U4090B Datasheet, PDF (12/34 Pages) TEMIC Semiconductors – Monolithic Integrated Feature Phone Circuit

U4090B
TLDT
TXA
SAI
TX
RLDT
INLDT
RX
RLDR
INLDR
94 8060
I_AT
I_ATAFS
AGA
IAGAFS
AGA
IGSA
TLDR
IATGSA
ATAFS
RATAFS
GSA
Figure 7. Accoustic feedback suppression by alternative control of transmit- and speaker amplifier gain
The attenuation of the controlled amplifiers TXA and SAI
is determined by the emitter current IAT, which is com-
prised of three parts:
I_ATAS
I_ATGSA
I_AGAFS
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
I_AT = I_ATAFS - I_ATGSA - I_AGAFS
DG = I_AT * 0.67 dB/ mA
Figure 11 illustrates the principal relationship between
speaker amplifier gain (GSA) and attenuation of AFS
(ATAFS). Both parameters can be adjusted
independently, but the internal coupling between them
has to be considered. Maximum usable value of GSA is
36 dB. The shape of the characteristic is moved in the
x-direction by adjusting resistor RATAFS, thus changing
ATAFS_m. The actual value of attenuation (ATAFS_a),
however, can be determined by reading the value which
belongs to the actual gain GSA_a. If the speaker amplifier
gain is reduced, the attenuation of AFS is automatically
reduced by the same amount, in order to achieve a
v ^{constant loop gain. Zero attenuation is set for speaker}
gains GSA GSA0 = 36 dB - ATAFS_m.
12 (34)
TELEFUNKEN Semiconductors
Preliminary Information
Rev. C1, 28-Oct-96