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PBL38813

Voice-switched Speakerphone Circuit with built in loudspeaker amplifier

Ericsson  

PBL 388 13

Speech IC

1

TX

+

C+

19

Vc

18

17

2 IB

Z≈0

22 -

IC IE 21

+

R VR

IR

VRE RE

IL

20

2V DC-

supply

23

2,6V Level shift

16

+

-

Vs

PBL 388 13

+ Line

audio input

from Rx

channel

- Line

VRE = VR;

V R = I R x R;

I L = I R = I B + I C ; I C < 50 x IE = 50

VS = 0 then IC = 0

x

VR

RE

= 50

x

R

RE

x

IR

Figure 23. Loudspeaker amplifier current supply system.

Some optional features using

the dc. set volume control on

the loudspeaker amplifier of

PBL 388 13.

The DC set volume control has an

wholly internal function to lower the gain at

low supply voltages. This is to avoid that

the power stage dies and causes breaks in

the output signal at long line lengths ie. low

currents in combination with high input

signals. This DC controlled volume is

externally accessible in the PBL 388 13

and can thus be utilized in several ways.

PBL

388 13 19

This resistor sets

+

the max. attenuation

a).

This resistor sets

the min. attenuation

position on the pot.

PBL

388 13

+

Weighted

resistors

19

b).

Three bit

digital

signal

Figure 24. DC - volume control options.

a). To control the loudspeaker volume

with a DC- voltage from a potentiometer.

b). To control the loudspeaker volume

with a digital signal ( for ex. 8 - levels ).

c). An AGC can be combined with the

volume control by connecting a resistor

from the DC - control pin 19 to the output of

the receiver detector at pin 9. Care has to

be taken not to disturb the speech switching

balance. If the resistor is made too low

ohmic the same value has to be applied on

the transmitter detector output at pin 6 as

well as that the capacitors at the detector

outputs have to be made bigger.

19

PBL

388 13

9

c).

To volume

control

Resistor that is added

and which determines

the dynamics of the AGC

19

+ + pin 4

PBL

10µF

388 13

To volum

control

Resistor that sets the

"softclipping" level

d). A ”softclipping” with a fixed level

can be combined with the volume control.

A draw back with the fixed level is that

when setting it in to inhibit clipping distortion

at a long line ie. low level, the level will not

increase with short lines even if the supply

voltage would allow it. In the other case

when setting the level for a short line some

amplitude clipping on long line can be

expected.

11

d).

18

PBL 20

388 13

17

Sets the steepness

of the "softclipping"

Resistor that sets the

"softclipping" level

e). A ”softclipping” that is controlled by

the ”real” output level that means that the

"softclipping" will follow the line current

changes and will at all times give the opti-

mum distortion limiting performance.

19

e).

+

10µF

To volume

control

Figure 25. DC - volume control options.

11

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