January 1998
PBL 388 13
Voice-switched Speakerphone Circuit
with built in loudspeaker amplifier
Description.
The PBL 388 13 contains all the necessary circuitry, amplifiers, detectors,
comparator and control functions to implement a high performance, voice-switched,
loudspeaking, ”hands-free ” telephone. The gain dynamics (attenuation between
channels) is selectable (25dB or 50dB) via a separate pin. A background noise detector
in the transmitting channel reduces the influence of continuous external noise signals
to the switching .
The PBL 388 13 is designed for telephone systems that are either powered from
the telephone line or from a mains powered constant voltage dc. supply. The circuit
contains a transformerless audio power amplifier with a current supply circuitry
(patented) that eliminates the need of inductors. Automatic volume attenuation in the
power amplifier extends the operating range at low line currents. A special feature in
this circuit is that the power amplifier volume control can be implemented either as an
ac. potentiometer control or as a digital control by a
µ-processor
(dc. control).
Filtering is possible of both, the audio and the speech switching control signals,
in both transmitter and receiver channels.
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•
•
•
Key Features
Minimum of external components
needed for function.
Selectable gain dynamics. (25 or 50
dB)
Direct telephone line powered solution
(patented).
Low power consumption:
≈1mA
at 3.3V
(typical) for speech switching, audio
power amplifier quiscent current
≈1mA.
Drives an 25 - 50 ohm loudspeaker
without a transformer.
Background noise compensation in the
transmitting channel with hold function
at receive.
Input amplifiers of both channels have
balanced inputs.
Exellent noise performance.
Encapsulated in 24 pin plastic ”skinny”
DIP and 24 pin SO .
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•
•
•
•
17
18
19
20
22
–
+
23
21
4
24
15
16
+
Control
F3
F6
12
10
P
B
L
3
11
8
8
PBL 388 13
1
3
24 pin SO
3
5
–
2
F1
+
7
6
8
9
1
Ref.
–
F4
13
+
14
P
B
L
F2
F5
3
8
24 pin DIP
Figure 1. Block diagram.
1
8
1
3
ERICSSON [ ERICSSON ]
