U4090B Datasheet, PDF (10/34 Pages) TEMIC Semiconductors – Monolithic Integrated Feature Phone Circuit

U4090B
3. During ringing the supply for the system is fed into V_B
via the ringing power converter (RPC). The only
functional amplifiers are found in the speaker
amplifier section (SAI, SA, SACL).
4. In an answering machine the chip is powered by an
external supply via pin V_B. This application demands
a posibility to activate all amplifiers (except the
transmit line interface TXA). Selecting IMP-
SEL = high impedance activates all switches at the ES
line.
Acoustic feedback suppression
Acoustical feedback from the loudspeaker to the handset
microphone may cause instability in the system. The
U4090B offers a very efficient feedback suppression
circuit, which uses a modified voice switch topology.
figure 8 shows the basic system configuration.
Handset
microphone
Loudspeaker
TX
Att
Log
Att
contr
Log
RX
Att
Figure 5. Basic voice switch system
Hybrid
Line
94 8956
Two attenuators (TX ATT and RX ATT) reduce the
critical loop gain by introducing an externally adjustable
amount of loss either in the transmit or in the receive
path.The sliding control in block ATT CONTR
determines, wether the TX or the RX signal has to be
attenuated. The overall loop gain remains constant under
all operating conditions.
Selection of the active channel is made by comparison of
the logarithmically compressed TX- and RX- envelope
curve.
The system configuration for group listening, which is
realized in the U 4090 B, is illustrated in figure 9. TXA
and SAI represent the two attenuators, whereas the
logarithmic envelope detectors are shown in a simplified
way (operational amplifiers with two diodes).
10 (34)
TELEFUNKEN Semiconductors
Preliminary Information
Rev. C1, 28-Oct-96