u4089b-m ATMEL Corporation, u4089b-m Datasheet - Page 9

• Manufacturer Part Number: u4089b-m
• Description: Multi-standard Feature Phone Circuit With Voice Switch
• Manufacturer: ATMEL Corporation

A detailed diagram of the AFS (acoustic feedback
suppression) is given in figure 9. Receive and transmit
signals are first processed by logorithmic rectifiers in
order to produce the envelopes of the speech at TLDT and
RLDT. After amplification, a decision is made by the
differential pair which direction should be transmitted.
The attenuation of the controlled amplifiers TXA and SAI
is determined by the emitter current IAT which is
comprised of three parts:
I
I
I
I
DG = I
Figure 10 illustrates the principal relationship between
speaker amplifier gain (GSA) and attenuation of AFS
(ATAFS). Both parameters can be adjusted indepen-
dently, but the internal coupling between them has to be
considered. The maximum value to be used of GSA is
36 dB. The shape of the characteristic is moved in the
x-direction by adjusting resistor RATAFS, thus changing
ATAFS
however, can be determined by reading the value which
belongs to the actual gain GSA
gain is reduced, the attenuation of AFS is automatically
reduced by the same amount in order to achieve a constant
loop gain. Zero attenuation is set for speaker gains GSA
v GSA0 = 36 dB – ATAFS
Rev. A3, 06-Mar-01
ATAFS (dB)
Figure 10. Reducing speaker amplifier gain results in an equal
ATAFS
ATGSA
AGAFS
AT
ATAFS m
ATAFS a
GSA o
= I
ATAFS
AT
m
sets maximum attenuation
decreases the attenuation when speaker
amplifier gain is reduced
decreases the attenuation according to the loop-
gain reduction caused by the AGA-function
. The actual value of the attenuation (ATAFS
– I
0.67 dB/mA
reduction of AFS attenuation
ATGSA
– I
RATAFS
GSA a
AGAFS
m
.
RATAFS
a
. If the speaker amplifier
36 dB
GSA (dB)
not usable
94 8957
a
),
Operating Range of Speaker
Amplifier
The basic behavior is illustrated in figure 11. Actual val-
ues of ILON/ILOFF vary slightly with the adjustment of
the DC-characteristics and the selection of the internal
line impedance.
Figure 12. Comparator thresholds depend on DC mask and line
94 9131
SA
SA
off
on
7
6
5
4
3
Figure 11. Threshold of speaker amplifier
10
= ILON
= ILOFF
= ILON
= ILOFF
12
at line impedance = 600 W
at line impedance = 900 W
impedance
14
IL
I
U4089B-M
L
off
( mA )
16
R
R
R
IL
DC
DC
DC
on
=
=130kW
=68kW
18
IL
13284
20
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