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PBL38814

Voice - switched 2-channel Circuit with loudspeaker amplifier

Ericsson  

PBL 388 14

Functional Description

Speech control section

Transmitter and Receiver

Channels

GNDA LSP + VA VOL

LL

18 19

22

21

20

CTR 24

Txout 4

V+ 16

3

5

+

F3

PBL 388 14

Control

23 LSP in

17 GND

11 Rxout

F6

13

11

F2

-Txin 1 -

+Txin

2

F1

+

8

N Det R5

+

C4

6

TxDet

9

CMP

Ref.

10

7

RxDet

+

C3

+

C1 C2

F5

15 -Rxin

F4

+ 14 +R xin

Figure 5. Passive networks setting the speech control function.

PBL388 14

F2

100k 100k

F1 +

3k

Tx

1

3k

2

~ VTxin

I

Ref.

17

F5

120k 120k

F4

+

20k

20k 15

Rx

14

~ VRxin

Figure 6. Receive and transmit channel input arrangement.

The transmitter and receiver channels

consist of three amplifying stages each, F1,

F2, F3 and F4, F5, F6. The inputs of the

amplifiers must be ac. coupled because

they are dc. vise at the internal reference

voltage (≈ 2V) level. F1 and F4 are fixed

gain amplifiers of 29.5 dB and 15.5 dB

respectively, while the rest of them are of

controlled gain type amplifiers.The gain of

F2, F3 as well as F5 and F6 is controlled by

the comparator. Ac. loading the channel

outputs F3 and F6 will lessen the dc.

current consumption, maximum load 10

kΩ. The output capacity can be increased

somewhat in case needed, by coupling a

10 kΩ resistor from the respective output

pin directly to ground (before the optional

capacitor).The comparator receives its in-

formation from the summing point of the

transmitter, receiver and background noise

detectors at CMP input. The control input

CTR, controls the gain dynamics (25 or 50

dB). Amplifiers F2 and F3 have the maxi-

mum gain when the transmitter channel is

fully open, consequently the amplifiers F5

and F6 will have minimum gain and vice

versa. See figure 5 and figure 11.

The positive input on each channel

has a high input impedance. It renders a

good gain precision and noise performance

when used with low impedance signal

source . The negative input of the receiver

channel should be returned to ground with

a capacitor. The differential input of the

transmitter channel can be used to sup-

press unwanted signals in the microphone

supply, see figure 7. Also see application.

Signal Detectors and the

Comparator

The signal detectors sense and rectify

the receiver and microphone signals to

opposite polarities referenced to the internal

reference voltage of approx. 2V. The voltage

at RxDet will go positive and at TxDet

negative in the presence of a signal at the

respective channel input. In the idle (no

signal) state, the voltages at RxDet ,TxDet

and CMP are equal to the internal reference

voltage. Signal at Txin will result in a

decreasing level at TxDetout and hence

also at CMP input.

6

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