitor with the load. The receiver
(a)
(b)
(c)
(d)
17
-
+ _Rx
+
16
17
17
≈150Ω
-
+ _Rx
+
-
Rx
Z
150Ω
+
+
+
16
16
Figure 12. Receiver arrangements.
8
amplifier uses at max. swing (4-6) mA
peak. This current is drawn from DC2
that can supply 2 mA continuous
current, the C3 helping to supply the
peaks, this applies for speech signals
only. Continuous sinusoidal signals at
this level will load the DC2 down. If a
distortion appears in the earphone
amplifier output at high signal levels,
high line currents, low ohmic earphone
load or at low frequencies, the most
probable fault is that the filtering
capacitor of the earphone amplifier
supply C3 is too small. At low line
currents (normal case, IL < 10 mA)