U4092B Datasheet, PDF(12/32 Page) TEMIC Semiconductors – Monolithic Integrated Feature Phone Circuit

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U4092B Datasheet, PDF (12/32 Pages) TEMIC Semiconductors – Monolithic Integrated Feature Phone Circuit

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U4092

TELEFUNKEN Semiconductors

TLDT

TXA

SAI

RLDT

INLDT

RLDR

INLDR

94 8060

IAT

IATAFS

AGA

IAGAFS

AGA

IGSA

TLDR

IATGSA

ATAFS

RATAFS

GSA

Figure 10 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

comprised of three parts:

IATAFS sets maximum attenuation

IATGSA decreases the attenuation, when speaker amplifier

gain is reduced

IAGAFS decreases the attenuation according to the loop

gain reduction caused by the AGA-function

IAT = IATAFS â IATGSA â IAGAFS

DG = IAT * 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 ATAFSm. The

actual value of attenuation (ATAFSa), however, can be

determined by reading the value which belongs to the

actual gain GSAa. If the speaker amplifier gain is reduced,

the attenuation of AFS is automatically reduced by the

v same amount, in order to achieve a constant loop gain. Zero

attenuation is set for speaker gains GSA GSA0 = 36 dB

â ATAFSm.

Preliminary Information

Rev. A1: 24.01.1995

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