January 1998

Figure 1. Block diagram.

24 pin DIP

PBL 388 13

Voice-switched Speakerphone Circuit

with built in loudspeaker amplifier

24 pin SO

Description.

1

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.

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 .

+

F2

F3

F6

F5

–

F1

PBL 388 13

F4

22

21

3

1

2

76

8

9

13

14

11

23

16

17

18

20

24

4

+

+

–

–

+

19

10

12

Ref.

5

15

Control