PBL 3881
+ Line
IL
+
19
C +
Speech IC
2V DC
1
-suppy
audio input
22
16
V
17
21
from Rx
channel
T
X
2,6V
Level shift
20
18
-
+
-
I2
+
2
IC
I1
Vs
Z ≈ 0
PBL3881
R1
IR
VRE
VR
R2
- Line
V
RE = VR
= I1
+ I C
V R
IL = I R
= I R
x R
VR
x
R
R2
IC
x
V = 0 then IC = 0
< 50 I2 = 50
x
x
= 50
IR
R2
Figure 25. Loudspeaker amplifier current supply system.
transmitting mode periode. The capacitor
number of different power supply
220 µF from V+ via a diode to background conditions. Fig. 21 and 22. The highest
+Line
noise detector output is to rapid charge
the capacitor C4. This is needed in case
the circuit is started in hands free mode,
without it the transmitter function would
be delayed. See fig.18.
output swing is obtained if pin -C is
connected to ground (- Line) and pin +L
is connected to a stable DC supply. This
supply could be either mains powered or
power from the telephone line through an
inductor. Fig. 22. Current consumption is
directly proportional to the voltage
IL
IC
I
Speech
circuit
Loudspeaker
amplifier
CTR Input
between pins +L and -C. When using the
application according to figure 21, pin -C
is used as the negative floating supply
point for the amplifier. The output signal
of the loudspeaker amplifier is referred
to +L. The reservoir capacitor C makes it
possible for the amplifier to handle power
peaks that are much higher than would
be possible with continuous signal. The
optimal design without using a stable
supply is to balance it against the DC
characteristics of the speech circuit that
is working in parallel. See fig. 25. The
single ended loudspeaker amplifier has
an internal gain regulation that prevents
distortion in case of insufficient line
current.
For full speech control (50 dB attenuation
between the channels) this input can be
left unconnected. To set the function to
full ”duplex” (both channels fully open ≈
0 dB) the input has to be at a level 600
mV below V+. Intermediate values of
speech control attenuation are possible
to set within the 600 mV mentioned
above. See figures 19 and 20. To set the
circuit into a mute state current (35-40
µA) has to be drawn out of the input. By
lowering the voltage at the input below
0.9 V a shut down condition will emerge
where both receiver and transmitter
channels are closed. See fig. 17.
Z ≈ 0
R
IR
-Line
Figure 26. Current sharing system.
Loudspeaker amplifier
The loudspeaker amplifier drives a 25 -
50 Ω impedance loudspeaker directly.
The amplifier is designed to work under a
11
ERICSSON [ ERICSSON ]
