U4092B Datasheet, PDF (10/32 Pages) TEMIC Semiconductors – Monolithic Integrated Feature Phone Circuit

U4092
TELEFUNKEN Semiconductors
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 IMPSEL = 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
U4092B 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
Hybrid
Line
Figure 8 Basic voice switch system
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 U4092B, 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
Preliminary Information
Rev. A1: 24.01.1995