LS656 데이터 시트보기 (PDF) - STMicroelectronics

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LS656

TONE GENERATOR INTERFACE TELEPHONE SPEECH CIRCUIT WITH MULTIFREQUENCY

STMicroelectronics 

LS656

Figure 6 : Circuit Configuration of the Shunt Regulator.

Figure 7 : DC Characteristic.

2. Two to Four Wires Conversion

TheLS656 performsthe two wires (line) to four wires

(microphone, earphone) conversion by means of a

Wheatstone bridge configuration so obtaining the

proper decoupling between sending and receiving

signals (see fig. 8).

For

a

perfect balancing of

the

bridge

ZL

ZB

=

R1

R2

The AC signal from the microphone is sent to one

diagonalof the bridge (pin 6 and 9). A small percent-

age of the signal power is lost on ZB (being ZB

>> ZL) ; the main part is sent to the line via R1. In re-

ceiving mode, the AC signal coming from the line is

6/15

sensed across the second diagonal of the bridge

(pin 11 and 10). After amplification it is appliedto the

receiving capsule.

The impedance ZM is simulated by the shunt regu-

lator that is also intended to work as a transconduc-

tance amplifier for the transmission signal.

The

impedance

ZM

is

defined

as

∆V6 − 9

∆I6 − 9

From fig. 6 considering C1 as a short circuit for AC

signal, any variation ∆V6 generates a variation :

∆V7

=

∆VA

=

∆V6

⋅

Rb

Ra + Rb

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